Weber and Coulomb's MX-5

Post up a thread for your EV. Progress pics, description and assorted alliteration
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Post by offgridQLD » Thu, 13 Mar 2014, 14:58

Particualy if the old ICE motor had any oil leeks or carless person spilling oil. The rubber in the mounts turns to jelly.

+ one on not making them to stiff. Nothing more tiring than vibrations resonating through the chassis. Perhaps a custom made torque arm mount similar to what used in a FWD car that only comes into play when you really step on it (if it comes to that).

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Post by offgridQLD » Thu, 13 Mar 2014, 14:58

Edit : sorry double post Image

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Post by jonescg » Thu, 13 Mar 2014, 16:40

I recall Malcolm saying that his solid mounted motor in the 4x4 was very noisy, despite being very well balanced.
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Post by weber » Thu, 13 Mar 2014, 21:39

Thanks for the warnings re too-hard motor mounts, guys.

Here's a very small enjoyment-increasing mod. With our use of the ignition key's momentary start position as a more general purpose input device (for changing the tacho display in drive mode, and charge current in charge mode), Coulomb found an annoying side-effect -- accessories have their power cut off momentarily -- so your favourite song gets rudely interrupted.

I decided the easiest way to fix this was by adding a diode from the key-switch's START output to its ACC output. Well almost. I thought, "It would be bad if the diode failed short-circuit so that turning the key to ACC enabled drive mode". Fortunately there is another output from the key-switch in START (and ON) position, called IG1. So I ran the diode from IG1 to ACC (BLU to BLK-WHT). I had some 1N5401 diodes (3 A 100 V) bought for 12 V relay logic, so I used one of them.

Image

Physically, the mod was about as simple as you could get. I bent the diode leads to the right width, squashed their ends flat with pliers so they could get a wedge-like start, and jammed them into the back of the connector that plugs into the key-switch. You can see the diode's tinned leads through the translucent plastic of the connector in the photo below. They are jammed between that plastic and the brass quick-connects inside it. You can also see the two blobs of neutral-cure silicone I squirzled on to insulate the parts of the leads that were exposed. Then I plugged it back in, tested it, and Robert's-ya-father's-nearest-male-relative.

Image

[Edited both the text and the photo to avoid possible confusion about where the diode leads were, as pointed out by Coulomb.]
Last edited by weber on Sat, 15 Mar 2014, 04:49, edited 1 time in total.
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Post by coulomb » Sat, 15 Mar 2014, 03:47

Following Weber's movie of the motor moving under load, we decided that the motor mounts (original engine mounts) had to be replaced. It took most of today's EV day, but we finally removed the old mounts. It happened that the first one to come out was the one we thought was not too bad.

Image   Image

The first photo shows a smallish crack at the top left, where the rubber joins the metal. However, as shown in the next photo, when under a little stress (here provided by the screwdriver), the crack widens somewhat. Still, it seemed like it was not in too bad a shape. We certainly hoped for more damage in the other mount, to vindicate the expense and time of replacing the mounts.

Image   Image

The mount on the left of each photo is a new one. As you can see, the second mount didn't disappoint; it's in three pieces. Weber measured some 16 mm it would have been able to move, possibly plus about 5 mm of rubber that might have compressed a few millimetres more.

As discussed earlier, we decided we'd Sikaflex the gaps in the new mounts.

Image

Here is the new mount after filling, with cling wrap around it to prevent it from leaking out, and to smooth the outline somewhat.

The astute reader at this point may be asking 'Why "the new mount (singular)"? Don't you mean "one of the two new mounts (plural)"?' In fact, it is our intention to replace both mounts, considering the effort of getting to them. On EV days we often seem to miss deliveries; they don't seem to make much effort to see if anyone is home out the back. But when we went to pick up the package from the post office, it seemed too small and light for two mounts. Indeed, there was only one mount there. The Ebay listing showed two mounts, so Weber assumed that "quantity one" meant "one pair". But it turns out that MX-5 mounts are the same on either side. Indeed, the text clearly states this, and that if you want a pair, then specify quantity two. Somehow, Weber must have been so convinced about the pair idea, that he either skimmed over that text without reading it properly, or he didn't read it at all. Needless to say, he was quite annoyed with himself. He's ordered a second mount now; it should arrive on Tuesday.

[ Edit: mention left mount is a new one; reworded the astute questions. Added arrives Tuesday. More rewording of mount (singular) verses mounts (plural).]
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Post by weber » Wed, 19 Mar 2014, 00:16

The second Engine mount arrived today and was duly Sikaflexificatorised(TM).

MX5 Mania, the supplier of the engine mounts (and several other parts used in the MX-5) were kind enough to credit me the cost of postage for the second mount when I pointed out the issue with the photo, even though I conceded it was my own dumb fault for not reading the description. They also improved the photo. Thanks MX5 Mania!

I looked up the cure time for the Sikaflex-227 we are using and it is exponential with depth (half a day for the first 2 mm and doubling for each additional 2 mm, at 23°C 50% RH).
http://aus.sika.com/dms/getredirect.get ... ka.com/107
This is because it cures by reacting with moisture from the air, and the cured outer layers form a pretty effective moisture barrier to the inner layers, and so it will probably take many months to cure right to the middle of our fill (20 mm), even though this is fast curing Sikaflex. However it should have a 7 mm thick skin within 3 days.

We should have filled it in say four 5 mm layers, letting each cure for 24 hours before adding the next.

By the way, the Sikaflex cartridge we're using is 5 years old and still flows as well as when it was bought, despite the fact that the above datasheet says it has a shelf life of 9 months unopened and "a relatively short space of time" once opened. The trick (taught to me by Jeff Owen) is to store it in the freezer. Of course you have to unscrew the nozzle and remove the solid plug every time you want to use it, but you don't have to wait for the cartridge to thaw. It doesn't freeze, it still flows at -18°C.
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Post by weber » Sat, 22 Mar 2014, 18:14

I did a desperate thing to speed up the curing of the innermost Sikaflex we put into the new motor mounts. After only 1 day of ambient curing for one mount (3 days for the other) I put them in a gas oven (combustion of natural gas produces plenty of water vapour), on the lowest heat setting I could manage without the flame going out, hoping for about 90°C. Then I managed to forget about them for an hour or so, after which I measured them at 120°C with an IR thermometer, turned off the gas, tossed in some water and left them in the oven to cool for another few hours.

We put the new mounts into the MX-5 and put everything back together and took it for some test drives yesterday. It's funny how all three of us have different attitudes to each of the MX-5's remaining foibles. I was disappointed that it was still possible to provoke the 5 Hz drive-train oscillation by stomping on the accelerator in first or second gear below about 2000 rpm. Coulomb was pleased that it was nowhere near as easy to provoke, and nowhere near as bad when you did. Newton demonstrated that his (front-wheel-drive) EV has not the slightest hint of any such drive-train oscillation, by producing wheel-spin in first gear, then letting me try a hard launch in second gear.

I figure it is due to drive-train wind-up in the MX-5, and as Newton pointed out, probably contributed to the failure of the old mounts, and still contributes to the clutch slipping, because it may produce torque peaks significantly higher than the maximum steady-state torque.

Has anyone else experienced this or does anyone have any theories about how we might reduce or eliminate it? I note that the (front-engine rear-wheel-drive) MX-5 does not have a rear gearbox mount or a front diff mount, but instead connects the two together by an aluminium-alloy truss beam. So this unified drive-train is hung on four mounts -- the two motor mounts and the two diff "wing" mounts.

In theory we could control the motor in such a way as to damp out these oscillations. But it's not entirely clear to me how.

We needed to remove the under-bonnet battery box so we could lift the motor high enough to replace the mounts. This gave us a rare opportunity to photograph some never-before-seen wildlife behaviour -- the frenzied mating rituals of the Orange Engine-bay Snake.

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Post by TooQik » Fri, 28 Mar 2014, 01:40

weber wrote:Has anyone else experienced this or does anyone have any theories about how we might reduce or eliminate it?
It might be worth talking to someone who uses their MX-5 for motorsport to see what drive train options are available and whether they might help eliminate it, assuming that you haven't already done so.

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Post by PlanB » Sat, 29 Mar 2014, 15:38

It could maybe be related to the clutch slip itself fellas? In high torque ICE mods where the clutch is right on the raggedy edge of holding/slipping you can get some nasty little resonances as the plates slip/grab/slip. Most of the petrol heads seem to deal with it by using dual plate clutches to handle the extra torque. You could maybe test this hypothesis by wiggling your big toe on the clutch as you tramp it with your other foot. If the shudder improves with a bit less friction then weaker plate springs & a bit more slip might be a temporary, if inelegant, fix?

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Post by weber » Mon, 14 Apr 2014, 00:07

After much web searching and experimentation we have had some real success in reducing the MX-5's drivetrain oscillations, subjectively by a factor of 2 or 3. The nice thing is that the solution is zero cost, consisting of 5 lines of C code (preceded by 4 lines of declarations).
    const  float a2 = 0.685124545
    const  float a1 = 1.626788294
    static float b1 = 0.0;
    static float b2 = 0.0;

    float g = command.current + b2;
    float h = a1*b1 - a2*g;
    b2 = b1;
    b1 = command.current + h;
    command.current = (g - h) / 2.0;
"What the hell is that doing?", I hear you ask. And I must answer that it is the purest form of magic. It is certainly not something that any one person could have dreamed up overnight. It was hard won by generations of the truest kinds of mages, sorcerers and wizards. But first ...

Thanks TooQik and PlanB for your suggestions. Sorry to take so long responding.
TooQik wrote:It might be worth talking to someone who uses their MX-5 for motorsport to see what drive train options are available and whether they might help eliminate it, assuming that you haven't already done so.
PlanB wrote:It could maybe be related to the clutch slip itself fellas? ... If the shudder improves with a bit less friction then weaker plate springs & a bit more slip might be a temporary, if inelegant, fix?
We have established that the drivetrain oscillations occur independent of clutch slip. And yes, it's quite possible that clutch-slip would damp the oscillation. This may explain why such oscillations don't occur on launch with a petrol motor (where you are forced to slip the clutch). I can't find any reference to such drivetrain oscillations in the MX-5 forums although we do find reference to "wheel hop" or "axle tramp", which may be related to our drivetrain oscillation, but seems more likely to be related to compression of the rear wishbone bushes.

The general inefficiency of internal combustion engines may explain the absence of the oscillation under the other conditions where it is now occurring for us, such as when we are travelling at medium speed in first or second gear and jab the accelerator, or when we are trying to crawl very slowly in a carpark. If you've ever tried to turn over a petrol engine by hand, even with all the spark plugs out so there's no compression, you'll know how much drag there is, compared to the ease of spinning an electric motor by hand.

A mechanical oscillation requires that there be a combination of inertia and springiness without sufficient drag, also referred to as mass and compliance without sufficient damping.

So what is that magical incantation doing above? Well, the truest mages, sorcerers and wizards are mathematicians, scientists and engineers, and this kind of magic spell is referred to as a recursive digital notch filter, in this case tuned to 4.2 Hz with a damping ratio of 0.7.

So how does that help to solve our problem?

That will have to wait for another post.

[Edit: Changed parameters a1 and a2 to those required for a notch frequency of 4.2 Hz and a damping ratio of 0.7 when the sample rate is 100 Hz.]
Last edited by weber on Mon, 14 Apr 2014, 14:53, edited 1 time in total.
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Post by 7circle » Mon, 14 Apr 2014, 06:26

Neat!

Suprised (or perhaps delighted) you can customise the control interface like this. Could you have done it with the Telemecenique industrial controller?

Just 2 multiplies and a divide by 2 if handled smartly by the complier shouldn't be to many instructions for the interface u_controller.

But wondering are you just effecting the accel_pedal algorithim output? (... that's the command.current)

Which would mean the oscilation is caused by the drivers foot bouncing on the pedal... ??

I thought using rpm or position feedback would be needed. But feedback delays over CAN bus com's would be a tough.
When the wiring was tidied up at hidden connector, I can't remeber if you had RC filters on the pedal signal.
Did you see wobbles in the logged data for the command.current?

Is the pedal arm too sensitive to free movement?

Throttle cable runs would have plenty of drag to pedal bounce. But fly by wire cars are so common now that type of problem may be solved in the ECU.

I was thinking the rotory ICE crowd may have similar problem having a smaller lighter motor and a center of mass closer to the crank than a twin cam 4-pot inline ICE. And maybe less drag. But rotary low rpm torque is very poor.

On the topic of currents, have you put the IMU boards on hold?

Might have another look over in your open code store room.

Thanks for the entertaining blog.
7C

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Post by 7circle » Mon, 14 Apr 2014, 08:19

Found your posts about the pot box in 2010 July 21 at 12:47am (can't grab the post id link with my phone touch screen)
Image

So there is a cable run.
But it is possible under forward acceleration the lever arm spring can compress due to the inertia of the cable and clamp mass being aligned in with motion of car and allow slack in the cable to the pedel.
The idea may be unlikely but 350Nm torque you guys are pushing has got some kick to it.
This would change the pedal signal.

On the coding of the DCU TI MPS430 in C how are the floats handled. Are the 24bit plus 8bit mantisa?
The soucre forge site is very handy allowing browsing of code with coloured formatting.
DCU as BMS master at Source Forge

And how did you resolve the ADC reading problem back when James mentioned the ADC block in the chip could be damaged. Your posts stoped then so I assume you replaced the DCU. The Pedal Puzzle


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Post by coulomb » Mon, 14 Apr 2014, 20:35

7circle wrote: On the coding of the DCU TI MPS430 in C how are the floats handled. Are the 24bit plus 8bit mantisa?
I thought 23 plus 7 plus two signs; it is standard IEEE 754 32-bit. I think they define doubles as the same size as floats, so all FPU functions return a 32-bit float/double.
And how did you resolve the ADC reading problem back when James mentioned the ADC block in the chip could be damaged.

We bought a replacement processor, but the suppressed memories of the pain of the last replacement seems to have prevented us from actually replacing it yet. So only one DCU can read ADC inputs at present. We found the car won't start without two DCUs working anyway; we have not investigated why that is yet.

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Post by weber » Mon, 14 Apr 2014, 21:35

Hi 7Circle,
It takes some time and effort to try to craft a post that is both entertaining and enlightening (and free of error), and sometimes it's hard to know whether others find it worth the read, so it was wonderful to find your response this morning, full of ideas and questions. Made my day.
7circle wrote:Surprised (or perhaps delighted) you can customise the control interface like this. Could you have done it with the Telemecanique industrial controller?
Yes, we could have. Although it's actually by Control Techniques (a subsidiary of Emerson). We specifically chose the Unidrive SP series because they did offer this level of programmability, although not in C. You can see an example of their Drive Programming Language here, from Ross Pink (see the fourth link, "source code").
viewtopic.php?title=ac-drive-programmin ... ing&t=1859
Just 2 multiplies and a divide by 2 if handled smartly by the complier shouldn't be to many instructions for the interface u_controller.
Yes. It blows me away. Nearly two centuries of math/science/engineering wisdom, beginning with steam engines and Joseph Fourier in 1822, distilled down to two multiplies, two adds and a divide-by-2. Brings tears to my eyes.

Much like the fact that our induction motor, capable of 200 kW peaks* in a volume of 264 x 264 x 470 mm, is nothing more than a geometrical arrangement of two different metals, one conductive of electricity, the other conductive of magnetism, no magnets and no brushes. [* It should achieve 200 kW if given 900 Vac (from 1300 Vdc) which would allow it to maintain 350 Nm out to 6000 r/min.]

But in both cases it is the arrangement of those math operators or those metals that has been so hard won, and their precise dimensions or constants that have been so highly optimised.
But wondering are you just affecting the accel_pedal algorithm output? (... that's the command.current)
Yes. That's exactly what we are doing. So this is described as "open loop" control as opposed to feedback or feedforward control. We are in effect predicting what the response to the control signal will be, and modifying that control signal to compensate for the bad response, to give us the response we want.

We tested it initially with Tritium's bog standard pedal mapping: command.rpm = RPM_FWD_MAX; command.current = pedal. Which means the notch (or band-stop) filter could have been implemented by a capacitor, an inductor and a resistor between the analog pedal signal and the EVCU [my generic term these days for ECU-like EV system controllers (as opposed to motor controllers) like the Zilla Hairball and Tritium's "EV Driver Controls", which I used to call a "DCU"].
Which would mean the oscillation is caused by the driver's foot bouncing on the pedal... ??
Kind of. But not in the way you are thinking. Given that one of the ways we can provoke it is simply by slamming the pedal to the floor and holding it there, resulting in a launch with multiple lurches and pauses, then it can't always be due to the driver's foot bouncing on the pedal, although if the driver does deliberately bounce on the pedal in synchrony with the oscillation, some truly horrendous lurching does occur, which feels likely to break something if persisted with.

By logging the pedal signal we see that when we mash it, it goes cleanly from 0 to 100% within 50 ms, while the oscillation, at its fastest, has a period of about 200 ms (5 Hz). That proves that the mechanics of the accelerator cable and return spring are not involved in the problem in general. Good thinking though, because what we see when we try to move the pedal very slowly, as we do when trying to maneuver in a carpark, are a series of little steps in the signal (jumps of 10 to 20 ADC counts, out of 4095) as the cable sticks and slips, instead of the smooth ramps you would expect. I plan to lubricate the accelerator cable and linkages to eliminate these mini-steps.

So why did I say "Kind of" in regard to your suggestion that the oscillation was due to pedal bounce? Joseph Fourier showed us that a simple step effectively contains all frequencies, including the one that will excite the mechanical resonance of our drivetrain.

We don't already have any significant RC (low pass) filtering on the pedal input. And yes, by filtering out all frequencies except those below our mechanical resonance we would avoid the problem too, but this would come at a serious cost in responsiveness. According to the literature, the time lag introduced by using a low pass filter for this job is typically 5 times that of a notch filter.

Here's something like our acceleration vs time curve when we put a torque step into the system (i.e. when we mash the pedal without the notch filter). It has "ringing" i.e. the aforementioned lurches and pauses.

Image

When the notch filter receives a step on its input it generates an output that has "anti-ringing" as follows.

Image

The result of combining the two (i.e. putting the above torque waveform into the system) is a nice clean (but slightly rounded) step in the acceleration vs time curve, with no ringing, which is exactly what you want from a step in the pedal signal.

So what we describe in the frequency domain as filtering out a small band of frequencies, appears in the time domain as this "anti-ringing" associated with any sudden change. These are two complementary ways of describing the same thing, and many of you will know that the mathematical operation that lets us switch our point of view in this way is called the Fourier Transform.
I thought using rpm or position feedback would be needed. But feedback delays over CAN bus com's would be a tough.
Yes. The motor has a 360 pulse-per-revolution encoder fitted and this feeds into the WaveSculptor motor controller. Our WaveSculptor sends r/min telemetry every 40 ms, and I imagine the latency is something of that order too. However the WaveSculptor itself already has a fast velocity control loop built in, wrapped around the torque control loop (really quadrature-current control), so we tried two different ways of using that, neither of which gave satisfactory results. You could eliminate the oscillation, but only at the cost of side effects that were worse, and in the case of velocity ramps, positively dangerous, as Newton will attest!

That's when I started searching for solutions to this general problem outside the automotive field, in the field of industrial control. Half the problem with such searches is figuring out what words they use to describe it. When I eventually homed in on "controlling compliant loads" I hit the mother lode, and learned about the use of notch filters. Here's one particularly awesome paper:
compliant_loads.pdf

Then I needed to know how to implement such a notch filter in software and eventually found this, in which you will find a dataflow diagram of the filter and the equations for calculating the coefficients (which we do in a spreadsheet):
http://www.dsplog.com/2013/07/14/digital-notch-filter/

I am very grateful to these authors, and the authors they have relied on, and so on, back centuries.

"So why isn't the notch filter a 100% solution?", you may ask. "And what or where exactly are the mass and compliance that cause this resonance?" Those are topics for another post.
On the topic of currents, have you put the IMU boards on hold?
We had put the IMUs on hold for quite a while, but last week we finally populated two of them. That will be the subject of another post (perhaps by Coulomb?) when I've caught up on describing our drivetrain oscillation research and experiments so far.

We've also been designing and laying out the PCB for a new "universal" version of our cell-top cell monitoring units (CMUs). It will fit all common sizes of prismatic Lithium-ion cell used in DIY EVs (and solar power systems) and will perform monitoring of bolted joints as well as cells, without requiring a third wire between cells.

[Edit: Fixed broken first-image link.]
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Post by weber » Tue, 15 Apr 2014, 00:10

7circle wrote:And how did you resolve the ADC reading problem back when James mentioned the ADC block in the chip could be damaged?
coulomb wrote:We bought a replacement processor, but the suppressed memories of the pain of the last replacement seems to have prevented us from actually replacing it yet. So only one DCU can read ADC inputs at present.

Suppressed painful memory must be right, Coulomb. I note that 7Circle is asking about the first ADC blowup (the last replacement), since he linked to a June 2011 post, and since I had not admitted publicly to blowing up a second one (in November 2013). But you've blown the whistle on me now. Image

I note that Tritium fixed the problem in 2012 in rev 4 of their "EV Driver Controls" EVCU by adding a diode/TVS module to protect all the analog inputs. But ours are both rev 3 (one being originally a rev 2, upgraded by us).

My memory of the events following the first ADC blowup is that Coulomb used a hot-air rework station at work to replace the MSP430F247 microcontroller (a quad flatpack with 64 pins on a 0.5 mm pitch) and indeed he described it to me as a painful experience and thought he had completely ruined it (presumably by misalignment of pins and pads), so he bought me a second replacement processor and handed it to me with the EVCU, saying it was my turn now.

However I found that Coulomb had in fact done a pretty good job, and it only needed a few pins cleaning up with some gel flux and a soldering iron, and we've been using it ever since.
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Post by weber » Tue, 15 Apr 2014, 04:50

I wrote: "So why isn't the notch filter a 100% solution?", you may ask. "And what or where exactly are the mass and compliance that cause this resonance?"
Here are some clues:

I logged the velocity telemetry (r/min) from the WaveSculptor while driving the MX-5 in various ways that provoke the drivetrain oscillation. Then I analysed the logs by counting the number of samples between peaks for several consecutive oscillation cycles (typically 3) under the various conditions. I got the following results:
               1st     2nd gear
carpark crawl  1.8 Hz  2.7 Hz
mid speed jab  2.8 Hz  4.2 Hz
hard launch    3.4 Hz  5.2 Hz
Resonant frequency is
f = 1/[2*pi*sqrt(m*C)]
where
f is frequency in hertz (Hz)
m is mass in kilograms (kg), and
C is compliance in metres per newton (m/N),
or where
m is angular mass (moment of inertia) in kilogram square metres (kg m^2), and
C is angular compliance in radians per newton-metre (rad/(Nm)).

So frequency goes down when either mass (inertia) or compliance (springy "give") go up.

From these clues you should be able to see why the 4.2 Hz notch filter is not a total solution. And you should be able to figure out whether the relevant mass comes before or after the gearbox. Similarly for the compliance.

And given Valenzuela, Bentley, Villablanca and Lorenz, 2005, and the following image, you may be able to guess what we plan to try next.
Image
[Edit: Changed "From this" to "From these clues". Fixed broken image link]
Last edited by weber on Fri, 09 Dec 2016, 12:23, edited 1 time in total.
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Weber and Coulomb's MX-5

Post by 7circle » Tue, 15 Apr 2014, 06:01

No... the XYZ gives it away...
I just came accross "SolderCore" embedded systems who have little bitty boards like that.
One has Magnectics, Gyro and accel for $50.

and I thoight you were going to dump the clutch and gearbox.... Image

Will you mount it on the Motor or chassis?


I was wondering what the DCU MSP430 clock runs at?
Do you have a way to tell if the code routines are running low on execution time?
I noticed the charger PID routine uses FRACT datatypes.
So the project uses a big range of data types.
Have you enabled the compiler to use hardware multiply?
I am assuming the MSP430F24x has it, but did't see it listed on the TI description page.
But the MSPgcc looks like it doesn't use it.
Bit the IAR toolchain looks.like it has it as an option and choosing long or double precision.

Back too your bait ... jaycar sells them hey.... from the picture filename.

Looks like you like the closed loop feedback from sensor approach.

Image when there is a ? get an answer as close to the source as possible.

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Post by coulomb » Tue, 15 Apr 2014, 16:50

7circle wrote: I thoight you were going to dump the clutch and gearbox.... Image
That was one of the on again, off again, on again questions in the first few months of the project, way back in early 2009. With the low voltage configuration (360 V verses the planned 720 V, which was actually more like 750 V with the originally planned 228 cells), we are very glad of the gearbox. We can always take it out later.
I was wondering what the DCU MSP430 clock runs at?
It's 16 MHz. In checking this number, I realise that there is a connection to the CAN chip's clock, but it's unused. Comments suggest that it's easier to sleep the processor when the internal (software trimmed) oscillator is used. We don't attempt to sleep the processor, so we could avoid clock frequency calibration issues by enabling the external clock. One of the mods on a recent EV day partly obscured the frequency calibration value, which we have stored in the form of four hex digits engraved onto the PCB Image
Do you have a way to tell if the code routines are running low on execution time?
I think I did this over a year ago, and it was lightly loaded. Tritium chose one of the largest members of the MSP430 family so they didn't have to worry about running out of horsepower.
I noticed the charger PID routine uses FRACT datatypes.
Yes.
Have you enabled the compiler to use hardware multiply?
Yes. There was some problem with an early version of gcc, which used it but incorrectly somehow. That's long fixed now.
I am assuming the MSP430F24x has it, but did't see it listed on the TI description page.
It's an MSP430F247, and it certainly has it. It also has an accumulate feature on the multiplier, but gcc doesn't use it. We actually use some assembler to force use of the multiply-accumulate in our PID code.
But the MSPgcc looks like it doesn't use it.
We use this version:
$ msp430-gcc --version
msp430-gcc.exe (GCC) 4.6.3 20120301 (mspgcc LTS 20120406 patched to 20120502)

As mentioned above, it certainly uses the hardware multiplier, but not the multiply/accumulate feature.
But the IAR toolchain looks like it has it as an option and choosing long or double precision.
We don't use the IAR compiler, since we would have to pay for it, and the Tritium code assumes use of the gcc compiler. We could have made minor changes to get it to work, but it didn't seem to be worth the effort. So we edit in IAR, then switch to a Cygwin window to compile, and sort of half use the IAR system for crude debugging (you can see values in registers and memory, but there is no source code correlation other than the "list" (tri86.lst) file (just a disassembly created with msp430-objdump -dSt).

[ Added "We don't attempt to sleep the processor". Removed confusing question about "charger project". CAN chip -> CAN chip's clock. ]
Last edited by coulomb on Thu, 17 Apr 2014, 07:22, edited 1 time in total.
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Post by weber » Tue, 15 Apr 2014, 20:21

So the notch filter isn't a 100% solution because we have a wide range of different drivetrain oscillation frequencies to deal with, as shown in this table:
               1st     2nd gear
carpark crawl  1.8 Hz  2.7 Hz
mid speed jab  2.8 Hz  4.2 Hz
hard launch    3.4 Hz  5.2 Hz
Why are there so many different frequencies? Running down the columns shows that the frequency varies with the average or "DC" torque. This suggests we are dealing with a highly nonlinear compliance. The further this "spring" is displaced, the stiffer it gets and so the lower its oscillatory or "AC" compliance. Lower compliance means higher frequency.

Going across the rows shows that the frequency changes with gear ratio. That indicates that the mass and compliance that are causing the resonance are on opposite sides of the gearbox. If the interaction between them did not have to pass through the gearbox, changing gears would have no effect on the frequency. The frequency changes by about a factor of 1.5. The ratio between these gears is 1.66.

We could use a microswitch to detect what gear we are in, and we already have the average torque information (based on motor currents), so we could change the notch frequency on the fly, but that would be a fair bit of work with the microswitch and in any case we found that setting it to 5.2 Hz or 6.0 Hz made no improvement to the 2nd gear launch oscillation, and we don't have a clue why.

So what are the mass and compliance that cause this resonance? We already figured out that they are on opposite sides of the gearbox. The fact that the frequency goes down when we go from second gear to first gear tells us that the mass is before the gearbox and therefore it is the angular mass of the rotor and flywheel. How do we know that? A mass after the gearbox, i.e. the mass of the vehicle, looks to the motor like a smaller mass when in first gear. Smaller mass means higher frequency, the opposite of what we see.

However a compliance that comes after the gearbox will look to the motor like a greater compliance when in first gear, because the motor will turn further for the same amount of wind-up of this torsional "spring". Greater compliance means lower frequency. Exactly what we see.

Our first thought was that this compliance must be in the motor and diff mounts. Having just replaced the motor mounts, and stiffened them with Sikaflex injections (sounds a bit like Botox injections), suspicion fell on the diff mounts.

With the demise of the under-car video camera (and its low resolution and frame rate anyway), Coulomb and I came up with the idea of locking the rear wheels (with handbrake and footbrake) while applying stationary torque in first gear and just _looking_ for the compliance with the assistance of a large mirror laid on the ground, and a torch (flashlight).

We took turns being the "driver" and the looker. Whoever was in the driver's seat would slowly ramp the torque up and down between about 17% and 50% of maximum (staying away from zero to avoid confusing the looker with backlash). We saw about 90° of wind-up at the pulley on the front of the motor, but any rolling of the motor on its mounts was barely perceptible. The diff roll was more noticeable but clearly there was nothing wrong with its mounts. This was just as well, as I had priced a set at $400.

Diff pitch? Nothing visible, the PPF takes care of it. Tailshaft universal joints, front and rear? Nothing visible there. Sure the whole tailshaft was turning about 30°, but no wind-up was visible in the uni-joints. The half-shafts? Some small rotation there, maybe a degree or two, but it was about the same as the tiny amount of wheel rotation we were getting because the wishbones were copping the torque via the brakes, something that doesn't happen when the oscillation occurs while driving. So the wind-up was happening somewhere between the tailshaft and the half-shafts. Inside the diff? No. It was in the inboard CV joints on either side of the diff. Their green outer casing was rotating back and forth by 5° to 10° while the half shafts were barely moving!

Apparently the outboard CV joints are Rzeppa joints (6 balls) while the inboard CV joints are called "double offset (end motion type)" (also 6 balls). These are similar to Rzeppa joints except the cage and balls ride in a cylindrical cup instead of spherical, with parallel axial grooves allowing a plunge motion in the joint (like the lower one in the cutaway image below).
http://forum.miata.net/vb/showpost.php? ... stcount=18
http://en.wikipedia.org/wiki/Constant-v ... ppa_joints
http://web.mscsoftware.com/support/libr ... _joint.pdf

Image

But we have no reason to believe our inboard CV joints are faulty (although possibly worn), so we continue with our attempts to add damping to the system via intelligent control of the motor.

You got it, 7Circle. We are planning to try acceleration feedback. The photo is of a $20 3-axis accelerometer from Jaycar. We only need one axis, so it will connect directly to the Tritium EVCU with 3 wires (GND, +5V, analog in). We will mount it to the chassis near the EVCU.

Valenzuela, Bentley, Villablanca and Lorenz, 2005 is the awesome paper I linked to in a previous post. It mentions acceleration feedback as an alternative to the open-loop notch filter. Only, in their case, they have to use a very complicated scheme called an "observer", to obtain fast enough and noiseless enough angular-acceleration information from the rotary encoder. In our case we can measure linear acceleration directly (as suggested by Ross Pink in email).

We just have to figure out how best to filter it or otherwise modify it to become an acceleration error signal, before subtracting it from the torque setpoint.

[Edit: Added cutaway image of the type of CV joint where most of the compliance is occurring.]
Last edited by weber on Tue, 15 Apr 2014, 10:53, edited 1 time in total.
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Post by coulomb » Wed, 16 Apr 2014, 00:47

Last EV day (a week and a half ago now), we made some progress.

First up, we made the modification to the EVCU (Driver Controls) boards to allow us to measure the auxiliary (12 V) battery voltage. It's one of those things that almost worked perfectly straight off; Weber had written the code for this weeks ago, but it was waiting for the hardware mods. It was giving the correct display most of the time, but was then displaying zero for about a third of the time. Weber spotted this clue, found a bug in the software, and had it fixed in ten minutes. It was assuming that when not in driving mode, we are in charging mode, and so displaying charge current (two thirds of the time showing charger A current, and one third charger B current.)

This allowed us to notice vividly how much the auxiliary battery was sagging under the various loads. With everything turned on, it was sagging to below 11.5 V, even with the DC/DCs pouring in 30 A. We had bought a pair of automotive relays some weeks ago with the idea of putting them in series with the DC/DC outputs, so they don't drain the auxiliary battery when the keyswitch is in the off position. We could also let the DC/DCs power the car from under the bonnet, instead of using cables that run all the way to the back of the car where the auxiliary battery is, then all the way back to the bonnet again, where the fuseboxes are and therefore all the loads. We found an 80 A fuse that was largely or entirely unused, and was connected to the power steering pump (which can draw over 30 A when the steering wheel is locked). This point is where we would put the output of the DC/DCs, which would hopefully reduce the sags. Now this change suddenly seemed like a really good thing to do, so we proceeded to do it.

There was a "deluxe" version of this mod that used the cables still running to the back of the car to disable the relays if the cable to the auxiliary battery was chopped, say by fire service personnel in the case of an accident. However, we decided that would take too long; we wanted a quick fix for the voltage sag. Indeed, the voltage sags did improve. I believe that the worst case was now about 12.0 V, and other load combinations improved as well.

The reason we used the two long leads to the back of the car, apart from the sheer pleasure of squeezing cables into conduits and running them under the car Image , was to ensure that the DC/DCs shared the load of charging the auxiliary battery fairly evenly. It doesn't matter much at the moment, with the two half-packs in parallel, but if we go to 720 V, then we want even loads, otherwise the DC/DCs could unbalance the half-packs, possibly quicker than the small bypass current could correct it (when charging). Fortunately, the two DC/DCs seemed to be sharing the current (on the 12 V side at least) quite well.

This only brought us to lunch time, so we decided to populate two of the IMUs (Insulation Monitoring Units). These are like specialised CMUs (celltop cell monitoring units), but these will be given the ID zero, and won't respond to cell functions like voltage request or the like. Instead, they will respond to new commands, not written yet, that will initiate touch current tests.

So these have most of the parts of a standard CMU, along with two reed relays, two large resistors, and a few other changes. So 80% of the parts are surface mount, so we needed to remember how we did the production run of CMU boards. Oh, and find the solder paste. A quick search of Weber's fridge failed to find the paste... not good. Eventually, we found some, marked "old, leaded". I think the unleaded paste we used for the CMUs had gone bad, and had been discarded. This old stuff was looking pretty solid; in fact, we could not get anything though the nozzle even with prodigious pressure on the plunger. Time to search for how to make old paste more usable, if that is indeed possible. Some sites say to just throw away the old and get new, but paste is expensive, we're unlikely to use much again for a long time, and we wouldn't be able to make the boards that day. One video showed miraculous rejuvenation using "flux", and we have flux, but their flux was liquid, and ours a gel. It was even difficult to find out what the solvent is in solder paste; the best suggestion seemed to be some special alcohol. Well, we have flux and metho, let's try them both.

Our gel flux didn't seem to work very well, and metho didn't seem to work quite the same as on the video, but after we finally managed to clear the major cloggage in the nozzle, the solder paste seemed pretty usable after all (or maybe the flux and/or metho did have some effect). So we have usable solder paste! But did we have all the parts we need? While Weber started hand pasting solder paste to the boards (for just two boards, a stencil is not justified), I started locating bins, and indicating what designation each part has on the ICUs. For the most part, the designators (e.g. "R17") were the same as for the CMUs, and we had already marked the parts with thise designators. Fortunately, we found all the parts we needed, although we found that we were using 1% resistors where the circuit called for 0.1% tolerance... oh well. Sorting out the parts took way longer than I thought it would, so as soon as I was finished, I looked to to set up the oven.

We don't use a proper reflow oven, or even an Arduino controlled toaster or frypan. This is because our CMU boards are so long, and we seem to have trouble getting the heating even, so we have to wait for all the solder to go silver before stopping the process.

I had a lot of trouble inserting the thermometer-plug into the frypan, and thought I may have disturbed the temperature setting in the process. Damn! I threw in some avocado oil under the heat spreader, turned it on, and furiously searched our careful laboratory notes for the settings. Well, when I say laboratory notes, I mean an exercise book with "MX-5" written on the front, where we jot down notes whenever we think about it. Alas, we didn't see fit to document this. So now I'm searching our build thread, and found that Weber had to disassemble and modify the thermometer-plug thing, to get a high enough temperature. But we didn't say what that temperature was.

When I got back to the lab, I noticed it was hard to see. The room was rapidly filling with smoke! A fan was hastily set up and windows opened, and I quickly looked up the smoke temperature of avocado oil. It seems it depends a lot on whether it is refined or not, and the virgin type often is unrefined. It makes a huge difference; ~ 200C verses 271C! A quick check of the label indicated that we had bought virgin oil, so it would be unrefined, and would smoke at around 200C. Our former selves let us down again! Ok, so we work though the smoke. I read a little later that we only ended up using the avocado oil at the edges of the frypan, where the temperature was lower.

A quick search indicated that "liquidus" for reflow soldering is typically around 217 C (it didn't say if that was for leaded or unleaded, I assume the former). The IR thermometer read around 220-230C, so we decided it was ready to *carefully* insert the boards. My recollection from last time was that it took some minutes before all the solder was silvery in colour. But these boards seemed to go silvery in tens of seconds. Maybe this was because we were using unleaded solder paste last time, and leaded this time, or because we took so long fussing about with the temperature that it was well and truly ready in there.

For reference in future: we used a thermostat setting of 10.

OK! It's ready! Switch off! Now, where is that fan? We used the fan to try and cool down the boards rapidly, so when we can remove the boards with gloves. The processors were a disaster area, but we expect that. Fortunately, flux did its usual magic trick, and all the connections looked great after a little rework with a soldering iron. I can't emphasise too much how important gel flux is when soldering surface mount parts with fine lead spacing. There were a few other touch-ups required, as well as the usual suspects, the LEDs. The LEDs are worse than usual, because we had to go with a round hole rather than a routed slot (part of the price of cheap low-volume boards), and the holes were not much smaller than the diameter of the LEDs.

It was a simple matter to add the through-hole components after that. We had no ready way of testing the IMUs, so that's where we left it for the day.

[ Edit: griller -> fryer -> frypan; flux -> solder paste in a few places (seniors moment); "leakage tests" -> "touch current tests" ; oil goes under heat spreader; thermostat setting ]
Last edited by coulomb on Thu, 17 Apr 2014, 05:39, edited 1 time in total.
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Post by weber » Mon, 19 May 2014, 17:53

Since our last update, effort on improving the MX-5 has been on suspension and chassis issues, and on ensuring she looked her best yesterday when she appeared in the "Special Interest" section of the "Maclean's Bridge" sports and classic car display in Meadowbrook.

Being the only electric sports car there, she got plenty of interest. And those of you who helped build her, but weren't there yesterday, (and indeed anyone who has worked on any EV conversion) should know that your work got a lot of respect, something I did not expect from a crowd of "petrol heads". But guys who have done petrol engine swaps or installed turbos, superchargers etc, know how much work it is, and most understand now, that electric drive is the logical progression.

Image

The car in the foreground is not Mexy, but her identical twin, ... until you lift the bonnet (which the owner was kind enough to do for us). Both were born in Hiroshima in 1990. Who knows, they might have been one after the other on the assembly line. You can see Mexy in the background, with Mexy-builders-extraordinaire Coulomb (Mike Van Emmerik) in tartan shirt, talking to an interested show-goer, and Newton (Jeff Owen) in black shirt far left, talking to our beloved Chairman SuziAuto (Graeme Manietta).

[Edit: More of my photos of the show in "The Lounge" here viewtopic.php?title=mcleans-bridge-spor ... val&t=4166]
Last edited by weber on Tue, 20 May 2014, 12:22, edited 1 time in total.
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Post by Johny » Mon, 19 May 2014, 18:54

Great to see you guys showing here off!
Yes, it's surprising where the interest comes from - and sometimes where it doesn't!

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Post by offgridQLD » Mon, 19 May 2014, 19:42

Yes,
    I think part of the appeal for the custom car guys is doing something different and ending up with a car thats unique.

Thats getting harder and harder to do with the ICE cars as everything has been done before. Electric drive opens up a whole new avenue for them.

Kurt

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Post by weber » Mon, 19 May 2014, 22:29

The insulation and current monitoring units (IMUs) are on the back burner again. The software for the cell management units (CMUs), both the "monitor" and the "TestICal" program, has been updated to include code to allow testing of the IMUs, but no testing has been done yet. The IMUs and CMUs will have identical software, but certain parts of this "DNA" will only be "expressed" in xMU's with ID=0 (IMUs).

Image
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Post by weber » Mon, 19 May 2014, 23:44

In a previous post, I mentioned we'd been working on suspension and chassis improvements. Back in March I listed 4 areas needing improvement:

1. Full power not yet available
2. Bottoming out the rear suspension
3. A torsional body oscillation (shudder)
4. A drivetrain oscillation

Re 1. We're still waiting for Tritium to complete testing of the 900V WaveSculptor, and we haven't yet stiffened the clutch pressure-plate spring.

Re 4. You may remember we posted earlier about the huge improvement in the drivetrain oscillation produced by a digital notch filter on the torque setpoint (5 lines of code). We have not yet installed the accelerometer, which is the next thing to try in the hope of further improvement.

The rest of this post is about no. 2 -- bottoming out the rear suspension. I decided to try installing stiffer springs.

You may remember we installed a YellowSpeed Dynamic Pro Sport coilover kit with adjustable everything. These had spring rates of 8 kg/mm on the front and 6 kg/mm on the rear. So I ordered two more of the 8 kg/mm springs from MX5mania, and mentioned I was going to put them on the rear. I'm fairly impressed by MX5mania. They pointed out that the 8 kg/mm springs were longer than the 6 kg/mm (180 mm vs 160 mm) and asked if I was sure they would fit. I explained that it was for my electric MX-5 conversion, and because it is about extra weight the extra length would be a positive benefit. [In case anyone with a conventional MX-5 is reading, I don't think it would be a problem there either.]

The problem with the 6 kg/mm springs was that even when their seats were screwed all the way to the top of the shocky body they had only lifted the car a very small way above the stops. The rest of the ride-height adjustment had to be done by screwing the shocky body partly out of its base. It seems the ideal is to set the spring seat so the shocky is mid-way through its travel, when the car is fully loaded. [Edit: I later read the user guide for the coilovers. The manufacturer recommends 2/3 bump and 1/3 droop (rebound).] Here's the "before" shot with the car resting on its wheels.

Image

My main worry was not spring length, but whether I would have to unbolt the coilovers at the top, because these bolts are now underneath contactor boxes which are very difficult to move. They require the lid screws being undone with a right angled screw driver and then cable lugs unbolted and cables withdrawn -- or similar contortions. And I thought the only way to avoid doing that, might be to unbolt the lower wishbones from the car, which would have ruined the wheel alignment.

Fortunately I was able to change the springs without doing either of these things, as you can see in the next photo.

Image

The most tedious part of the job was unwinding the preload on the old springs and putting the preload on the new springs. I calculated the preload in millimetres to put the shocky in the middle of its 80 mm of travel when the spring is supporting a wheel load of 395 kg via a motion ratio (lever ratio between wheel and spring) of 0.73. 395 kg is something like the rear wheel load with the car fully loaded.

Preload (mm) = 395 kg / (8 kg/mm * 0.73) - 40 mm = 23 mm.

With a thread pitch of 1.5 mm and 8 notches per spring seat nut, and only being able to pull it one notch at a time, that meant 23 * 8 / 1.5 = 123 pulls to apply the preload. And despite oiling the spring seats there is a lot of force on there and so a lot of torque required to overcome the friction. As I say, tedious.

I forget why, but for the record, I ended up using a 28 mm preload and the shockies are now screwed into their bases as far as they will go (about 36.5 mm). The road clearance is 100 mm (minimum legal) when the car is fully loaded, as it was before. Here's the "after" photo.

Image

I tested them by driving back and forth (twice) over the two speed bumps in our street, at ever-increasing speeds. Mexy now laughs at speed bumps. "Ha ha" she says, as she bumps over them at 50 km/h. Beautiful. Oh wait a minute, remind me what those speed bumps are there for again?

There is a bit of undershoot on the rear now, when coming down off the bumps - a bit of a wallow - so I will need to increase the rear damping slightly. But that was expected. The original YellowSpeed setup recommended 8 clicks on the damping adjuster with the 6 kg/mm rear, and 12 clicks with the 8 kg/mm front. So I'll have to put 4 more clicks on the rear shockies. Unfortunately that will require moving those rear contactor boxes. Gulp! I've seen grown men bite their own heads off rather than ...

So we can cross number 2 off the list. Hoorah!
Last edited by weber on Sat, 21 Jun 2014, 11:17, edited 1 time in total.
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