AC drive programming and pedal mapping

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Post by Tritium_James » Sat, 20 Mar 2010, 00:40

Also with the Porsche: something else I'd forgotten about (this is making it look more and more like Weber's diagram!). It also has a slider on the dash to control how much 'full' regen is, when you've got the pedal all the way up. Even with it set to max I think it's about 25 or 30% torque.

The reason I'd forgotten about it is that everyone driving the car always ends up with it set to maximum and leaves it there.

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Post by woody » Sat, 20 Mar 2010, 00:55

You can probably calculate the accelleration / engine braking from an ICE given the engine rpm, camshaft timing, throttle valve size, displacent, valve size, compression ratio, peak torque, fudge factor etc.
A simple approximation would be a function of rpm and throttle opening. More braking at higher RPM, braking effect kicks in at a more open throttle at higher RPM.
With direct drive that wouldn't be natural as no-one is used to driving in one gear. (yet)
I see Ross didn't quite get there with one input - a separate "power" knob on the dash - so he's just one of us mortals :-)
Toyota has the "B" gear in the prius, plus the brake pedal.
Auto transmissions usually let you shift into a lower gear.
I'm not sure the one pedal is going to be ICE-like ...
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Post by Squiggles » Sat, 20 Mar 2010, 01:46

So when you chaps manage to sort this regen control out I expect to see a set of generic instructions/rules so that every one can implement their own version.

Or should I read these post again and realise you've done that already?

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Post by a4x4kiwi » Sat, 20 Mar 2010, 13:33

hi Guys,

Here is the way that Azure Dynamics does it. http://www.azuredynamics.com/products/f ... Manual.pdf

Cheers,
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Post by coulomb » Sat, 20 Mar 2010, 16:00

Squiggles wrote: So when you chaps manage to sort this regen control out I expect to see a set of generic instructions/rules so that every one can implement their own version.

Yes, sure.

But I suspect there might be more than one sets of instructions, since the community seems to be divided at present into
* all regen on throttle pedal, it's really cool
* make it just like the ICE, faking compression braking, else it's not safe
* foot off means coast, you can't hypermile properly without this.

There may be a reasonable way of accommodating all three, even if a little more expensive. You need a pot or slider on the dash, setting the amount of regen with the pedal up, as TJ has in his client's Porsche. For the first scenario, you set it to maximum. If TJ is right, that's where experienced users will end up putting it. For new drivers or those who prefer ICE-like driving, it's put part-way, maybe at 1/3 (maybe it would be good to have a detent there). For the hypermilers, or if you happen to be driving on ice or the like, set it to zero. Regen is implemented on the brake as well, progressively increasing with brake pressure.

I see two problems. The first is that the throttle response has to change and "expand" from the last 60-70% in the first scenario, to almost 100% in the latter, and it all has to be smooth and natural. Whether this can best be done in controller software, or a custom throttle micro, or custom hardware, remains to be seen.

The second is that in a rear-wheel drive vehicle, where regenerative braking is entirely on the rear wheels, there may be situations where you want to disable regenerative braking due to the driving conditions (again, ice or other slippery surfaces). That means at least another switch as well as the first pot/slider, and the user interface starts to get complex. Maybe the controller could detect lack of drive wheel traction, and automatically reduce or disable regenerative braking as needed.

That would imply some mechanical braking with regen at all times. I believe that the Prius is this way, meaning you don't ever get to avoid using mechanical brakes. However, if you really want to save mechanical brake wear, you can use the high regen on throttle option. The problem with that is that if the controller has to reduce braking due to rear wheel slippage, it can feel unnatural, and also the driver won't be getting the braking he expects, and has to switch to the brake pedal, costing time.

Some may argue that a lot of these problems go away with front wheel drive, and it's true that things are simpler in the FWD case.

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Post by Squiggles » Sat, 20 Mar 2010, 16:11

coulomb wrote: The second is that in a rear-wheel drive vehicle, where regenerative braking is entirely on the rear wheels, there may be situations where you want to disable regenerative braking due to the driving conditions (again, ice or other slippery surfaces).
Maybe the controller could detect lack of drive wheel traction, and automatically reduce or disable regenerative braking as needed.


Having absolutely no first hand experience with the phenomenon, wouldn't regen breaking tend to be self regulating/antilock? If the wheels lock there is suddenly no regen and no load so they start to rotate again?? Actually won't this happen as you approach 0 rpm, surely the slower the rotation the lower the regen torque (after a certain point is reached)

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Post by coulomb » Sat, 20 Mar 2010, 19:05

Squiggles wrote: Wouldn't regen breaking tend to be self regulating/antilock? If the wheels lock there is suddenly no regen and no load so they start to rotate again??
My understanding is that you request a negative torque with a minimum speed of zero (i.e. don't keep going into negative speed, i.e. reverse). So locked wheels slipping in the wet fulfils those criteria nicely, so the drive would be happy to stay with zero speed and locked wheels.
Actually won't this happen as you approach 0 rpm, surely the slower the rotation the lower the regen torque (after a certain point is reached)

Well, yes, below some 10 km/h, regen will start to diminish on a typical AC conversion. But at rest, regen has finished its job, and zero speed is what you want. Am I missing something? As far as the drive is concerned, both situations are the same; mission accomplished. But with locked wheels, the driver disagrees with this assessment.

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Post by weber » Sat, 20 Mar 2010, 20:33

coulomb wrote:... the community seems to be divided at present into
* all regen on throttle pedal, it's really cool
* make it just like the ICE, faking compression braking, else it's not safe
* foot off means coast, you can't hypermile properly without this.

There may be a reasonable way of accommodating all three, even if a little more expensive. You need a pot or slider on the dash, setting the amount of regen with the pedal up, as TJ has in his client's Porsche.

Good summary. But there's another dimension on which we differ: The accellerator position that corresponds to zero torque, and whether this should vary with rpm as it does with an ICE. The only reason I can see to use a fixed position is that it is easy to implement.
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Post by Squiggles » Sat, 20 Mar 2010, 21:40

coulomb wrote: Well, yes, below some 10 km/h, regen will start to diminish on a typical AC conversion. But at rest, regen has finished its job, and zero speed is what you want. Am I missing something? As far as the drive is concerned, both situations are the same; mission accomplished. But with locked wheels, the driver disagrees with this assessment.

Edit: fix mangling through multiple browser crashes (separate download)

The thing I don't understand is how the wheels lock up. The resistance to rotation is from regen torque (am I wrong), when rpm is very low there is virtually no regen so the resistance is low so the wheels continue to rotate.....come on you AC experts help me out here! If you set your drive to zero revs presumably the best it can do is apply 3phase at 0hz, if there is no rotation of the rotor there is no inductance so no rotor field so no torque....am I missing something?

I can see how it could happen with a synchronous motor but we are talking induction aren't we!

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Post by Electrocycle » Sat, 20 Mar 2010, 22:50

the controller can drive the motor in reverse up to the slip speed, for full reverse torque at zero rpm.
It's not regen any more, but it can brake down to zero.

Usually issues with "lockup" under brakes are not a true lock up but a loss of grip due to the wheels spinning at substantially less than the road speed.
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Post by Johny » Sun, 21 Mar 2010, 02:09

I get where you are going with this Squiggles. At low speed the regen braking is mostly being supplied from back EMF from the switching devices. Logically there is a point where the back EMF is so weak that regen braking ceases to provide much torque. My understanding is that it is around 5 to 10km/hr on a direct drive.
ABS primary function is to stop the wheels from locking under braking(???). Regen braking can't lock the wheels (but can make them rotate much slower than the road speed).
So if the wheels are rotating at 5km/hr and the vehicle is doing 60km/hr, does this cause significant loss of control over the direction of said vehicle?
Hmm. Answered my own question - yes. Adhesion to the road has been broken. Same as rear wheels doing 70km/hr with vehicle doing 50km/hr - needs much driver attention.
My conclusion is that regen braking does not intrinsically give you traction control or any usable ABS function - it must be added.

So is it the ABS doesn't stop the wheels from locking, it allow them to freely rotate, then provide braking force, then freely rotate etc.

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Post by Tritium_James » Sun, 21 Mar 2010, 02:33

An intelligent drive will be able to regen all the way down to zero speed, if commanded to do so. To do this may require putting power into the motor (in the reverse direction) at slow speeds. So it's not really regen at slow speeds as it will be using energy from the batteries, but you will be getting constant negative torque all the way down to a dead stop.

If you want an ABS-like function you'd have to command it to do so explicitly.

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Post by Squiggles » Sun, 21 Mar 2010, 03:09

Electrocycle wrote:
Usually issues with "lockup" under brakes are not a true lock up but a loss of grip due to the wheels spinning at substantially less than the road speed.


Very nice, critical point that I had missed entirely.


There seems to be a bit of a cloud around the difference between regen and reverse drive though.

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Post by Tritium_James » Sun, 21 Mar 2010, 11:07

Squiggles wrote:There seems to be a bit of a cloud around the difference between regen and reverse drive though.


From the controller's point of view they're exactly the same thing - negative slip. If the motor's going fast enough you'll get current out, otherwise you'll have to put current in.

Then it's up to the controller's speed control loop to make sure once it slows down to 0 it doesn't start accelerating in the opposite direction.

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Post by coulomb » Sun, 21 Mar 2010, 14:49

Johny wrote: I get where you are going with this Squiggles.

And I certainly didn't; my fault. Sorry for the pretentious post that resulted.
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Post by coulomb » Sun, 21 Mar 2010, 15:01

Tritium_James wrote: Then it's up to the controller's speed control loop to make sure once it slows down to 0 it doesn't start accelerating in the opposite direction.

Right. But what if you changed the loop to terminate at zero frequency, or zero regen current (edit: not let it go out of the battery, only in), rather than at zero speed? So then regen braking would be constant down to 5-10 km/h, then taper to nothing as the frequency went down, and stop altogether at a few km/h. Then I think you'd get that "automatic ABS" behaviour that Squiggles was talking about.

I'm guessing people would hate it for ordinary braking, since regen (edit: by itself) would quit just before the vehicle completely stopped, so when you wanted to stop with regen only, it would instead dribble to a near stop, and overshoot where you expect you would stop by enough distance to be annoying.
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Post by weber » Sun, 21 Mar 2010, 15:39

coulomb wrote:But what if you changed the loop to terminate at zero frequency, or zero regen current (edit: not let it go out of the battery, only in), rather than at zero speed? So then regen braking would be constant down to 5-10 km/h, then taper to nothing as the frequency went down, and stop altogether at a few km/h. Then I think you'd get that "automatic ABS" behaviour that Squiggles was talking about.

I'm guessing people would hate it for ordinary braking, since regen (edit: by itself) would quit just before the vehicle completely stopped, so when you wanted to stop with regen only, it would instead dribble to a near stop, and overshoot where you expect you would stop by enough distance to be annoying.
Why would it be annoying? It's exactly what people are used to with engine braking. You use the brake pedal for the actual full stop. So I think it's a fine idea for the regen you request by backing off the accelerator.
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Post by Johny » Mon, 22 Mar 2010, 14:42

coulomb wrote:I'm guessing people would hate it for ordinary braking, since regen (edit: by itself) would quit just before the vehicle completely stopped, so when you wanted to stop with regen only, it would instead dribble to a near stop, and overshoot where you expect you would stop by enough distance to be annoying.
What you are describing is pretty much what an automatic transmission with torque converter does.

Slushbox autos provide engine braking when the vehicle is doing significant speed, coast at lowish speeds, and try to drag you forward at near zero speed. Modern autos are a lot better than the older boxes but the tendency is still there. By the time you get down to 5km/hr I don't think that the loss of regen. braking would bother you much at all. If it's a panic stop then the main part of the work is already done.

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Post by Huub35 » Thu, 25 Mar 2010, 07:56

Dear all,

having driven a Tesla Roadster this weekend (yes !!), I can say that regen via the throttle is very natural (to me, at least).

I would state the decelleration level to be stronger (ca. 2 x) than a gasoline engine, more or less comparable to a diesel in low gear. I did not try or need to modulate the regen level while driving, just lifting the foot from the pedal was OK for a decent stop towards a traffic light. I have not remembered how far the "regen stroke" of the throttle was.

Very cool was the power meter, instead of rpm, in the dash. Whereas the max positive position is ca. 200 kW (from memory), I think I recall that the max regen was -40 kW (colored green, nice detail). From this we can already work out something on the brake levels I think. Driving in the city I think I had max regen in the vincinity of -20kW.

Nothing special happened, but I did not ride on ice, so I cannot comment on that.

Furthermore I noticed that accelerating away from traffic lights did not cause wheelspin. Either the road and tyres were really grippy, or the torque is controlled in some way. Did not elaborate on that one.

The remark of the tesla guy was that he really relied on the regen for braking, only using the brake pedal for standing still (like an automatic transmission). He also remarked on the fact that regen does not work with full batteries, and that he was sometimes surprised by this when riding away in the morning.

The tesla has a "missing regen" warning light in the dash, just like the "missing abs" or "missing ESP" lights. I can see that this light could be driven from a HV signal of the BMS, or should we rather try to get this from the controller itself?

Hope this helps a little bit in the discussion,

Regards,


Huub

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Post by Johny » Thu, 25 Mar 2010, 14:45

Huub35 wrote:having driven a Tesla Roadster this weekend (yes !!), I can say that regen via the throttle is very natural (to me, at least).
You lucky person.
The tesla has a "missing regen" warning light in the dash, just like the "missing abs" or "missing ESP" lights. I can see that this light could be driven from a HV signal of the BMS, or should we rather try to get this from the controller itself?
This is easliy overcome by adding brake resistors" and a controller that switched them in when the DC voltage exceeded a certain amount. Ramping the resistors in (via PWM or chopper control) over a 1 to 2% range of pack voltage would provide a smooth transition that would be unnoticable from "normal" regen.

It is worth noting that even though a few EVers with regen. have the capability of doing this easily - they don't bother with it. I originally procured resistors for this purpose but no longer intend to use them based on the experience of others.

The light is probably a good idea though.

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Post by bga » Thu, 25 Mar 2010, 23:40

weber wrote:
Johny wrote: Curiously, being a embedded system's software developer, I am implemeting my throttle interpreter in hardware.

Cool. Yeah, no reason why something like Ross's schematic coudn't be implemented in actual analog hardware.

I notice you are trying to do it without any feedback of the motor's actual speed or torque (or active current), so I don't understand how it can give the "moving zero" of torque that an ICE has, as described above.
Or analoged in actual digital software Image

I've posted a suggestion like this previously:
It sounds like a couple of modes are needed to cope with varying traffic conditions, but really these are vaiarions on the width of the deadband and limits on the braking effort and torque slopes.

I would expect that a tight deadband would be more useful in stop-start and creeping traffic and a wider deadband for good coasting on free dflowing roads, which is more efficient than trying to recover the energy through regen.

In another thread, AC Motor suggested a clutch operation in forward (or reverse) so that when the car is stopped on a hill, the motor will apply torque to keep the vehicle stationary and prevent roll back. I thought that the controller should allow a creep backwards very very slowly, say 1 or 2 RPM, so that when the brakes are applied and the backwards motion ceases, the motor will unload and go to zero current.
Safe operating limits on the regen and motor current are needed to keep the drive system happy and not too hot.

An emerging behaviour would be that, if the direction is changed when moving, the vehicle would run to a stop and sit there waiting for accelerator input. Above 5kph, as suggested in the NCOP, the direction change could be ignored, or wait unitl the speed is in range. Maybe a beeper is needed to alert the operator to the inappropriate input.

A lamp when regen isn't going to work very well would be a good idea.

In this sutuation, the controller could resort to plug braking (DC current injection) which will slow the vehicle up by making the rotor hot and help to discharge the battery so it doesn't happen on the next braking event.

It's not the end of the world, but I can see it from here.

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Post by coulomb » Sat, 01 Jan 2011, 17:50

I just happened to be reading this old thread recently.
Electrocycle wrote: the controller can drive the motor in reverse up to the slip speed, for full reverse torque at zero rpm.
It's not regen any more, but it can brake down to zero.
Ah, interesting point. So maybe for maximum range, you don't want to command zero speed, you want to command a low speed such that you don't need to put power into the motor at all. When you get to that speed, or close to it, you'd switch to zero torque (speed irrelevant). (I'm assuming a controller like the Tritium WaveSculptor that has speed and torque settings.) So you'd need mechanical brakes for the last few km/h.

I wonder if this effect is part of the reason Jack Rickard reported such low range extension from regen. He found that on one run, it actually cost a tiny bit of range with regen turned on, compared to leaving it off altogether. Maybe he was using pack power to keep the motor at zero RPM while waiting at lights and intersections. Especially if you have a few stops facing uphill or downhill; it would take some pack power to keep the vehicle still, while just putting your foot on the brakes (and demanding zero torque) would require no power.

In fact, I notice something like this effect on my Prius. I find that the display shows battery power being used if I don't put my foot on the brake. This is even at level roads. It's annoying to see battery power being wasted, so this causes me to use the foot brake at lights. Maybe Rickard wasn't watching the ammeter when stopped.

This behaviour (not regen'ing down to zero speed) could be sensitive to a power/economy switch, if you had one. Another complication to take care of when designing pedal response software or hardware. With our controller, we have an MSP430 in the Drivers Controls box between the throttle box and the CAN bus, so it's no trouble for us (within limits) to implement something a little more complex.

Edit: this effect -> something like this effect
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Post by Johny » Sat, 01 Jan 2011, 21:56

Funny your should bring that up. I was concerned that the motor used power to hold it at zero RPM so in my Lenze function block programming I have put in a system to kill the torque below 50 RPM (at the moment). I was kind of hoping I didn't need it as I could use another Compare FB (I only have 4), to operate brake lights.

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Post by weber » Sat, 01 Jan 2011, 23:48

One thing that confuses me is that VF drives only let you specify the magnitude of the torque(active current), not its direction, while you can specify the magnitude and direction of the angular velocity. And we don't even care about negative velocities because we have reverse gear.

I understand that the torque is a "symmetrical limit" as Ross Pink says, and its direction or sign is determined by the difference between requested and actual speed.

But surely the situation is dual, like Norton and Thevenin equivalents in electrical circuits, and it could just as easily have been signed torque and unsigned speed that we got to specify. So the speed would be a symmetrical limit and its sign would be determined by the difference between requested and actual torque(signed active current). For some reason I find it easier to think about it that way.
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Post by weber » Sun, 02 Jan 2011, 04:25

I think I'm starting to understand this stuff. What do folks think of this latest attempt?

Requested torque (normalised) is a function of both pedal position and actual speed. In this case
T = p^2 + (p^2-1)*k*s
where T is requested torque, p is pedal position, s is actual speed and k is the max regen torque which is 0.25 for the chart below but could be controlled by a slider.

Requested speed (rpm, normalised) is based purely on pedal position, and corresponds to that actual speed at which the requested torque would be zero (for the same pedal position).
S = Min{1 , p^2/[(1-p^2)*k]}
where S is requested speed, p is pedal position and k is the max regen torque which is 0.25 for the chart below but could be controlled by a slider.

Image
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