MX-5 progress report:
Accelerator pedal signal
We have not had any further problem with intermittent noise in the accelerator pedal signal. Hoorah! And I even took someone-I-wanted-to-impress for a drive. So here comes the final attempt to provoke the fault to return: I hereby pronounce it fixed.
With that problem fixed, we turned our attention to other cases of torque oscillation. Next in severity was a shudder at about 5 Hz that ocurred briefly whenever we stomped on the accelerator when stopped or travelling at low speed. "So don't stomp on the accelerator", you might say, but where's the fun in that. I was unsure whether it was the WaveSculptor somehow "losing sync" or whether it might be our new you-beaut ceramic button clutch going slip-stick-slip-stick.
It did not show up in the ordinary WSConfig logs, but then they have a sample rate of 5 Hz (200 ms between samples). Tritium_James suggested I use the more general-purpose CAN logger so I could see the RPM updated at the new rate of 25 Hz (40 ms between samples).
As it happened, when I did the test, it became blatantly obvious that it was the clutch slipping. I could see it on the tacho, because it chose that moment to slip continuously for several seconds! Once again, the WaveSculptor was blameless.
Here's the graph of RPM versus time showing the clutch slip. The solid line shows what the motor was doing, and I've drawn in a dashed line to show what the gearbox, and hence the car, was doing.
The more logs I examine, and the more graphs I plot from them, the more I see that the WaveSculptor is always in complete control of the motor, and makes it do exactly what we ask it to. A superb piece of engineering.
So, for now, we have simply told the WaveSculptor to go easy on this clutch by winding the maximum motor current and hence the maximum torque back to two thirds of its ultimate maximum. 200 A rms down from 300 A rms. This is at a battery voltage of around 330 V under load, so still quite drivable.
We'll wait and see how it goes when we get the 900 V WaveSculptor, as it will probably be limited to around 200 A anyway. With that, we'll get our power from the doubling of the RPM out to which we get full torque.
The fallback, suggested by Graeme from Suzi-Auto, is to take out the new pressure-plate and double up the diaphragm spring, stacking the spring from the original pressure plate on top of the one in the new pressure plate. I'm afraid I have to doubt the claims of the clutch manufacturer that this clutch can take 380 Nm. Tritium measured our motor at 350 Nm at 300 A.
We can now answer a question that dates from the earliest pages of this build thread. Should we have left out the gearbox and done "direct drive" to the diff? The answer is:
As it is now, we drive around most of the time in second gear. When we go from 360 V to 720 V with the WaveSculptor upgrade, we may want to spend more time in first
gear. Diff is 3.636, First is 3.136, Second is 1.888, Third 1.330. To go direct drive, we'd need a 6:1 unobtainium diff (120 km/h at 6800 rpm) which would be like driving in a gear between second and third now, and we wouldn't have the option of first gear for a faster launch.
One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).