Thursday, September 16, 2010

Throttle Detail

I had to take the throttle out of the car because the rear motors were trying to go even at zero throttle. This is a function of the kind of potentiometers I am using. They are supposed to be 0 - 5k ohm, but actually they are ~750 ohm - 5.2k ohms. The Alltrax controllers I am using for the rear motors don't have the ability to set the dead zone on either the top or bottom of the sensor range, probably because they expect the controller to be switched off at zero throttle. However, I am using the built in monitoring functions of the controller (a big plus for the Alltrax controllers) to get the motor current, battery current, battery voltage, and controller temperature. If the controller is off at zero throttle, that information isn't available.

Anyway, I solved this issue by putting in microswitches connected to 22 ohm resistors. When the accelerator pedal is not depressed, the microswitches are activated and the 22 ohm resistor is put in parallel with the output of the potentiometer. This lowers the resistance seen by the controller to ~23 ohms, which the controller reads as zero throttle. Problem solved! Anyway, here is some video...


  1. Why don't you use Digital Potentiometer instead?

    Here is simple exemple, 4 Digital Potentiometer in one chip from MAXIM-IC.

    You could do your differential steering the easy way! No mechanical stuff. Using your own microcontroller, you could trim and adjust the steering by simply changing numerical value in your code. Fine tuning.

    You can also get single pot with way more resolution and different parameter. There is other company that makes those pots like Microchip, TI, etc. All that using SPI, I2C interface. Get a Arduino to control all that.

    Keep up the great work!


  2. Jerome,

    I thought about using a digital pot but I didn't want to give up the simplicity of mechanical. With my current setup, even though I haven't implemented the servo controls, the throttle still works as a purely mechanical system.

    With digial pots, I be relying on the microcontroller to set the throttle position (after sensing pedal position from a mechanical pot). If the code controlling the microcontroller is bad or the comms to the pots is interrupted or something happens that I haven't even considered, I could lose control of the throttle, and that would be a bad thing. However, if I could ensure that the system would work correctly 99.999% of the time, a digital pot would be the way to go.

    But thanks for the suggestion. I hope the project is as interesting for others as it is for me.

  3. I listen again to the video and understand that the pedal will physically be connected to the middle plate.

    I totally agree of the aspect of safety but you have to admit that microcontroller are everywhere, planes, car, train, MTR. The majority have redundancy to prevent accident but still, electronic is very reliable.

    You have watch dog that will tell you when the micro hang up. You can have backup capacitor of keeping a charge when battery power is lost. The CRC is good for the communication. There is plenty of technics to have faulty free system.