Controlling Position of toy car with a DC motor

Hi everyone, and thanks for reading this. For anyone that can help me with this, thank you in advance!

Here’s the task:

I have to design and build a small car that can roll forwards and backwards a precise amount, based on the position of an input potentiometer. The car’s feedback is a 10-turn high precision wire-wound pot. So, the maximum distance the car can travel forward or backwards is about 12 feet because the diameter of the wheel that the feedback pot is attached to is only ~4inches.

As I alluded to in the subtitle, this is a project for a senior-level mechanical engineering design course. While I do understand the feedback systems’ flowchart pretty well I can’t seem to translate that into circuitry.

Here are the design constraints:
The input pot is a 1meg-ohm 1-turn pot (bourns)
The feedback pot is a 500Ohm, 10-turn pot (military surplus, Duncan electronics)
We are supposed to use Burr-Brown OPA547 high-voltage/high-current Op-Amp to drive the motor, and we also have a National Semi LF412 Dual JFET Input Op-Amp for summing or inverting, or whatever.
The motor is a little bitty DC deal; looks like it was originally designed for a cassette tape deck. It’s got good torque, and slapping a 9-volt battery on it makes our little car haul across the carpet.
The power supply is supposed to be a +/- 9Volt setup, courtesy of two 9volts, but it seems that there should be a way to accomplish that with just one…?

The car should be able to go forwards and backwards from it’s starting place assuming it’s not backed up against a physical stop on the 10-turn pot. It should also be “know” when it’s approaching the end of it’s traversable distance (0th or 10th turn of the feedback pot) and stop before it slams into the end of the range.

Ultimately, we’re supposed to build a PID controller, but I’d like to start out with a simple proportional controller, and witness the overshoot phenomenon, and understand where (electronically) that comes from. EG: I understand that fundamentally the device must overshoot, in order to know that it’s overshot, and then back up to the correct position a few times, but I’d like to see how noticeable that is on a 1-pound car with a very tight drivetrain, and excellent response.

Once I’ve got that circuit working I’d like to incorporate the integration circuitry and operate it as a PI controller. Finally, if time allows, I might like to take it to the PID level and see if there’s any difference between the PI and PID controls on such a simple device.

If anyone could provide some form of help with this, I’d greatly appreciate it.

For whatever its worth, the car is actually a little trike (3wheels) with the large front wheel being driven on the OD by the little motor (providing a 60:1 reduction) and the wheel is mounted to the shaft of the 10-turn pot.

Thanks!
drew*

Seems like you need kind of a DC servo motor circuit.

I had developed a quite simple single supply circuit on that way. Put me an email and I'll send you the schematic. (as soon as I could find it somewhere in my PC!)

[email protected]
www.CodeFreaks.Net

Drew,

Personally, I have not implemented an analog pid from scratch but I have used micros to do it. I offer my suggestions away:
Use the input pot signal to set your distance reference. Use the feedback pot signal to indicate the actual position.
1. Get the op amp and setup it up as a difference amplifier. The input pot signal gets tied to the ref. which would be the + or noninverting pin. The
feedback would be tied to the - pin (inverting).
The output is the error. Note the output voltage will swing somewhere between near the +supply voltage to the -supply voltage or ground for single supply.
2. Proportional part would be your op amp with the gain. You take the error signal from the error amp and adjust it accordingly to scale your response just right.
3. Integral part is when you take the error signal and you run it through an integrator, again you can build this with the op amp, resistors and capacitor.
4. Derivative part: build a differentiator and feed it the error signal.
5. Sum up all the P, I, & D and you get a PID output signal. You use a summing amplifier to do this. You might put a clamping circuit to clamp the maximum correction signal just in case your pot fails or picks up noise though for a tiny toy car it probably does not matter. Take the PID output and drive the amplifier that drives the motor. Actually, your motor amp should have it's own PI or PID to comply with your PID command signal. Normally, this is a current control type. [you may try w/o it]. You'd need to tweak the P, I, & D gains to get it just right.
For actual circuits, you could buy the Opamp Cookbook, Forrest Mims project books at Radio Shack, or you can visit National Semiconductor, Analog Devices, etc. and visit their application notes section.
my guess is that when you set a distance the car will race towards the setpoint and oscillate back and forth if you use only P.
Good luck, oj