SMC03 and POT for Analog Position Feedback

[ALIBE]

I am at a point where I want to hook up my 10K POT to the motor axis. I read in this forum that by jumper clipping to Analog, will make convert the speed commands to pos commands.

I have a few ?s on how to setup the PID params. And, if there was a pre-tested set of steps that I can/should follow to make this process as easy as possible.

Can someone shed some thoughts on what is the best approach to setting up the PID params and what is the best flow to follow.

I am controlling two 12V Windshield wiper motors using 2 SMC03 motor controllers.

Thank you in advance

[nexisnet]

There are two approaches to generating good PID parameters. A theoretical approach involves either extensive physical characterization and mathematical modeling of your system (although computer programs like Matlab can do a lot of this numerically, but it’s still a lot of work to set up).

I prefer a more empirical approach (i.e. try and see), and there is a short description of a good (I think) technique in this thread.

You can determine good values experimentally by making small changes to the coefficients one at a time. The user guide briefly mentions a very good way to do this:

Start with the integral and differential terms set to zero, and test different values of the proportional term. If this term set low, the motor will move slowly, and it’s position will not scale linearly with the position command (with a purely proportional controller there will always be an offset from the desired position proportional to that position). As you increase the proportional term, your motor will move to the commanded position more quickly, and the offset will decrease and eventually become negligible, but if there isn’t enough mechanical damping in your system you will start to see oscillations about the commanded position.

If you do see oscillations before the motor speed and final position offset are to your liking, lower the proportional term until you see no oscillations and start to increase the integral term. This part of the loop will try to move the motor to it’s commanded position in response to it sitting in the wrong place for too long. Whereas the proportional term commanded the motor to move with a speed proportional to the position error, the integral term will command increases to the motor speed as it integrates (adds up over time) the error.

As you start to increase the integral term the motor will start to creep towards it’s true commanded position. As you set this term higher to get there faster, it may start to overshoot and cause oscillations again. This is when you can bring in the derivative term. This term will tend to slow the motor down as it approaches the commanded position, to prevent it from overshooting. As you set this term higher it will limit the speed your motor can reach as it approaches the commanded position (too high and it will never get there!).

The main thing is to change values slowly, and one at a time. Setting P or I too high can make the system completely unstable, where the jitters will increase with every oscillation! Also, don’t feel like you need to use both the I and D terms. You may find that P (I=0, D=0), PI (D=0), or PD (I=0) control is sufficient for you.

So, what’s your plan for these motors; are they still controlling your windshield wipers?

-Adam

[ALIBE]

thank you Adam. I will take a shot at these recommendations.

My project is codenamed, “Home Motion Deck”. Goal is to provide a DIY route of building a home motion simulation and “rider” platform soln at low costs. We will see where that goes :). I will provide more details on my projects site : nagiprojects.crosscity.com/

Also, many thanks for Jan (and Pololu) for patiently answering several of my questions earlier this am - w/ recommendation that SMC04 would be a better MC for my project