Electronic – Choosing a PID controller

An unloaded DC engine runs with a speed proportional to the voltage, ideally without drawing any current (In practice, internal friction will cause the engine to draw some current). As you start loading the engine, the current will increase, and some power will be lost due to resistant losses in the windings. The applied voltage will now split into two parts. One part, the electric losses, is current times winding resistance, this part only generates heat in the engine. The other part is the voltage turning the engine. With load, current will increase, and your motor will slow down, unless you compensate by increasing the total voltage.

There are basically two things that can cause your engine to break down. If the engine is stalled, or you put too much energy into it, the resulting heat will burn of the windings and/or the brushes. A good way to avoid this is to have some current limit or fuse to limit the current if the engine is overloaded.

If you let the engine go too fast. Centrifugal forces will cause the rotor to collapse. Typically the windings gets pulled out, until they touch the stator and gets ripped apart. This means you can apply more than the rated voltage on a loaded engine as long as you A) Don't let the engine over-rev. B) Don't let the engine get to hot.

First, since steppers are great at positioning (there is no need for a position feedback), you should certainly limit their movement as you've said yourself. I am not sure how the motor shaft is engineered right now, but if it was fixed to the motor, letting it continue spinning would risk damaging the equipment.

Next, 200ms transport delay in your sensor will probably be too slow, otherwise you will need to slow things down a lot in order to slow down the ball itself. Similar to what Rocket Surgeon said, you should simplify the image processing algorithm to calculate the path only once, and then quickly calculate only the position of the ball in each frame. If you want to skip this step quickly, find a red ball instead of this one, and then check only the red component in your RGB picture, until you've found a better algorithm.

For the PID control, start with the fact that you actually need two separate PID controllers, one for the east-west motor, the other one for the north-south one. If you have two exact motors, their parameters must be equal.

For a PID controller to act, it needs to know the error: difference between the desired position, and the actual position of the ball. X and Y components of this offset will be the inputs for two PID controllers (one for each motor). To get the error, you need to have the desired position on your path first: a trajectory.

To get the trajectory, you need to process the image and get the path, as well as its starting and ending point. I am not sure if your algorithm is capable of distinguishing the path from the rest of the board right now, but if not, note that this is an algorithm of its own to handle before continuing. Again, you can skip this part by manually entering the junction points, if you are eager to see some results quickly. In any case, you should be able to define the setpoint speed, and have your software move the desired coordinate position over the path, from start towards the end. Obviously, you will start with a low desired speed.

So, before starting with control, you should go through the following checklist first:

• Simplify your image processing algorithm to get faster response
• Create an algorithm which creates a trajectory over your path using a predefined speed
• In each frame:
  • Calculate the difference between the trajectory and the ball position
  • Pass the delta-X component to the east-west PID, pass the delta-Y to the north-south PID

It may turn out that it is better to create the trajectory one segment at a time, and continue with the next segment when that ball ends the previous one. Otherwise, you will need to take care that the ball doesn't overshoot the desired trajectory (which may be hard to accomplish)