Sunday, January 27, 2013

Turret Gun: Stall Checks

Stall checks are a failsafe step which increases reliability and decreases maintenance.  When electrical motors are stopped but power is still applied the windings on the inside of the motor can heat up too much, leading to burnouts as the windings melt through insulation and begin shorting, or melt completely, increasing the resistance until the motor smokes.  The tetrix motors are particularly bad in this sense; they smoke whenever they get stuck.

A current sensor is probably the best option for preventing stalls.  The current sensor reads the current that the motor is drawing.  Motors have a rated stall current; if the current sensor reads this current, then power can be cut to the motors for some period of time before allowing the program to "try again".

Optical encoders provide another easy way to check for a stall; if power is applied to the motor, then the encoder reading should be changing over time.  If the reading is static then the motor is stalled and power should be cut.  The code and the algorithm shown here do exactly that.

The issue with a stall check is not really in detecting the stall, although you do have to be careful about how often you check, and how much the encoders must turn before you are satisfied that the motor has not stalled.  Instead, the issue is how you recover after you have stalled.  Once power is cut to the motor, it will not be able to spin, meaning the encoder values will not change.  If your only algorithm is the one shown above, then you will have to reset the program completely once any stall occurs.

Another related issue occurs when you want to apply power for the first time.  Before power is applied to the motor, the default for a current sensor algorithm will be "OK, you can power the motor", because the current is zero.  However, the default for an optical encoder algorithm will be "No, you can't power the motor" because the encoders aren't changing values.

To get around these problems, you need to work harder with encoders than you do with a current sensor.  You need to consider both a) is the motor stopped because no power is applied or because it is stalled, and b) is the motor stopped and ready to startup again.  Fixing a) is pretty easy by adding in the question of "is power applied?", and only running the stall check if it is.
The much tougher question is how to get the motor running again.  What I did was the following.  Consider that for the motor to begin again, we want the user to recognize a stall and remove power to the motor.  After this point we want the program to be allowed to attempt to run the motor again without implementing the stall check.  There should be a "grace period" where the motor gets a shot to start moving again, BUT the grace period can't be too long in case the motor is truly stalled.  So first we need an algorithm to recognize if the joystick has been returned to a zero position, indicating recognition of the stall.  One such algorithm is as follows.  We keep track of the last time the motor power was non-zero... if more than 200 ms have passed since the last non-zero motor power reading, we assume that the joystick is centered and we raise a "stopped flag".

Once that is all in place, we can put it together with the stall check algorithm as follows.  If the stopped flag is raised and the joystick is now in a nonzero orientation, we apply power to the motor for 300ms without running a stall check.  Once we've done this, the "stopped flag" is lowered and the stall check will be occurring again.

The only trick is the ordering.  The checks for the stop flag and for the current motor power need to happen first in the loop, followed by the stall check algorithm, and ending by resetting the timer.  This order allows for the stop flag to be raised at the same time that a non-zero power is applied, allowing us to enter the 300ms grace period where the stall check is not run.

The algorithm does add a level of latency due to the delay which stops the microprocessor while a motor is run.  But this was the best I could come up with at this point.

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