Electronic – arduino – Decoupling issue with DC Motor

Your schematic is correct, basically.

Although it is not explicitly shown on your diagram, I assume you have the Arduino plugged into a USB port which supplies it with power?

What happens if you connect the 9V power supply directly to the motor? Of course the motor should spin constantly at full speed. This tests that the voltage of this power supply is appropriately matched to your motor, and the power supply can source enough current to run the motor.

Now, what happens with the wire disconnected from pin 9 on the Arduino and plugged into +5V on the Arduino? If the transistor and other parts are wired up correctly this should also cause the motor to spin up constantly at full speed.

If this doesn't work double check the pinout of the transistor.

Now, in your software, try turning the motor on with a single digitalWrite(MotorPin, HIGH) statement in the setup part of your program. Don't put anything inside the loop at this stage. This should just cause your motor to turn on and stay on all the time at full speed.

Now, if that works, try something like analogWrite(MotorPin, 128) executed once in the setup part of your program, with nothing in the loop. The motor should now turn on, and stay on constantly, at reduced speed.

Is the motor current too much for the 2N2222 to handle? (Once you know what the motor's actual specifications are for current draw, check the collector current in the 2N2222 datasheet.) You may need to consider substituting a different, higher-current transistor such as a TIP120 or BD675.

You shouldn't be using the PID output as a velocity command in your system, as it will simply become unstable and eventually oscillate between one end limit and the other, or have no response.

Using a linear actuator as you are, you should use the output of the PID as a position command, and have the actuator follow the output that way. By setting the PID min and max limits, the end of travel switches are only safeties if something goes horribly wrong.

Also, if you detect that your PID is at either limit (which it might have to be if the response is very slow), you want to inhibit the integral term for as long as it is at a limit so that you don't suffer from integrator wind-up.

PID's work fine on motor controls, true, but I use them industrially to control temperatures easily on systems with several minutes of lag. Tuning is everything... Remember the basics, Proportional gain will only get you (approximately) half to three quarters way to the setpoint, Integral gain will take you the rest of the way. Derivative tempers the change.

For temperature control loops with a long lag, I find it much easier to ramp the setpoint up slowly to the final setpoint rather than jump it immediately, it allows a tighter tuning of the PID without having it go into oscillation.