Controlling a DC Motor
Small DC motors are easy to model and control. Fortunately for us, they can be reduced to simple terms so our control algorithms are easy to code. The permanent magnet DC motor can be modeled as a device that produces torque proportional to the current flowing through it. It also produced a voltage proportional to the rotational velocity. One last bit of modeling completes the picture: there is a small series resistance in the model. Hence, if one stalls a motor, the current draw and torque produced will be the supply voltage divided by the resistance.
Commonly one reads that the RPM of a motor is proportional to the voltage across it’s terminals – and for most purposes that is true: at any given voltage, the motor will spin up in speed until the generator portion of the motor model matches the supply voltage. At that point no more current will flow into the motor and it will produce zero torque. Of course, there is some amount of friction, so there will be some amount of torque required to spin the motor, thus some amount of current needed. This current causes a voltage drop across the small series resistance in our motor model. This voltage drop takes away from supply voltage and causes the motor to spin a bit slower than the supply voltage would indicate.
In industrial controllers one typically sees a variety of control methods: constant speed, where the applied voltage is "adjusted" for the IR voltage drop across the internal resistance (the controller measures the current though the motor, calculates the voltage drop across the internal resistance and bumps the supply voltage to compensate); Constant torque, which is simply a constant current supply, and the incremental encoder or tachometer feedback systems which, of course, give absolute control over position and speed.
This is from - http://www.barello.net/Papers/Motion_Control
Commonly one reads that the RPM of a motor is proportional to the voltage across it’s terminals – and for most purposes that is true: at any given voltage, the motor will spin up in speed until the generator portion of the motor model matches the supply voltage. At that point no more current will flow into the motor and it will produce zero torque. Of course, there is some amount of friction, so there will be some amount of torque required to spin the motor, thus some amount of current needed. This current causes a voltage drop across the small series resistance in our motor model. This voltage drop takes away from supply voltage and causes the motor to spin a bit slower than the supply voltage would indicate.
In industrial controllers one typically sees a variety of control methods: constant speed, where the applied voltage is "adjusted" for the IR voltage drop across the internal resistance (the controller measures the current though the motor, calculates the voltage drop across the internal resistance and bumps the supply voltage to compensate); Constant torque, which is simply a constant current supply, and the incremental encoder or tachometer feedback systems which, of course, give absolute control over position and speed.
This is from - http://www.barello.net/Papers/Motion_Control
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