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PIC Based Speed Controller

PIC Based Speed Controller

Real world applications often call for controlling small to medium sized DC motors from digital circuits. For smaller motors it is usually economically infeasible to buy a commercial speed controller as the cost of the controller will far outstrip the cost of the motor itself.
The PIC high speed, low cost, and low power requirements lend it to being an inexpensive smart chip controller for DC motors.

Background

Many modern speed controllers for DC motors use a technique called Pulse Width Modulation (PWM) to control the speed of the motor. Using a switching device such as a power transistor or enhancement mode MOSFET, power is switched on an off rapidly to the motor. The natural inductance and resistance of the motor acts as a low pass filter and makes the effective voltage seen by the motor to be the average value of the voltage over time. By varying the duty cycle (width) of this switched voltage, the effective average voltage can be lowered (narrow width pulse) or raised (wide pulse). This produces the effect of a linear change in voltage, however since the switching device is either off or in saturation it is never required to dissipate too much power internally. This increases the overall efficiency of the system as well.

The other half of the problem is how to tell the motor controller what speed you would like the motor to be set to. Any of a number of schemes would work, however the Radio Control (R/C) community has a standard for controlling servos that is particularly apt for this application. This standard uses a pulse, of variable width, repeated periodically to specify the “position” of the servo. The position variable in the servo can take on values from “full retract” to “full extension” and any position in between. Where the servo is positioned is determined by the width of this incoming pulse. In this application we will use this same technique, however rather than full retract or full extension our controller sets the speed of the motor it is controlling from full reverse to full forward.

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