14-02-2013, 01:53 PM
Speed Controllers
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Introduction
The purpose of a motor speed controller is to take a signal representing the demanded speed, and to drive a motor at that speed. The controller may or may not actually measure the speed of the motor. If it does, it is called a Feedback Speed Controller or Closed Loop Speed Controller, if not it is called an Open Loop Speed Controller. Feedback speed control is better, but more complicated, and may not be required for a simple robot design.
Motors come in a variety of forms, and the speed controller's motor drive output will be different dependent on these forms. The speed controller presented here is designed to drive a simple cheap starter motor from a car, which can be purchased from any scrap yard. These motors are generally series wound, which means to reverse them, they must be altered slightly, (see the section on motors).
Below is a simple block diagram of the speed controller. We'll go through the important parts block by block in detail.
Theory of DC motor speed control
The speed of a DC motor is directly proportional to the supply voltage, so if we reduce the supply voltage from 12 Volts to 6 Volts, the motor will run at half the speed. How can this be achieved when the battery is fixed at 12 Volts?
The speed controller works by varying the average voltage sent to the motor. It could do this by simply adjusting the voltage sent to the motor, but this is quite inefficient to do. A better way is to switch the motor's supply on and off very quickly. If the switching is fast enough, the motor doesn't notice it, it only notices the average effect.
When you watch a film in the cinema, or the television, what you are actually seeing is a series of fixed pictures, which change rapidly enough that your eyes just see the average effect - movement. Your brain fills in the gaps to give an average effect.
Now imagine a light bulb with a switch. When you close the switch, the bulb goes on and is at full brightness, say 100 Watts. When you open the switch it goes off (0 Watts). Now if you close the switch for a fraction of a second, then open it for the same amount of time, the filament won't have time to cool down and heat up, and you will just get an average glow of 50 Watts. This is how lamp dimmers work, and the same principle is used by speed controllers to drive a motor. When the switch is closed, the motor sees 12 Volts, and when it is open it sees 0 Volts. If the switch is open for the same amount of time as it is closed, the motor will see an average of 6 Volts, and will run more slowly accordingly.
As the amount of time that the voltage is on increases compared with the amount of time that it is off, the average speed of the motor increases.
This on-off switching is performed by power MOSFETs. A MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) is a device that can turn very large currents on and off under the control of a low signal level voltage. For more detailed information, see the dedicated chapter on MOSFETs)
The time that it takes a motor to speed up and slow down under switching conditions is dependant on the inertia of the rotor (basically how heavy it is), and how much friction and load torque there is.
Choosing a frequency based on motor characteristics
One way to choose a suitable frequency is to say, for example, that we want the current waveform to be stable to within ‘p’ percent. Then we can work out mathematically the minimum frequency to attain this goal. This section is a bit mathematical so you may wish to miss it out and just use the final equation.
The following shows the equivalent circuit of the motor, and the current waveform as the PWM signal switches on and off. This shows the worst case, at 50:50 PWM ratio, and the current rise is shown for a stationary or stalled motor, which is also worst case.
Speed control circuits
We will start off with a very simple circuit (see the figure below). The inductance of the field windings and the armature windings have been lumped together and called La. The resistance of the windings and brushes is not important to this discussion, and so has not been drawn.
Q1 is the MOSFET. When Q1 is on, current flows through the field and armature windings, and the motor rotates. When Q1 is turned off , the current through an inductor cannot immediately turn off, and so the inductor voltage drives a diminishing current in the same direction, which will now flow through the armature, and back through D1 as shown by the red arrow in the figure below. If D1 wasn’t in place, a very large voltage would build up across Q1 and blow it up.
Regeneration
In this circuit, energy can flow only one way, from the battery to the motor. When the speed demand of the motor drops suddenly, the momentum of the robot will drive the motor forwards, and the motor will act as a generator. In the circuit above, this power cannot go anywhere. Although this isn’t a problem, it is desirable that this power be put back into the battery. This is called regenerative braking and needs some extra components.