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Controlling a Small DC motor
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For our immediate purposes, let s define a small DC motor as one that s about an inch or two in diameter and two to four inches long. We will use ones that run on about 12 volts and draw a couple of amps. The amperage and voltage values have to match the capacity of the amplifiers we have chosen for running the motors. The two-axis Xavien amplifier needs over 12 volts to operate properly and will handle 3 amps continuously and 6 amps for short bursts at up to 55 VDC for two separate motors or coils. This amplifier is used for all the experiments in the book. Like all DC motors, the small motors shown in Figure 25-1 provide high speed and low torque. They provide no feedback in regard to the distance traveled (revolutions
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Figure 25-1 Small electric motors suitable for our experiments
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note These are examples of small DC motors of the type under discussion. Motors with shafts on both ends allow us to mount encoders on them for our later experiments.
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completed) or the speed of the motor. (Under certain conditions, the back EMF generated by a motor can be used as speed feedback, but we will not use this in our experiments. This use is more common with analog control schemes.) On almost all DC motors, we can control the following parameters:
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On/off control. Control of power to the motor. Polarity of the power provided (direction of movement). Minimum power delivered at starting set point (power needed to start motor). Maximum power delivered when running as a set point. (The max RPM control depends on the load.)
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Essentially, we can have comprehensive control of both the speed and the direction of these motors. Let s design a system that will give us this control of the motor, from a potentiometer and the Propeller chip/LCD system we have been using. We will design the system so that the middle position of the potentiometer will be the zero speed position, and as we turn the potentiometer in either direction, the motor will run in the selected direction. Turning the potentiometer all the way in either direction will give us full speed in the selected direction. We will provide variables in the software to control both the minimum and the maximum speed of the motor.
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Potentiometer note In order to use a potentiometer, we need to read the position of the potentiometer wiper to get a value we can input into our control scheme. We will be using a 12-bit value for the potentiometer, so the reading will go from 0 to 4,095. We will select 2,048 as the point that gives us zero power to the motor.
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The outputs to the motor amplifier/driver will be as follows:
An enable bit (brake=0) A PWM value (0 to 4,095) A direction bit (0 or 1)
These three values are sent to the motor amplifier/driver over the three control wires. Exactly how this is managed in the driver is a function of the motor driver we use, but most drivers do have the following three control wires for each motor:
Enable/inhibit bit enables the driver (the Brake bit) PWM input for speed Direction bit
We can extract the direction and PWM by interpreting the 0 to 4,095 value of the potentiometer as follows (set the direction bit as detailed here):
If the value is below 2,048, set the direction to negative. Set the direction bit to 0. If the value is 2,048 or above, set the direction to positive. Set direction bit to 1.
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Set the speed so it will always be between 0 and 4,095:
If the value is 2,048, set the PWM value to 0. If the value is above 2,048, set the PWM value to (Pot value-2048)*2. If the value is below 2,048, set the PWM value to (2048-Pot value)*2.
We decided earlier that we will use the larger two-axis amplifier made by Xavien. This is a very easy-to-use and fairly powerful amplifier that can handle up to 6 amps (maximum) at 55 volts (maximum) for short bursts. It accepts the three signals we need to control the motor without modification. Having two independent amplifiers on this device is a useful convenience that will allow us to use this amplifier to run our stepper motors in the various stepper experiments later in this book. The frequency that the industry uses for the PWM signal is selected so that it is above the hearing range of humans and domestic animals. The noise is caused by loose laminations and other magnetically sensitive components in the motor. High square wave frequencies are extremely irritating to the human ear and are to be avoided. As far as the control of the motors goes, 60 Hz is completely useable (but higher frequencies do work better). We will use a frequency considerably below 20,000 Hz for our frequency because these little motors do not have a lot that will start vibrating in them at our power levels. However, do keep this in mind when you need to run a larger motor. The software we are using could easily operate at higher frequencies.
animal Hearing range Most industrial amplifiers run at 40,000 Hz to keep the noise that may be generated above the hearing range of domestic animals as well.
The circuitry needed to run our motor is shown in Figure 25-2. This circuitry reflects what needs to be wired where to run the motor with the Xavien amplifier and a Propeller chip with an attached LCD. For power, we need two wall transformers. One to provide 7.5 to 9 volts at one amp for the Propeller power supply and one to provide between 13 and 50 volts at about one amp for the motor power supply at the amplifier. The 7.5-volt power supply provided by Parallax for the education kit is adequate for the Propeller as used in this experiment. All the power supplies we use must be wired so that their positive terminals are at the center and the negative terminal is on the periphery of the connector. Also, 2.1mm-diameter connectors will be used throughout. Using this standard arrangement will keep us from getting our power connections made incorrectly and thus damaging the electronics. The connections to the Propeller will be the standard education kit layout that we always use, with the following added:
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