barcode reader sdk vb.net Figure 28-1 Small DC electric motors with encoders in Software

Generation Quick Response Code in Software Figure 28-1 Small DC electric motors with encoders

Figure 28-1 Small DC electric motors with encoders
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Running DC MotoRs with AttACheD inCReMentAl enCoDeRs
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Not about Motors
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This is the most sophisticated program development in the book, and you need to understand up front that it is not really about running motors. It is about understanding any control system with some form of feedback, a very important skill to develop. In this case, the feedback is coming from an optical encoder, but it could be anything. Even though we are controlling a motor, it could be any control point of interest to us. A motor has been picked because this is a system that is easy to purchase and put into operation, and you can see what is going on without any other instruments. It is a fast system in that we can see the results of what we are doing almost immediately. Having said that, we can begin work on our encoded motor, but keep in mind this chapter is about closed-loop feedback systems, not encoded motors! Because all this is fairly complicated for beginners, we will create a series of programs that allow us to creep up on the solution. The relevant programs are developed over the course of this chapter. The programs functions are listed here:
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Hold a motor at its present position; fixed gain to return to position. Hold a motor at its present position; fixed and proportional gain to return to position. Hold a motor at its present position; fixed, proportional, and integrating gain to
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return to position. Motor position controlled by a potentiometer. Motor speed controlled by a potentiometer. Motor speed and direction controlled by a potentiometer. Motor goes back and forth; potentiometers control gain and distance moved. CASE statement used to control gain. Determine gain versus speed table for one motor. Program segment to determine gain with CASE statement. Rudimentary velocity path specification. Slow motion, fast motion, slow motion, stop. Repeat. Ramp up and down slowly. Constant rate. Repeat. Ramp up, run at constant speed, ramp down, and stop. Repeat. Turn a DC motor into an R/C servo. Motor speed controlled from an R/C servo signal.
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Each program is listed in its entirety because it is very difficult to try to understand a program while looking for program segments here and there at the same time. It is hard enough to keep everything sorted out in your mind as it is.
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Discussion
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Adding an optical encoder to a motor allows us to determine how fast the motor is spinning, the direction in which it is spinning, and how far it has spun. With this scheme we can control how far the motor moves, how fast it gets to its target location, and what velocity profile it follows on its way to its destination. This essentially is
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what we define as comprehensive motor control. This is the type of control we need for sophisticated robotic and computer numerically controlled (CNC) machine applications. This is what we need to run a fast, pen-based plotter or, for that matter, any device that needs to coordinate the movement of more than one motor for its competent operation. All multidimensional CNC machines fit this definition (pen plotters, lathes, mills, laser cutters profilers, and many others). With encoded motors, the first question we are invariably asked is, How are you going to keep track of the encoder (Every one understands that this is a very timeintensive undertaking, so this is an easy question to ask. The hard part is the answer!) The question is intended to point to the fact that the hard part of running a motor with an attached encoder is reading the encoder constantly and still having time to actually run the motor. It takes up a lot of processor time in a single-processor system. A popular solution is to use a separate chip to read the encoder or to use a processor with built-in hardware to read the encoder. Our solution will be to dedicate one of the cogs to keep track of the encoder and provide us with the current encoder reading whenever we need it. This is equivalent to using a separate chip or processor, only much better because it s all internal to the Propeller chip we are using. We will not need any external wiring to get the job done (other than connecting the encoder up to the Propeller). The usual scheme used to control the DC motor is called a PID loop. The letters P, I, and D represent the three components of the feedback loop that control the motor. A constant, K, is needed to take care of friction components. In layman s terms, these items are defined as follows:
P represents the proportional part of the loop. I represents the integrating function in the loop. D represents the derivative part of the feedback equation. K (when used) represents the overall system friction.
For most DC motors, the speed under constant load is proportional to the power sent to the motor. This is the proportional part of the gain equation P. If you are running a motor under a variable load, the speed the motor attains will be approximately proportional to the load on the motor. If the load on the motor is increased (and we want to maintain the same speed), we have to add more power to the system to keep the motor speed constant. This is done by adding a little power at a time (again and again in the control loop) until we get to the desired speed (and/or the desired position in time). This is the integrating component of the equation I. If we are interested in maintaining the speed of the motor within very tight limits and we want the motor to be where it is expected to be in time, we need to make a calculation about how fast the speed and position of the motor are changing and add an appropriate power component to the motor to keep it within the desired limits. If the motor speed is falling off sharply or its position is falling behind in time, we have to add the power right away instead of integrating it in a little bit at a time, as we were doing with the integrating component. Another way to do this it to calculate where the motor is supposed to be at any one time and make a gain correction based on how great the positional error is. This is how we will do it in our parallel environment. This part of the need for power adjustment is the derivative component of the equation D.
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