Basics

Motion control is a broad term that can be simply defined as the precise control of anything that moves. Motion control on the PC is the new standard for intelligent motion control integration. The combination of advanced performance, real-time, embedded CPU-based controllers, PCI bus throughput, and Windows graphical programming make PC-based motion the best choice for powerful, easy-to-use solutions.

Regardless of the motion application, there must be a device to provide brute motive force, and a device to control the motion. In this "brawn and brain" combination, a motor provides the brawn, and a motion or motor controller provides the intelligence. The power drive translates the low-power control signal to a higher power form appropriate for driving a motor.

The Brawn - Motors and Power Drives
There are many types of electric motors, each with its own power drive requirements. Three common types are alternating current (AC) servo motors, direct current (DC) servo motors, and stepper motors.

AC servo motors operate from alternating current (AC) power sources. These motors can be inexpensive to build and operate, are reliable, and usually operate at standard line voltages and frequencies. Since the speed of an AC motor is difficult to control, AC motors are most often used for simple motion rather than for sophisticated motion control applications.

DC servo motors operate from a direct current power source and are suited for complex motion tasks because the speed and torque (twisting force) in DC motors are controlled by varying the voltage and current.

Stepper motors produce motion in discrete steps. These motors have a specified number of steps per revolution, but more advanced stepper drives can provide microstepping - additional stops between the normal step locations. Microstepping greatly increases the resolution of the stepper motor.

The Brains - Controllers
Most modern motion controllers use the power of the microprocessor to generate and execute complex motion profiles. These controllers are usually programmable, and programming is accomplished with hardware switch settings or keypads on the control board inside the PC or through some external computer with a communication link.

With programmable motion controllers, the user can input a motion profile and specify motor acceleration, maximum velocity, deceleration, and ending position for many successive moves. Controllers often provide the ability to add delays between moves, to wait for inputs from external digital signals, or to set digital outputs based on the motor's position.

A controller has four fundamental tasks: plan what to do, start doing it, monitor what is happening in the real world, and correct for errors. Controllers that perform all four tasks are known as closed-loop or servo controllers. Open loop controllers perform only the first two functions and are used when the system parameters are well known, or where precise control is not critical.

An encoder monitors the real-world status of the motion. Encoders mount to the motor shaft or the load, and provide digital pulses as the shaft or load moves. Encoders are specified by lines per revolution or counts per revolution. Velocity is determined by measuring the frequency of the incoming pulses, and position is determined by counting the number of pulses that have occurred.

The controller takes information from the encoder and compares this value to the calculated value. If the values are the same, the controller simply outputs the next planned point. However, if there is error, the controller compensates by adding to or subtracting from the output signal.

Software
Sophisticated controllers heavily depend on software, not only for the low-level control calculations, but also for high-level user interaction. Most motion control companies go to great lengths to provide user-friendly access to the advanced functionality of their boards. One popular way to accomplish this is through a graphical programming package such as LabVIEWä from National Instruments. LabVIEW virtual instruments (VIs) for motion control are available from most PC-based motion control boards and significantly reduce the learning curve and development time associated with motion applications.

Motion Control Example
For example, to locate a star using a motion control system, you need to move a telescope, with two degrees of freedom - right ascension and declination. You must coordinate these functions to accurately find and track an object in the sky.

First, you retrieve coordinates on the star from a database. The software then translates the coordinates into motion coordinates and maps the target point. Then, the software downloads the motion coordinates to the controller on the PC. The motion controller now knows the location of the star, how far to move the motor, and the safe speed to move without damaging the telescope. Next, the control card sends a command signal to the power drive. The power drive then moves the motor. The encoder measures the location of the motor, and the controller compares the encoder signal with the calculated trajectory and provides a corrected signal. As a result, the controller removes any error, and the telescope accurately locates the star.

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