Motion control applications generally are considered a specialty subset of controls engineering, and really do require a different way of thinking. Controls engineers could be experts in general machine control, but they generally lack the knowledge and experience to design and implement a robust, motion-control system.
It's imperative that the proper components are chosen for a motion control project, and the selection really has to start with the mechanical side of the project. The mechanical components and their motion requirements determine what other components are selected.
I've learned to break this process down to some basic decision-making segments:
1. Is the required motion linear or rotary? If final motion moves something linearly, what is the load that's moving and at what speed? The mechanism that turns a motor's rotary motion into linear motion would typically be an acme or ball screw or maybe a belt drive. If the motion required exceeds the capabilities of a screw or belt system, then perhaps a linear motor is required. Given the load and speed requirements, motor speed and torque requirements can be determined.
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[pullquote]Does the final motion need to be rotary? If it needs to be an "endless" axis that moves in one direction for an infinite distance, then you'll need to consider encoder rollover. Don't ignore rotary inertia, as this will determine motor selection. A motor might meet the torque and speed requirements, but it could have too low a rotary moment of inertia, and your motor will not be controllable. Even with a linear motion system, inertias need to be determined.
2. Do you need a stepper, servo or induction motor? A stepper motor is the least costly option. Stepper motors don't have to use an encoder, and that means less cost for the motor and control system, but they have some serious limitations. The biggest problem is that stepper motors develop their peak torque at zero speed. As soon as a stepper motor starts moving, its torque capabilities drop off. Running the motor at higher voltages helps this situation, but no matter what, torque drops off with speed. In general, don't use stepper motors for applications that require more than 1,000 rpm. Most reputable stepper motor suppliers will have torque/speed curves available to help you make your selection.
A servomotor typically is a permanent magnet motor capable of high speed and/or high torque. It has a resolver or encoder to provide position feedback to the control system. The drive (amplifier) also uses a feedback device to determine motor rotor position for commutation (current timing of the three phases). Unlike a stepper, a servomotor can produce full torque at zero speed and rated speed.
Historically, induction (standard ac) motors haven't been used in motion-control applications. That changed with the advent of advanced drives. With an encoder, induction motors can be used in motion-control applications. Servomotors still will give higher performance, but in many higher horsepower applications, a properly rated induction motor can be used instead of a servomotor at a much lower cost.
3. Do you need a dumb drive or a smart drive? In the old days, a drive was nothing more than an amplifier. It took a current (sometimes a velocity) command from a motion controller, and controlled the current (or velocity) of the servomotor. Nowadays, drives are smart. You can still successfully use a dumb drive, but there are other choices. If your application just needs to move a single axis to preset positions, you can use an indexing drive. All motor (and motion) control is handled by the drive. Positions and speeds can be changed via some form of communications if necessary. If you only have a single axis, but still need higher-level programming, you can get a drive with a full-blown motion controller built in.
If you want a networked motion system, the drive takes motion profile commands from a motion controller over a network. The motion controller coordinates all motion profiles (of multiple axes also), and the drive provides current, velocity and position control for the motor. Most of these facts about servodrives apply to induction motor drives, but choices are more limited.
The same is basically true for stepper drives, except that dumb stepper drives take either step and direction pulses or sometimes an analog velocity command. You also can get an indexing stepper drive, or a stepper drive with an onboard motion controller. Networked stepper drives are available, but theyre not as popular.
I'm out of space, so I'll cover the remaining decision-making elements here next month.
Brian Beal is president of Highland Controls, an integrator and distributor of industrial motion control systems and related products.
