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SELECTING THE MOTOR THAT SUITS YOUR APPLICATION
Motion control, in its widest sense, could relate to anything from a welding robot to the hydraulic system in a mobile crane. In the field of Electronic Motion Control, we are primarily concerned with systems falling within a limited power range, typically up to about 10HP (7KW), and requiring precision in one or more aspects. This may involve accurate control of distance or speed, very often both and sometimes other parameters such as torque or acceleration rate. In the case of the two examples given, the welding robot requires precise control of both speed and distance; the crane hydraulic system uses the driver as the feedback system so its accuracy varies with the skill of the operator. This wouldn’t be considered a motion control system in the strict sense of the term. Our standard motion control system consists of three basic elements:
Fig. 1 Elements of motion control system
The motor，This may be a stepper motor (either rotary or linear), a DC brush motor or a brushless servo motor. The motor needs to be fitted with some kind of feedback device unless it is a stepper motor.
Fig. 2 shows a system complete with feedback to control motor speed. Such a system is known as a closed-loop velocity servo system.
Fig. 2 Typical closed loop (velocity) servo system
The drive，this is an electronic power amplifier that delivers the power to operate the motor in response to low-level control signals. In general, the drive will be specifically designed to operate with a particular motor type – you can’t use a stepper drive to operate a DC brush motor, for instance.
Application Areas of Motor Types
Stepper Motors
Stepper Motor Benefits
Stepper motors have the following benefits:
• Low cost
• Ruggedness
• Simplicity in construction
• High reliability
• No maintenance
• Wide acceptance
• No tweaking to stabilize
• No feedback components are needed
• They work in just about any environment
• Inherently more failsafe than servo motors.
There is virtually no conceivable failure within the stepper drive module that could cause the motor to run away. Stepper motors are simple to drive and control in an open-loop configuration. They only require four leads. They provide excellent torque at low speeds, up to 5 times the continuous torque of a brush motor of the same frame size or double the torque of the equivalent brushless motor. This often eliminates the need for a gearbox. A stepper-driven-system is inherently stiff, with known limits to the dynamic position error.
Stepper Motor Disadvantages
Stepper motors have the following disadvantages:
• Resonance effects and relatively long settling times
• Rough performance at low speed unless a micro step drive is used
• Liability to undetected position loss as a result of operating open-loop
• They consume current regardless of load conditions and therefore tend to run hot
• Losses at speed are relatively high and can cause excessive heating, and they are frequently noisy (especially at high speeds).
• They can exhibit lag-lead oscillation, which is difficult to damp. There is a limit to their available size, and positioning accuracy relies on the mechanics (e.g., ball screw accuracy). Many of these drawbacks can be overcome by the use of a closed-loop control scheme. Note: The Comp motor Zeta Series minimizes or reduces many of these different stepper motor disadvantages. There are three main stepper motor types:
• Permanent Magnet (P.M.) Motors
• Variable Reluctance (V.R.) Motors
• Hybrid Motors
When the motor is driven in its full-step mode, energizing two windings or “phases” at a time (see Fig. 3), the torque available on each step will be the same (subject to very small variations in the motor and drive characteristics). In the half-step mode, we are alternately
energizing two phases and then only one as shown in Fig. 4. Assuming the drive delivers the same winding current in each case, this will cause greater torque to be produced when there are two windings energized. In other words, alternate steps will be strong and weak. This does not represent a major deterrent to motor performance—the available torque is obviously limited by the weaker step, but there will be a significant improvement in low-speed smoothness over the full-step mode.
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