1、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, typic
2、ally 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 o
3、f 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 wouldnt be considered a motion control system in the strict sense of the term. Our standard motion control system consists of three basic elements:
4、 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 contro
5、l 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 speci
6、fically designed to operate with a particular motor type you cant 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 reli
7、ability 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. Step
8、per 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 eliminat
9、es 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 ste
10、p 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). Th
11、ey 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 Serie
12、s 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
13、), 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, th
14、is 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 performancethe available torque is obviously limited by the weaker step, but there will be a significant impr
15、ovement in low-speed smoothness over the full-step mode. Clearly, we would like to produce approximately equal torque on every step, and this torque should be at the level of the stronger step. We can achieve this by using a higher current level when there is only one winding energized. This does not over dissipate the motor because the manufacturers current rating assumes two phases to be energized the current rating is based on the allowable case temperature). With only one phase energized,
