Machine Design

Positioning Components

A number of positioning components are commonly employed in industrial closed-loop systems today. These components fit the general categories of controllers, mechanical positioning hardware, feedback transducers, and motors.

Outputs from closed-loop positioning systems are rotary or linear motion. At power levels under 100 hp, dc motors are generally used. However, stepping motors are sometimes operated in closed-loop mode, though they are torque limited and generally are available only up to about 3 hp. Thus, stepping motors cannot accelerate as rapidly or run as fast as some servomotors.

In addition, there is also high interest in powering ac induction motors in closed-loop mode. When used with the proper controller, induction motors can provide dynamic response equal to or better than that of brushless dc drives for applications above about 10 hp. However, the drives for these motors tend to be more complicated than for dc, often making this approach more expensive.

Many types of position transducers can be used. Common transducers include lasers, potentiometers, optical encoders, resolvers, and magnetic scales. Transducers can be functionally divided into absolute and incremental types. Incremental encoders typically generate a pulse for every increment of motion over which they travel. To gauge position, a controller counts the pulses. The most common incremental encoders are rotary optical encoders, linear optical encoders, and linear magnetic encoders. Other types also used (particularly for high resolution) include resolvers, synchros, and devices called Inductosyns.

Absolute transducers indicate position directly, but are normally quite costly. They typically gauge position by generating a discrete code for every increment of motion over which they travel. A controller then uses a translation table in memory to relate the code read from the transducer into system position. Absolute encoders are available in both rotary and linear optical types.

Tachometers are probably the most widely used velocity transducer. In addition, velocity transducers are sometimes designed with Hall-effect sensors for special applications. Finally, some systems differentiate the position feedback signal to compute velocity. Thus, a velocity transducer is not needed.

Positioning hardware and mechanical coupling in both closed-loop and open-loop systems can take a variety of forms. Older motion-control systems generally used either belt or gear couplings between the driven shaft and the motor. But modern systems now often make the drive motor and feedback encoder an integral part of the driven shaft to avoid backlash and other instabilities.

Typical positioning mechanisms include ball screws, lead screws, planetary roller screws, and simple ball/nut mechanisms. Most systems that provide linear motion or translate rotary motion into linear motion incorporate these components. A typical example is x-y table movement.

In addition, devices called linear actuators combine ball screws with other actuation components such as slip clutches and feedback control. Special positioning mechanisms such as voice coils are used in applications demanding fast response. The typical use for such hardware is high-speed hard-disk drives.

For positioning extremely heavy loads, hydraulic actuators are sometimes used. These devices, referred to as electric cylinder jacks, can move about 20 tons and may be supplied with integral positioning or speed controls.

There are both hydraulic and pneumatic actuators with built-in positioning controls as well. Hydraulic valves are available with embedded closed-loop controls. These devices are used to provide proportional control of valve position. They are also commonly embedded into linear and rotary hydraulic actuators to provide closed-loop actuator systems. Typical applications are those where loads on the order of hundreds of pounds or more must be positioned to accuracies of a few tenths of an inch. Flight simulators, for example, use these devices.

In addition, closed-loop systems comprising a pneumatic cylinder, controller, and linear position sensor are available. These devices generally handle loads of a few pounds and provide better accuracy, on the order of 0.001 to 0.0001 in. They are often alternatives to mechanical positioning equipment such as ball-screw systems.

The controllers range from single-chip microcontrollers to full-blown industrial computers. The approach depends on such factors as positioning speed and response to changes in external loading.

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