Machine Design

Sizing up Electric Band Cylinders

Electrically powered linear-motion-control systems scan and track paint sprayers, perform tool drilling, clamp and brace components in assemblies, and exercise a variety of pick-andplace operations. They are usually subsystems in larger, modular automation systems that satisfy a wide range of motion needs.

Rodless cylinders are one kind of linear actuator that provide the mounting platform in these electrically powered systems. They are often selected because they can handle relatively large loads and bending moments produced by off-center loads, and they provide easily controllable and cost-effective linear-motion systems.

When planning an electric motion system, the size of the actuator and its load and speed are typically specified along with motor, driver, and controller. The performance of a linearcontrol system depends on the type of motor, drive, and controller used with the actuator. The motor provides the torque and speed necessary for the actuator to meet the requirements, and the drive converts the local power source (typically 115 Vac, 60 Hz) to the power required by the motor. The power ratings for motors and drives must match the peak and the rms requirements of the application, and the controller converts inputs from an operator, supervisory system such as a PLC, or a computer to control the motor and generate the motion profiles.

The procedure to size and select the cylinder is relatively easy. First, list the requirements for stroke length, load weight, load speed, thrust force, resolution repeatability, actuator orientation, moment loads, and duty cycle. Determine whether a screw-type actuator, belt drive, or other type is best suited for the application. Maximum operating speed of screw actuators is limited by the critical speed of the screw. Beltdrive actuators are used where speed and stroke requirements exceed the screw’s critical speed. Screw actuators provide finer resolution and better repeatability than belt-drive units and are preferred for short-stroke, high-move repetitions.

To select a motor for the actuator, compare the qualities for dc, stepper, servomotor, and drive for meeting the requirements of the system. First, openloop dc motors and drives are a lowcost and mature technology for straightforward applications. Acceleration and deceleration are easily controlled, and dynamic-braking is usual for higher inertia loads. End-of-stroke switches provide simple position control. However, dc drives do not generate holding torque at standstill.

Stepper motors, by comparison, are compact and provide moderate-cost speed control with high-resolution positioning and excellent repeatability. They provide holding torque at standstill and are best applied where loads are predictable and unchanging. Endof- stroke limit switches and a home position switch are usually necessary. Multiple move sequences are programmable and selectable.

Finally, servomotors offer the highest performance, but typically cost more. For linear-actuator applications, most servos are dc brushless. High-positioning accuracy with a servosystem is attained through a closed-loop feedback system using either a linear or rotary encoder. Finding optimum performance at optimum cost with a stable servosystem requires tuning and selecting the feedback parameters to match the load dynamics. Servosystems are totally programmable and are often selected to provide curvilinear motion or interpolation for multiaxis systems.

This information was supplied by Tol- O-Matic, Hamel, Minn. For more information on applying actuators

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