100-Percent Duty-Cycle Actuator

DN Staff

July 15, 2009

3 Min Read
100-Percent Duty-Cycle Actuator

As areplacement for air cylinders, while also offering advanced linear motioncapabilities, new integrated actuators are utilizing servos to guarantee a 100-percentduty cycle, enhanced performance and reliability.

The ICR Plus SmartActuator fromTolomatic offersmachine designers a programmable, linear rod-style actuator for complexindustrial applications. The unit integrates a digital drive, servomotor andactuator into a compact package for low- to medium-force applications. Designed to compete with air cylinders andtraditional electric systems, the actuator can be built to order and ready toship in five days from factory order.

"The heart of the ICR Plus is anintegrated digital drive that provides the brains for the motor-actuatorsystem," says Aaron Dietrich, electric products manager for Tolomatic. "It provides the user with flexible tools forprogramming a wide range of complex motion profiles, along with options fornetwork communications including CANopen and DeviceNet."

Use of a servomotor, versus a stepper motor, was a designchoice to achieve a 100-percent continuous duty cycle and better performancecharacteristics. The design also usesballscrew technology rather than a solid nut or Acme screw to reduce friction. Dietrich says those solutions typically onlyprovide duty cycles in the 30- to 40-percent range.Using stepper motor technology would result in an operating duty cycle of50 percent or less, and problems generating the amount of force required forapplications operating at higher speeds.

To compete against air cylinders and stepper-based systems,the design team looked carefully at costs and "design for manufacturing." Thehigh thrust bearing that is normally part of the rod-style actuator is directlyintegrated into the servomotor, and the screw couples directly to the motorshaft. This approach reduces costs byeliminating the motor coupler and motor mount.

Two ball screw selections are available for the ICR Plus,providing up to 80 lbs of continuous force (150 lbs peak) for speeds upto 23 inch per sec, or up to 200 lbs continuous force (400 lbs peak)for speeds up to 9 inch per sec. Maximum stroke length is 24 inch.

To produce long service life, designers carefully analyzedthe RMS or average thrust of target applications to predict expected life ofthe actuator in millions of inches. Withthe five-turn-per-inch ball screw operating at 400 lbs peak thrust, theestimated life of the unit is ten million inches. As the application requirements becomesmaller than the 400-lb peak thrust, the estimated life increasesexponentially.

Dietrich says the peak thrust target was chosen to competewith two- to three-inch bore pneumatic cylinders. A three-inch bore cylinder at 80 psi cantheoretically drive a 550-lb load but pneumatics typically build a 25- to 50-percentsafety margin into their sizing calculations, which is close to the ICR designgoal of 400 lbs.

Tolomatic also offers an ICR Basic model designed withdedicated functionality to complete as a direct replacement for pneumaticcylinders. Dietrich says the ICR Basicis unique because the servo drive is pre-tuned (no motor tuning required) andpre-programmed with limited dedicated functionality for simple air cylinder applications. No programming or software interface isrequired to integrate the unit into the machine.

Three potentiometers are used to independently adjust theactuator's extend speed, retract speed, and force output for push-and-holdapplications. Acceleration anddeceleration parameters are pre-programmed to create smooth controlled motionversus the typical "bang-bang" operation of an air cylinder extending andretracting against a mechanical stop. Dietrichsays with most programmable actuators, users need a laptop computer toprogram the unit. If the air cylinder application requires verysimple out and back motions, users can implement an electric actuator solutionwith the ICR Basic and not be required to learn new software.

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