Engineering Productivity Kit Motion Control

By: 
January 04, 1999

Dual-axis linear drive adds a new twist to pick-and-place

by John Lewis, Northeast Technical Editor

Hauppauge, NY--Imagine a direct-linear drive that moves in two axes. That's essentially what Anorad Corp. came up with to simplify multi-axis stage design. Called PSM (Planar Servo Motor) technology, the design consists of a motorized forcer that glides over a platen or defined work area.

Linear motors, air bearings, and encoders--mounted on the forcer's underside--work with a magnetic array embedded in the platen's surface. Together, these devices propel and position the forcer as it glides on a cushion of air over the platen. The design eliminates mechanical guides commonly used in stacked or planar X-Y systems, and allows more than one forcer to perform coordinated motion over a single defined work area.

Although developed in a planar form, PSM technology will make its commercial debut as a rotary device called the Cyclone Z-Theta stage. "Anorad gained its fame by essentially flattening out the traditional rotary motor to create a direct linear drive," says Anorad's Founder and CEO Anwar Chitayat. "Now we've taken a two-axis, planar version of our linear motor and inverted the forcer and platen function so that the motors, encoders, and air bearings are stationary, and the magnetic array is buried within the moving forcer. Then we rolled it back up into a cylinder to achieve rotary and linear motion."

Compact stacking. Cyclone is only 31-mm wide, and has flat sides for compact "ganging" or "stacking" in high-throughput, pick-and-place applications. A hollow-shaft forcer allows centerline vision for positioning correction without offsets. It lets engineers minimize space requirements by routing control wiring and pneumatic tubing for tool sensors and pickup hardware through the forcer for a neat package.

"Cyclone acts like an air cylinder, but its design offers much higher performance," says Chitayat. Capable of 1.0-m/sec velocities in the z direction and 60 rad/sec in theta; 2.0-g acceleration in z and 600 rad/sec2 in theta; the Cyclone 50 (50-mm stroke) provides 0.15-m positioning resolution in z and 0.0007 degree resolution in theta--at a rated payload of 100 gm.

The Cyclone series is totally non-contact. Coated air-bearings resist damage in the event of air loss. "Without wearing surfaces, position performance won't degrade," says Chitayat. "The design eliminates lubrication, and reduces maintenance. Moreover, the encoder's read head averages feedback from a large patch of the scale. This means that even large scratches or surface defects in the forcer won't affect accuracy."

Vacuum counterbalance. To neutralize vacuum force so the motor doesn't have to fight it during actuation, Anorad designed a "leaky" seal or filter for the vacuum generator. "It breathes to achieve higher forces, and so that the motor doesn't have to pull extra current and generate extra heat," explains Chitayat.

The design allows the Cyclone 50 to act as a pneumatic cylinder that produces 27 lb of downward force. While Cyclone's construction gives work-cell designers another option, customization is also available to meet additional requirements.


Flash News

Analog drive company goes digital

ORMEC Systems Corp. (Rochester, NY) announces the acquisition of Westamp Inc., Chatsworth, CA. Westamp operates as the UltraDrive Division of ORMEC Systems Corp. "Combining ORMEC's DSP-based motion-control experience with Westamp's skill developing and manufacturing a wide range of servodrive products represents an exceptional platform to produce innovative, digital servodrive products for tomorrow," says ORMEC's President and CEO Gordon E. Presher. Emphasis at UltraDrive will be on DSP-based digital drives in a wide range of sizes and formats. "We will finally deliver on the performance promise of DSP technology," says UltraDrive President Bradley Landseadel. UltraDrive continues to market, support, and service products under the Westamp brand name.

Motion system goes the distance

Amtec Automations Inc., (North Billerica, MA) builds a fully automated, economical "wet etch process station" using Warner Electric's Rapidtrak(R) linear actuators, SLO-SYN(R) servo motors (with encoders) and an MX2000 controller. This three-axis motion system frees personnel time, increases safety by eliminating exposure to chemicals, and optimizes yield through accurate process control. "Normally, our stations are three- to six-ft long. For Litton Airtron, (Morris Plaines, NJ) we needed a linear actuator that could move 15-ft, and one that was quick, accurate, reliable and affordable," says Amtec's VP, Manager of Operations Audrey Marsh.

Gearmotor contributes to ergonomic breakthrough

Dumore Corp. (Mauston, WI) designs a gearmotor for Ergodyne (St. Paul, MN) that provides safety, performance, and quiet operation for its new On3TM device. The On3 helps healthcare professionals move patients laterally, without risks of back injuries. The Red Group, a Minnesota-based product development company helped Ergodyne with the design. The design required a motor and gearbox combination that was small, quiet, and could handle a higher intermittent load than published industry standard 1/8-hp gearmotors. Capitalizing on its steel cut gearbox design and gear cutting capabilities, Dumore designed a gearbox that worked with its 1/8-hp motor to meet the performance parameters.


Need to save space and cost? Try a chipset

Freemont, CA--Space and costs are critical factors in every motion control application. Sunrise Technologies International Inc. saved both by using a chipset instead of a motion card for its LTKTM laser-treatment system for farsightedness.

The company is using Performance Motion Devices' MC1401A Servo Motor Chipset for the multi-axis controller. A single card commands the 15 motors that drive the X-Y motors and beam shutter controls that orient eight laser spots simultaneously.

"PMD's chip-based approach to motion control saved us a lot of space. The MC1401A motion chipset came with sample schematics and application notes that allowed us to develop a low-cost yet powerful motion controller relatively quickly," says Satish Herekar of Sunrise Technologies.

The MC1401A is an intelligent, programmable chipset that performs complete motion sequences. It consists of a DSP for profile and PID calculations, and an ASIC to handle motor-specific functions such as PWM.

To identify the best motion control methodology, Sunrise surveyed a variety of prospective motion solutions for the project. "We also looked at purchasing an off-the-shelf motion card. However, cost was an issue and by building our own, we were able to integrate the controller and the amplifier on a single card. In many medical applications this high level of integration is important because it makes servicing the product simpler and reduces cabling cost," explains Herekar.

Key to Sunrise's application was the ability to continuously track the motion platform using smooth, s-curve profiles. "By purchasing the motion ICs off the shelf, we were able to focus on the system software rather than low-level hardware development," says Herekar.


General dual loop improves backlash compensation

Jacob Tal, President, Galil Motion Control Inc., Mountain View, CA

Backlash between the motor and sensor is a nightmare that often leads to loop instability. It exists most commonly when a rotary motor drives a slide with a leadscrew, and the sensor is placed on the load.

A common approach to backlash compensation is a dual-loop scheme using two sensors--one on the motor and another on the load. The compensation filter's PID function is divided into two parts. The load sensor closes the main loop where the PI terms are applied, and the motor encoder closes the D term of compensation. The result is a damping term based on motor position

Although this scheme works well, backlash compensation improves if the PID operation is divided with I in the outer loop and PD in the inner loop as in the general dual loop While the outer loop is unstable on its own, the inner loop is stable on its own. Motor stability depends on the relative influence of the two loops. Selecting loop parameters that make the inner loop significantly stronger allow it to dominate motor behavior.

To speak with a Galil applications engineer, call (800)377-6329.


Products to watch

Move motion off the bus

An Ethernet connection on Compumotor's new 6K motion controller offers an alternative to traditional bus-based control systems (board-level controllers) that require cumbersome and confusing cabling. The 6K includes a Wizard-based Windows 95/98 Motion Planner software package that makes Ethernet support easy. 6K supports both stepper and servo control, expansion I/O, multitasking, and enhanced position following such as virtual master, following repetitive cycle, and web registration.
Parker Hannifin Corp., Compumotor
5500 Business Park Dr.
Rohnert Park, CA 94928
(707) 584-7558. (T)

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