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Optimizing reciprocating motion

Reciprocating motion is found in cutting, pumping, and positioning applications everywhere. Here are some ways to improve the performance of these cycling systems.

Think of reciprocating motion, and cams those kings of kinematic linkages may come to mind. For good reason: Cams have been the traditional gold standard for reciprocating motion since the dawn of industrialization. Even today, modern varieties (designed with software for sophisticated speed and acceleration motion curves) can outpace fluid power and electrical systems on response time. In fact, when a fixed system is acceptable, cams can be the best way to maximize cycles.

Historically, reciprocal motion has been generated by rotary devices (steam or gasoline engines and electric motors) turning a crank or a cam, or indirectly by turning a shaft, explains Phil Hollingsworth, director of engineering at California Linear Devices, Carlsbad, Calif. However, as he points out: In these mechanical systems, reciprocal motion is by necessity repetitive in its motion profile, repeating once per cycle. Adjustments to the motion profile can be very time consuming and costly. And due to the inertia of such a system, speed changes can typically only be achieved over the course of several cycles.

So if a system needs to perform varying functions over its life, electronically adjusted systems are more suitable. Electronic cam output, as with mechanical cams, is a function of some measured position. However, motion is delivered by synchronized motors driven by (re-programmable) controllers an approach free of the wear and inflexibility problems of mechanical types.

With electronic servo-driven actuators, motion profiles can be varied every cycle based on inputs provided by the machine, Hollingsworth adds. Programmable motion control profiles allow stroke lengths, speed, acceleration, and dwell times to be varied for every stroke, or altered occasionally as needed very versatile.. Change-over in product can be as easy as directing the controller to use different programmed parameters.

Direct driving

In reciprocating applications that need maximum responsiveness, linear-motor actuators generate propulsion power without inertia-creating leadscrews, gears, couplings, or drive motors. The resulting low inertia, coupled with high drive forces, produces acceleration and speed for at least twice the bandwidth of alternative positioners. This means they excel in applications involving repetitive short-distance, forward-and-reverse motion. And this isn t brute-force reciprocating motion actuator load positioning is under continuous feedback control for repeatability to within a few microns, says Stan Froud for Copley Controls Corp., Canton, Mass.

Some linear motors are very responsive, agrees Hollingsworth,, possessing high bandwidth expressed as very high acceleration rates with S-curved motion profiles. This type of profile can be called on to increase machine throughput without shaking things to pieces.

These S-curve motion profiles are different than trapezoidal motion profiles, which have constant acceleration and deceleration and produce a bump on starting and stopping, says Jim Woodward,, applications manager at Copley Controls Corp. (The bump is called jerk and is the rate-of-change of acceleration.) In trapezoidal moves, the jerk is theoretically infinite because acceleration and deceleration begin instantly and remain constant. In contrast, S-curve profiles ramp up the acceleration/deceleration, controlling jerk, and producing programmably soft starting and stopping. They are great for eliminating noise, wear on machinery, and handling parts that are either fragile or which might fall off of machinery when it starts or stops too quickly, Woodward adds..

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© 2014 Penton Media Inc.

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