Today's modern controllers have the processing power needed to map servo errors at the first (acceleration) and second (jerk) derivatives of speed. So they can solve not just acceleration errors but jerk errors like those created by friction and stiction. By adding offsets to the normal command, servoamplifiers like Parker's Compax3 can correct for many sources of resonance and other repeatable disturbances before the system actually triggers the disturbance.

The base assumption used by this new breed of amplifier is the Luenberg Observer. This type of control gets results that are six times more accurate than those of standard PID controls. Proactively chasing expected errors not only reduces errors in position but also stops resonances before they happen.

For example, a motor coupling winds up as it is accelerating and then, for a short time, overhauls the load as it unwinds. These changes are noted and mapped in the autotune sequence. Once mapped by the controller the low-frequency resonance never happens, nor does the error it would introduce.

Another application concerns an inertia mismatch between a linear motor and its load. That mismatch would be unacceptable with older controllers. At best it would produce large position errors at start and stop. Now that mismatch is acceptable. Its errors are controlled by mapping what happens and by dynamically introducing corrections.

Though quite technical, this type of control is easily implemented. There would be no further tuning necessary if the motion controller and amplifier were a single unit. With separate motion controllers and amplifiers, the cascaded PID control should only have to chase minor errors. Technicians should find its tuning to be easier than that of regular PID systems.

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Parker Hannifin Corp. Automation Group,
(800) 272-7537,
parker.com/automation