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A pause for pulses: a servoamplifier slows the transition rate of motor drive pulses and helps curb EMI/RFI radiation. (Design ideas: motion control).

EMI/RFI radiation is caused by the higher harmonics of a motor drive waveform. The amplitude of higher harmonics increases with the steepness of the drive pulse. Consequently, reducing wave form steepness can minimize radiation at interference frequencies.

The Model 7225ACF servoamplifier is designed to do just that. Metal cased and shielded to meet international EMI/RFI specifications, the device has cables that feed pulsed drive power to a servomotor in order to radiate interference energy, which may couple into adjacent circuits. The unit has an edge filter that curbs EMI/RFI radiation by slowing the transition rate of drive pulses. In addition, sinusoidal commutation eliminates cogging and maximizes motor positioning accuracy.

The servoamplifier's edge filter transforms its output from pulses with nanosecond rise times to pulses with rise times measured in microseconds, which reduces the steepness of the drive pulse. The edge filter is based on inductors connected in series with the amplifier's output. The inductors work in conjunction with a resistor/capacitor filter network and active circuitry to limit waveform peaks.

Meanwhile, the device delivers power in the form of a 20 kHz pulse train at constant voltage. Power output is controlled by adjusting pulse duration, which alters the effective load current. The servoamplifier energizes motor drive coils in a pattern, which sets up a powerful traveling magnetic field. This force provides motor acceleration and drive.

Most brushless linear motors are typically built around sets of U, V, and W magnetic coils. The motors are activated by energizing these coils in a special sequence, referred to as motor commutation. Model 7225ACF, however, provides sinusoidal motor commutation, which powers a motor's U, V, and W coils according to a full 360-degree commutation cycle based on a specific pattern.

Sinusoidal commutation achieves constant drive force free of ripple or cogging. The key is amplified linearity. Force variations are minimized by the unit's linearity and sinusoidal commutation. Force variations can induce vibrations in machinery, apply cyclically varying pressure to cutting tools, and produce uneven tension in spooling applications.

Model 7225ACF has 10 A of continuous output and drives ac brushless linear servomotors. Electrical safety is ensured by opto-isolators that provide a non-conducting link between the ground-based control and feedback circuits.

More information is available by contacting Copley Controls Corp., 410 University Ave., Westwood, MA 02090, calling (781) 329-8200, writing in 62 on our reader service card, or replying online at

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Publication:Product Design & Development
Article Type:Brief Article
Geographic Code:1USA
Date:Jul 1, 2001
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