issue: February 2008 APPLIANCE Magazine
Medical Drives in Compliance
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Servo drives takes the burden for FCC Class B compliance off medical device design engineers.
Advanced Motion Controls (Camarillo, CA, U.S.) created new, cost-effective design strategies for servo drives to meet or exceed medical equipment standards for stand-alone FCC (Federal Communications Commission) Class B compliance.
Drives today are pulse-width modulation (PWM) switching power devices made to switch current on and off at relatively moderate frequencies at or above 20 kHz. These transitions create noise signatures at much higher frequencies, typically tens of MHz to 1 GHz, and other sensitive medical devices nearby can be adversely affected. Therefore, when used in medical equipment, servos are also required to be low-noise devices. This can be a major concern for medical device designers.
“Engineers now do not have to concern themselves too much with other system components for specialized grounding, shielding, or encasing to meet these rigorous standards,” says Karl A. Meier, an engineer and marketing manager for Advanced Motion Controls. The supplier’s servo drives can be built to these standards using design techniques such as compact PCB-integrated flat-wire inductors for motor outputs and power inputs.
“These are inductive components that can be typically large and expensive but ‘choke’ the rate of change of current,” explains Meier. “They can be externally mounted and incorporated on or within the wiring. However, PCB-mounted units are placed on the circuit board itself and are completely contained within the housing of the unit. This, in turn, greatly reduces noise coming from or going to the power section of the servo drive and simplifies integration.”
Another technique is common-mode filtering on logic supply input, which also filters out higher-frequency signals by canceling noise common to both input terminals. “This applies to both the incoming and any possibly of outgoing signals, even though the connections are meant for simple dc input voltage supply for logic circuitry,” explains Meier.
PCB inter- and intralayer layout techniques are used, and ground planes act as shields as well as grounding.
Applications could extend well beyond medical equipment. “In fact, any equipment could have these requirements placed upon them if they are to be operated within the confines of sensitive instrumentation or devices,” says Meier.