Originally Published MPMN May 2006

PRODUCT UPDATE

Smart Motors Take Control

Servo drive systems offer power, positioning, and speed

Corinne Litchfield

Many medical applications place high demands on modern drive technology with regard to positioning, speed, torque stability, overload capability, and dynamic performance. Servo drive systems, which consist of a servomotor and drive, show accurate response over a wide speed range. The systems are also capable of handling overload situations. This article explores the features of these systems, as well as the latest offerings in servomotor technology. A buyers guide with a complete listing of motor and motion control suppliers can be found here.

High Power in Compact Dimensions: Servomotors

Available in both ac and dc currents, servomotors consist of a motor, gearbox, and regulating electronics. They have two inputs: an energy source for the motor and a command source, such as an encoder, to tell the motor what to do. Some servos accept speed commands and attempt to move at the requested speed, regardless of load. Other servos accept location commands and will attempt to rotate the output shaft to a particular position.

Typically, servomotors can deliver high power levels in compact sizes. The MR7 servomotor series from ESR Pollmeier (Ober-Ramstadt, Germany) covers a torque range from 0.4 to 40

N•m at flange dimensions from 40 to 188 mm. The motors are made from high-grade neodymium. Resolvers or optical encoders are used as motor position sensors. Options such as a holding brake or keyed shaft are available. The company also offers digital and analog servo drives, and multiaxis servo systems for direct mains connection to 230 V or 3 × 400–480 V.

Another servomotor offers protection against caustic cleaning agents and high-pressure washdowns. The Allen-Bradley MPS stainless-steel motor from Rockwell Automation (Milwaukee) can be easily cleaned without concerns about corrosion or motor damage. The smooth, cylindrical housing is designed to withstand 1200-psi washdowns for water and solid ingress protection. The motor is equipped with a shaft slinger, which protects the shaft seal against direct high-pressure spray and prevents abrasives from damaging the seal. It can be installed on the machine exactly where it is needed, eliminating the addition of a motor housing or parts, including chains, gears, and bearings. The MPS motor is available in frame sizes of 100, 130, and 165 mm. Peak torque ranges from 11.1 to 67.8 N•m. Factory-sealed flying leads allow users to make connections without opening the motor.

Getting in Control: Servo Drives

A servo drive supplies a motor with the exact amount of current needed to produce the desired torque and speed. It controls the current of the motor phases via a control loop. The control loop constantly checks to see whether the motor is on the right path, and will make the necessary adjustments if the motor is not working properly. While there are many types of control loops, the PID (proportional integral derivative) control loop is generally used for servomotors.

The TA330 linear three-phase servo drive from Trust Automation (San Luis Obispo, CA) can be used with several types of motors. It works with three-phase dc brushless servomotors using Hall-effect sensors, three-phase ac brushless motors using external sinusoidal commutation, single-phase dc brushed servomotors in bridged mode, and brushless linear motors.

Measuring 14.9 × 7.7 × 4.7 in., the drive can be used with high-inertia mismatched stages and low-inductance motors. It offers a current level of 150 V and 900–2700 W of power and can be configured to interface with any motion controller. High-performance systems require a motion controller that can generate the two ±10-V-dc command signals needed for sinusoidal commutation; the drive generates a third command signal to maintain high levels of precision. It can be set up to operate in trapezoidal mode, using Hall-effect sensors as feedback. The DTS control allows torque settings to be changed on the fly.

The drive offers zero cogging, no torque ripple, and smooth motion. It has very low electrical noise, making it suitable for systems that have noise-sensitive circuitry, such as transducers and sensors. Its flexibility allows motion system designers to integrate the drive into inspection and measurement systems. The unit requires a 24-V input to power logic and bias supplies at low noise levels. For applications that are not sensitive to electrical noise, an optional internal logic and bias supply is available.

Other features include thermally controlled, variable-speed forced-air cooling and a rugged housing. Connections are made using ribbon, SMB coaxial, and pluggable-terminal connectors.

A secondary controller is not necessary with a programmable, four-quadrant servo drive. The DSV405 motion control servo drive from EADmotors (Dover, NH) uses constant-current bipolar chopping with full on-board programmability for brushless dc motor applications. Rated at 12–40-V dc input and 5-A continuous output, the drive can operate in position, velocity, or torque modes. Eight input and output ports allow the drive to receive and respond to signals from switch closures and openings along with optocoupler outputs. This allows the controller to start and stop a motor based on switch closures or interruption of an optocoupler beam.

The drive’s programming capability eliminates the need for a secondary controller. The easy-to-use programming language allows users to create and store motion profiles, including acceleration, deceleration, hold and run current, position targets, and slew velocity. Other simple commands let the user respond to input signals from the external system and drive secondary devices, such as relays or solenoids, based on the needs of the system.

Programming is performed in a HyperTerminal environment or via a downloadable Windows GUI. Medical applications include chemical test stations and dispensing equipment.

Moving Forward: Linear Servo Slides

Servomotors can be used to power actuators. The linear ServoSlide from Intellidrives Inc. (Philadelphia) is a preengineered direct-drive linear actuator. A linear, brushless servomotor, designed to eliminate backlash, friction, and wear problems, is used to power the slide. It features low cogging for accurate positioning.

A double-rail bearing system supports the moving carriage and provides dynamic stiffness and straight travel. To decrease thermal drift, the carriage contains a noncontact linear position feedback. Resolution levels from 0.1 to 10 µm can be selected. Limit sensors are used to establish end-of-travel and “home” positions.

Slide components are mounted in an extruded aluminum housing. The housing is machined to enable straight, flat installation. It includes a panel to provide plug-in connectivity and quick disconnect for all signal and power requirements. A cable transport module with high-flex robotic cable is also installed and prewired to the connector panel. The servo slide offers force rates of 100–2000 N with a stroke of 100 mm to 5 m. Accuracy is 5–25 µm and repeatability is 1 µm. Medical applications for the servo slide include fluid-handling and dispensing systems, microarray assembly, and prosthetic manufacturing.

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