Motion Control Components Lend Hand to Biomedical Applications
- Tuesday, 01 April 2008
The need to be patient-friendly requires that motors start and stop smoothly. Chris Harman, Vertical Market Manager of Life Sciences for Omron Electronics LLC, noted that many lab automation applications have a relatively low number of axes and utilize point-to-point motion at relatively slow speeds. This suits them to stepper-type motors.
Chris Moskaites, an application engineer for Oriental Motor, added that microstepping motors, which can stop and hold a position between the full or half-step positions, eliminate the jerky character of low speed stepping motor operation and the noise at intermediate speeds. Oriental offers its RK Series, which combines the high resolution and smooth motion of a five-phase microstepping system with the simplicity of a full stepper. “In general, we need a higher resolution for medical applications, with smoother motion and more stopping,” Moskaites said. “When handling a sample, we don’t want an abrupt stop or spill.”
Because of the specialized nature of most biomedical applications, motor and control companies are called upon to customize the application. “We’ve worked with a customer on an evacuator pump for ambulances for first responders to clear an individual’s airway,” said Mike Lefevbre, Brush Motor Engineering Manager for AMETEK Technical & Industrial Products “Because of the vibratory nature of the pump and potential field abuse, we have to do significant modifications. We potted the armature windings where they are fused to the tank or commutator. We came up with a special arrangement to secure the bearing system to the shaft.”
To maximize the motor’s effectiveness, biomedical applications require encoder feedback. High-resolution absolute encoders provide the fine positioning control required for biomedical applications, such as positioning a bed in a magnetic resonance imaging system, noted Allen Chasey, marketing manager of encoder supplier Dynapar. “A high clock rate (up to 10 MHz) helps with low speed smooth control,” he added.
The ability to maintain a low level of conducted noise and radiated emissions also enters the equation. “We can install internal capacitors in the motor, or install ferrites in the leadwire assembly,” said Ametek’s Lefevbre. “The requirements are often unique to the application, dependent on operating frequency.”
Photoelectric sensors also can aid precision motion control, according to Steve Wong, business development manager for Banner Engineering. “A lot of times, our sensors trigger alignment or create an output signal to activate the motion control.”
Wong cites the example of filling tablet bottles. “For tablet counting, we have a photoelectric sensor that counts tablets moving at 20ms from the leading edge of one tablet to the next edge. The product is moving at a very high speed. We want to ensure the amount is correct. If it is wrong, the sensor activates the motion control device to reject the bottle.”
The ability to generate high amounts of torque – often from a smaller package – is increasingly required for biomedical uses. Maxon, for instance, developed a 6-mm motor to fit inside a tiny tube for a microprocessor-controlled liquid dispensing application. “Most motors cannot achieve high torque in this size range,” said Maxon’s Martino.
Likewise, laboratory instruments pose physical constraints to integrating motion components. “Lab instrumentation space is at a premium,” said Allmotion’s Goodin. “One factor is that the equipment cannot be more than 19 in. high.”
Maxon’s Martino added, “It is very difficult to get the Hall sensors on the pc board with the technologies we’re employing, given the tight sizes and tolerances required.”
Cramming a hot running motor into a small space increases the potential for significant heat dissipation, according to Lefevbre. “High-grade laminations can be used to minimize losses. One also needs to heat sink the device properly — any heat must be insulated from the person handling the biomedical device.”
Besides heat, there’s also the lingering concern about handling contaminants. Although motors can be sterilized, oftentimes they are not involved in materials handling. “Biomedical applications typically use our stainless steel products,” said Lori Colivas, marketing manager for Bishop-Wisecarver Corp., which supplies motion component guides and actuators. The company supplies an actuator, called LoPro, which is used in blood analyzing equipment. As it has no contact with the slides or blood, the main requirement is stainless steel construction for good wear resistance and long life, according to Colivas.
In most cases, motor components are qualified as part of the end application, leaving the onus of approval to the medical OEM. But control software must comply with medical agency approvals, according to T.J. Tanzillo, Biomedical Segment Lead for National Instruments. He noted that National and other software companies must document their source code to comply with FDA requirements. National supplies motor control software to the University of Nebraska for a training system that utilizes the robot-assisted da Vinci surgical system.