The worlds of biomedical and motion control are joining at the hip, with motors, controllers, actuators, and other motion control components becoming key elements in advanced biomedical applications ranging from fluid dispensers to imaging tables to surgical simulation systems.
The increasing use of sophisticated electromechanical and electronics technology in patient monitoring, laboratory, and other biomedical applications is opening the doors to the motion control industry, which has been developing smaller, higher precision motion components for industrial applications for years. But biomedical applications impose their own stringent needs.
Though speeds are lower and motion sequences are sometimes simpler, the need to gently move patients or accurately dispense samples requires motors to start and stop smoothly, with very high precision. Vibration, noise, and heat must be carefully controlled, and motion control components must often fit space-constrained products.
Reliability is the arguably the key requirement for the motion control system, because biomedical OEMS want to ensure that a product performs reliably for extended operating cycles in a laboratory or operating room environment. Given those constraints, cost becomes less of a primary concern.
“Our products are not the cheapest, but for what we provide they’re cost-effective,” said Joe Martino, a sales engineer for Maxon Precision Motors. Maxon makes a number of design modifications, such as using ceramic rather than metal inside gearboxes, for improved reliability. “You won’t find these materials in consumer applications,” he said.
Martino notes that roughly half of the company’s business comes from biomedical applications. One application for Maxon’s motors is a surgical simulator, developed by Immersion Medical, that enables the surgeon to simulate laparo-scopic procedures through a virtual reality system.
Johnson Medtech, the medical products network of Johnson Electric, supplied its Nanomotion motors to help the University of Calgary and MacDonald, Dettwiler and Associates Ltd. create the neuroArm, reportedly the world’s first MRI-compatible, image-guided surgical robot. The neuroArm uses 16 Nanomotion HR2-1-N-3 cermaic piezo ultrasonic motors, coupled with the company’s AB5 drive module.
The key advantage of Nanomotion’s motors for MRI applications is their non-magnetic design, unlike more common permanent magnet motors, according to Alan Feinstein, Nanomotion’s president. Motors that generate magnetic fields would lead to misleading images on the MRI.
The motors are used to regulate the motion of six rotary joints. Using the real-time visibility into the human body provided by the MRI, the neuroArm enables the surgeon to manipulate tools at a microscopic scale, performing surgeries previously deemed difficult or impossible.
Many biomedical applications call for simple, repetitive motion rather than executing complex sequences. David Goodin, president of Allmotion, which manufactures stepper drives, stepper controllers, servo drives and servo controllers, said, “All customers want to do is draw a sample, not machine a contour. There are controller designs we are working on for multi-axis configurations that don’t necessarily require coordinated motion.”
Other performance aspects are more critical. For one, vibration must be controlled to ensure patient stability. Motors and controllers that drive a patient table, for instance, must minimize vibrations that could make the patient uncomfortable and possibly upset sensitive organs. Likewise, motion control components regulating imaging systems must maintain stability, or images will be inaccurate. Galil Motion Control adapted its single-axis, DMC-1412 controller into an ultrasound system from Philips Medical Systems that can identify, for instance, distinct anatomic features of a developing infant. The controller incorporates a proportional-integral-derivative (PID) filter with acceleration and velocity feed-forward functionality for accurate control and smooth motion. The controller helps the ultrasound system probe with high accuracy and stability between frames, eliminating wobbly or bouncing images.
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.