Most motors are made of metal and they use permanent magnets, electromagnets, or a combination of both to induce motion. While these magnetic motors are perfectly suitable for the vast number of potential applications there are some applications, particularly in the medical industry, that require motors that are not made of ferrous materials and that do not use magnets.

Prime examples are medical instruments and the robots and other equipment that handle them when the devices are used in or around medical imaging devices such as a magnetic resonance imaging (MRI) machine. Here, traditional motors can pose safety and image quality concerns. This is an especially acute issue as these imaging machines become more prevalent and their uses expand beyond capturing multi-dimensional images of their subjects. Increasingly, MRIs are being used in real time with automated or semi-automated test systems, and by doctors remotely manipulating surgical tools, injection pumps, probes, manipulators, and other such devices while using the MRI images to guide their efforts.

These types of applications have put pressure on manufacturers to use motor technology that will be safe to operate within the confines of an imager. An MRI machine uses magnetic signals, rather than X-rays, to create image “slices” of the patient. To generate these signals, primary and secondary magnets are used and this can be problematic for devices that use traditional motors.

The trouble is that traditional motors contain ferromagnetic metals that can pose a safety hazard when used in machines that have high magnetic fields, such as an MRI. With the very high magnetic interaction forces at work, heating may occur in conductive materials by electromagnetic induction. Metal objects employed in or near an MRI system can be sucked into the machine, drawn by the strong magnetic forces employed. These devices use pulsed magnetic and radio frequency fields of very high density, commonly in the range of 3 Tesla. That is one reason that patients with metal in their body or who have certain implantable medical devices cannot use an MRI machine. In addition, the motor’s magnets inherently generate magnetic and RF fields during operation that could result in RF arcing and cause hardware damage and degradation of image quality.

The piezo effect produces a hula-hoop action that drives a nut around screw to induce precise motion with the Squiggle motor from New Scale Technologies.

Non-magnetic motors, those made from nonferrous materials and which do not generate magnetic or RF fields, are becoming the motor of choice for these applications. Most often, piezoelectric motors, using the reverse piezoelectric effect, are used, but other technologies are under development including a pneumatic motor.

The piezoelectric effect is the term used to describe how a piezo crystal will generate voltage in response to an applied mechanical stress. A simple and common example is a piezo igniter on a barbecue gas grill, where pushing the igniter button releases a spring-loaded hammer that hits the piezo materials to generate a spark that ignites the gas.

By contrast, a piezoelectric motor uses the effect in the opposite direction, using piezoelectric ceramic materials to convert electrical energy in to mechanical energy, known as the reverse piezoelectric effect. The external application of a voltage to a piezo crystal causes it to change shape, or distort by a small amount. This distortion is typically about 0.1 percent of the size of the original piezoelectric crystal. While the change is small, its cumulative effect is big.

Not all piezo motors are automatically non-magnetic. While they do not generate electromagnetic fields, to be non-magnetic the motors and all their parts must be constructed from non-ferrous materials.

Enlarge this picture The PneuStep motor developed by researchers at Johns Hopkins University uses compressed air to operate.

At least two companies, New Scale Technologies, Victor, N.Y., and Nanomotion, Ronkokoma, N.Y., are using the reverse piezoelectric effect for their miniaturized piezoelectric motors. Beyond the fact that they can be non-magnetic, these motors have other useful features. They are small, for instance, smaller than even the smallest electromagnetic motor. They are also very precise, with accuracy and motion control in the nanometers.

New Scale Technologies’ Squiggle motor uses the small movements of several piezo crystals to generate linear movement. Fundamentally, the motor is just a nut and a screw, says Dan Viggiano, vice president and general manager of New Scale’s OEM and Industrial Business.

The motor consists of four piezoelectric plates bonded in 90 Deg spacing to a non-magnetic metal tube -— the “nut” — that has been threaded on the inside. A matching threaded screw is inserted into the tube. The linear motor uses ultrasonic standing wave vibration in the threaded nut to directly rotate the screw.

The externally applied voltage pulses sequentially and bends the nut 90 Deg in one direction and 90 Deg in the other direction. The signals are synchronized so that the “nut” works its way up the “screw” in an orbital motion. The company refers to this as an orbital “hula hoop.” This action pushes the screw in a continuous motion so that the motor can be programmed to move in very small, and very accurate increments. The motor can realize sub-micrometer stepping and velocity control, according to Viggiano.

Enlarge this picture The Squiggle motor from New Scale Technologies is small enough to fit on the tip of a finger.

The piezoceramic plates are activated using a two-phase electric drive with a fixed frequency and a 90 Deg phase shift. The ultrasonic frequency depends on the motor model and can range from 40 to 200 kHz, but a typical frequency is about 150 kHz. At 150 khz, the 1.55 X 1.55 x 6 mm Squiggle motor can achieve speeds of 10 mm per second and can push 20 grams. At the low end, the motor can be set to move very slowly — nanometers per second, or even nanometers per hour. The top speed is around 7 mm per second. The larger 7 mm diameter Squiggle motor can push up to 500 grams but moves at slower speeds, about 2 mm per second.

The Stall force for the standard SQ-100NM non-magnetic motor is more than 5 Newtons. If the load is distributed over several motors, more force could conceivably be achieved, as long as the load on each motor does not exceed its stall force. New Scale’s focus has been more on applications requiring very small motors, as opposed to those requiring very large forces. A 1.8 mm motor that can push more than 30 grams (0.3 Newtons) is a significant force in miniature motion systems, says Viggiano.

Some Squiggle motors have been tested to more than 1-million cycles of continuous duty cycle. Failure comes from wearing of the screw threads, not fatigue of the crystals in the piezoelectrical material, says Viggiano.

Squiggle motors are being used in MRI applications and are being designed into endoscopes and miniature micro-fluidic pumps. In the later cases, the precision and size of the motor is more important than its non-magnetic properties.

Enlarge this picture This diagram shows the open-loop control of the PneuStep motor from Johns Hopkins University.

In an MRI application, researchers at the Roswell Park Cancer Institute, Buffalo, N.Y., use Squiggle motors in studying the whole bodies of small animals. Researchers used a syringe pump driven by a Squiggle motor to inject contrast media into a live mouse in the MRI machine during imaging. A motor was also used to drive a stage along the X and Y-axis in minute increments to reposition the mouse and capture different image slices of the animal.

The Squiggle motor is primarily used for linear motion such as in a syringe pump, but can also be configured to work in a rotary fashion. In addition to medical applications, Viggiano says the motors are used in many other applications. Mobile phones with high resolutions cameras, for instance, are an appropriate use for these small motors in that the motor can accurately adjust the zoom lens. Additional uses include electronic access control, micro fuel cells and cooling sytems, and mobile robots.

While traditional electromagnetic motors work well for many such compact applications, they are reaching the limits of their miniaturization. The piezoelectric motors, however, can be made small enough for even the smallest device. Basically, Viggiano says, the motor can be made as small as a screw and nut can be manufactured.

Nanomotion supplies piezomotors for medical applications and a variety of other industries such as the semiconductor industry and for test and metrology instruments. In essence, anywhere accuracy, non-magnetism, and small size are important, the motors can be customized to the application.

Nanomotion also uses the reverse piezoelectric effect, but implements the technology in a different way than does New Scale Technologies. Unlike New Scale’s nut that drives a screw, Nanomotion makes motors that are made from layered bulk piezo elements that are driven by a 39.6 kHz voltage to generate an ultrasonic standing wave. The wave causes a ceramic plate to vibrate, bending and contracting in place. The plate is compressed against a mechanical structure such as a gear head assembly, and the back and forth wiggling motion pushing against the structure propels the structure in either a forward or backward direction. It can work be used for a linear or rotary motor depending the bearing structure. If the bearing structure is linear, it moves in a straight line. If the bearing structure is rotary, than the motor rotates.

Enlarge this picture

Nanomotion’s PZT motors are based on stacks of piezoelectric crystals. Nanomotion uses two AC sine waves to generate the ultrasonic standing wave. An electrical field is applied in pulses to the crystal in the direction of its main axis. The pulse changes the shape of the crystal and results in a quick elliptical movement as the voltage is pulsed on and off. The ellipse is proportional to the voltage that is applied. The larger the ellipse, the faster the mechanical structure will go. At a maximum velocity of 300 mm per second the motor operates at 270 Vrms.

The piezo elements used by Nanomotion are of a standard size and generate 16 oz. of linear thrust. The company offers a single-bodied motor that will incorporate up to 8 piezo elements, all of which are the same size, and which generates 8 lbs. of thrust. It also offers a driver that can handle up to 32 elements, or four of the 8-element motors. This allows for a possible 32 lbs. of thrust.

Because the motor pushes against a mechanical structure, Alan Feinstein, president of Nanomotion, says that the structure itself must be stiff enough to withstand the pressure. The motor applies a force to one side of the structure that is five times greater than the driving force. Normally, the motor needs a stiffness of 40 N/micron, but that is not always possible.

Recently, Nanomotion’s HR-8 series of motors were used on the neuroArm robot for use in MRI applications. The neuroArm is a robotic arm that allows a surgeon to remotely operate on a patient’s head while the patient lies inside an MRI machine. The robot uses 16 Nanomotion ceramic motors per robot, spread over the robots’ six rotational axes and one linear axis.

The largest of these actuators, which is used in the base of the robot, exerts about 12 lbs. of force. The smallest motor, those for the robot’s wrist joint, exert about 1 lb. of force. Where the designers could, the structure was made of materials stiff enough so that there wasn’t any play or bending when force was applied. In other areas, designers used ceramic braces to counterbalance the pressure and eliminate any slip in the structure.

While the way that New Scale Technologies and Nanomotion use the reverse piezoelectric effect differs, both companies were able to overcome a problem that affects quality when a traditional piezo motor is used in an imager. Traditional piezoelectric motors use DC voltage to expand and contract the piezo crystal and the DC voltage levels can interfere with the electromagnetic field of the MRI’s imager and can cause artifacts in images. Traditionally, this meant that the piezomotors couldn’t be operated any closer than 0.5 m from the image isocenter, says Dan Stoianovici, associate professor of urology and Director of the Robotics Laboratory at Johns Hopkins University, Baltimore.

To get around this, both companies use AC voltage. Nanomotion uses two AC sine waves to generate the ultrasonic standing wave. Similarly, New Scale’s motor uses AC sine waves to generate ultrasonic standing waves.

Stoianovici led a group of researchers in the development of a non-magnetic, pneumatic motor that does not require high voltage and electrical frequencies to operate. In fact, it doesn’t require any voltage or electrical frequency at the motor. Their pneumatic motor, dubbed the PneuStep, uses compressed air generated by standard air compressors to drive the motor. The electric compressors are located away from imagers to avoid any interference. The pressure per square required to move the piston is about 10 psi.

The PneuStep is based on the premise that end-to-end motion within a cylinder is almost exact. The motor is made up of three cylinders, which are spaced 120 Deg around a motor gear head, and within each cylinder a piston moves linearly. Propelled by air, the pistons slide back and forth almost the exact amount with each stroke. When pressurized sequentially, they generate motion and each stroke moves the gear assemblage a precise amount. If the sequence is cylinder 1, cylinder 2, cylinder 3, the gear head moves forward in a clockwise rotation. If the sequence is reversed, the gear head moves backwards in a counter-clockwise rotation.

It becomes in effect, a stepper motor, or what Stoianovici calls the world’s first pneumatic stepper motor. The steps can be made in minute amounts; the robot in his laboratory has a one linear step of about 50 micrometers (0.05 mm). An electro-optical interface is used to verify positioning. Currently, the motors are being used in-house to move the robots that control medical probes and other devices used for urological testing. Researchers in Belgium, are using the technology to study the brains of lab rats. New versions of the motor are in development, and licensing of the technology is available, he says.

All of the part’s motors are plastic and can be made through injection molding. The basic motor is rotary, but the gear head can also be configured for linear motion and in various step sizes.

Like the piezo electric motors, the PneuStep can be used in applications other than those that require non-magnetic motors, as long as it can be air-operated. A household appliance is an unlikely use for the technology, he says, but air-operated power tools might work.

While these pneumatic and piezo motors will never replace traditional magnet-based motors, those devices that are made from non-ferrous materials and are non-magnetic will continue to have their place. In fact, much like the piezo crystal that grows when an external force is applied, these nontraditional motors may find new uses as electronics become smaller and their uses become more diverse.

For more information, email:
Johns Hopkins University, dss@jhu.edu
New Scale Technologies, dviggiano@newscaletech.com
Nanomotion, afeinstein@nanomotion.com