When designing a medical pump that is meant to administer the correct dose of medication to a body at the right time, precision, reliability and repeatability top the list of musts. If it’s a stepping motor application, the motor rotates the exact number of steps. It must make the lead screw drive the syringes in exactly the right amount. And, it must do it every time at the right time and at the right amount.

Whether designing a medical pump, medical imager or other product, motor choices vary. In some applications, a single criterion such as torque, speed, power efficiency, long life, or cost may supersede the others. In others, competing factors may require different capabilities to be used in conjunction. In this world of highly specialized, critical equipment, the unique needs of application determines the appropriate motor.

If the equipment will go into a clinical setting on a large machine such as a medical imager, long life may be most important. If it is a tool such as a angioplasty balloon or a rotorblator, which cuts away clogs in arteries, that is used once and then discarded, long life may give away to cost effectiveness. In some cases, such as in an oxygen or flammable gas pumping application, the motor must generate spark-free performance and electrical noise must be dampened so as to not to interfere with other nearby equipment. Home medical devices such as a glucose monitor may require small size and low weight.

For portable devices, efficiency becomes a big concern. “You want a motor that is as efficient as possible,” says Ted Severn, director of sales and marketing, MicroMo Electronics, Clearwater, Fla. “If you have a pump that is going to be worn on the body, it will typically be powered by a battery, so you would want the drive mechanism or the electronics that are used in the device to use as little of the battery power as necessary in order for it to live a long time.”

Besides precision and reliability, words often synonymous with quality, features that designers should consider include size, long life, low acoustic noise, EMI/RFI issues, clean operation, need for sealed operation, smooth torque profile, and controllability, including the ability to provide torque at low RPMs or in incremental steps.

While all of these factors are important considerations for medical applications, they can also be relevant to specifying motors in other segments. For example, foodservice appliances or water treatment appliances may have needs for similar levels of isolation of material from a pump mechanism for cleanliness and sanitation purposes.

“There are many common denominators that cut across applications,” says Mike Rogen, vice president of electronic sales and marketing, Maxon Precision Motors, Burlingame, Calif. “Many non-medical applications need to be sterilizable. And small and lightweight motors would relate to any kind of handheld device. And, long life of the motor is pretty standard across the board.”

A long life

Enlarge this picture ThinGap manufactures a line of brush and brushless motors utilizing a patented electromotive coil design and replaces the iron core and wire windings used in conventional motors with a precision-machined copper sheet.

While precision and reliability are essential in the medical arena, cost issues still play an important role too. In general, motors come in brush and brushless modalities. Suppliers offer models in either type and also by subgroup, such as stepper motors. MicroMo, for instance, offers DC-micro motors, miniature stepper motors and brushless motors.

The traditional brush motor often costs less than its counter part because it does not require the sophisticated control electronics needed to run a brushless motor. Brush motors are mechanically commutated, which means they steer voltages to the correct motor phases to obtain the optimum torque. Whereas, a brushless motor is electronically commutated.

With the brush motors, a rotor turns and commutator bars contact the brushes and keep the rotor turning. It is this action that makes brush motors more susceptible to arcing between the brushes and the commutator. In cases where low EMI is important, such as in an MRI or CT scanner scenario where interference with other diagnostic equipment can’t be tolerated, brush motors may not be the best choice.

On these types of motors, brushes wear out before anything else. Depending on how hard the motor is driven, brushes have a life of around 1,000 hours, although some companies promote as much as 5,000 hours depending on the brush type. According to Rogen, metal brushes can give between 1,000 and 3,000 hours of life and a graphite brush motor would exceed 5,000 hours.

Brushless motors are completely electronically commutated motors and therefore there is no brush wear. These motors lead long lives, a minimum of 10,000 hours, but much longer in some circumstances. With these motors, the bearings are the first to go.

In general, brushless motors are more expensive because of the electronics involved. “If cost is an issue, and lifetime is not as important, than a brush motor would be perfectly fine,” says Rogen. “If the opposite is true, and all other things being equal, go with a brushless motor in a medical application.”

A version of the brushless motor is the stepper motor, which is applicable for medical devices that require the motor to start and stop at specified intervals. With a stepper motor, a pulse moves the rotor a certain amount of steps. According to Douglas Jones, an expert on stepper motors and associate professor at the University of Iowa, Iowa City, most steppers, as they are also known, can be started, quickly spin, and with an appropriate controller, have the rotor stop at a precisely specified position.

These motors typically do not have the torque output of other motor types, but they often have a shorter package size. For instance, MicroMo’s stepper motors come in diameter sizes of between 6 mm and 22 mm, which are typical, traditional motor sizes. However, if a 6-mm stepper motor is compared to a 6-mm brushed or brushless motor, it has a shorter overall length. So, if package size or envelope size is the biggest concern, it could be a good solution.

In some applications, such as where higher torque is required, a gear box or transmission is required. However, gearboxes may limit product life because they can wear out before the motor. In such applications, a different type of motor can eliminate the need for a gearbox. For instance, ThinGap of Ventura, Calif., offers a DC motor that it says can eliminate gearboxes or transmissions because of its higher torque capability and high-inertia rotor.

Coil designs differ

Enlarge this picture The MicroMo Series 2057 Brushless DC motor is sterilizable. It has a motor-diameter of 20 mm and a length of 57 mm and can achieve a continuous output torque of up to 18 mNm at speeds up to 58,000 rpm.

ThinGap’s motor differs from many others on the market. Traditionally, motors are constructed with wire-wrapped iron-cores. Brushless motors have a stationary lamination to hold the coils, usually on the outside of the motor, but ThinGap has developed a freestanding coil. The coil does not use laminations for structural support, which means there are no laminations to interfere with the magnetic field. This allows the stator to reside in a very thin magnetic gap. In ThinGap’s motor, the stator is stationary and the permanent magnets rotate. The entire magnetic circuit rotates. “We have a purely inverse image of the true ironless core brush motor that other companies have,” says ThinGap's chief technology officer Greg Graham. “The coil is stationary, while the magnetic assembly, including the inside magnets and back iron of the motor and the outside iron on the rotor turns. The magnets do not sweep laminations, which create a circulating current generating heat and limiting the current into the motor and therefore its performance. In turn this limits the top end performance of the motor. The design of our motor eliminates the inefficiencies caused by laminations, allowing much higher performance in the same size package.”

The entire magnetic circuit is rotating together so the magnetic field that the steel parts are being subjected to literally never changes while the magnet assembly rotates. Traditionally, when a magnet sweeps the iron laminations, heat is created in laminations resulting in eddy current losses.

Graham says that this means that the motor provides a linear current-to-torque profile at all speeds, better speed control, very low breakaway torque, high stopping torque, rapid acceleration and higher peak power. He adds that the motor is more efficient because it eliminates most eddy current losses caused by iron-laminated cores. These losses are an exponential function of RPM, Graham says. The higher the RPM, the more loss the motor has. Without these losses, the ThinGap motors have a wider speed range. By increasing efficiency, less energy is left in the motor; or for the same amount of energy left in the motor output can be increased. (See Fig.1).

Smooth torque control is especially important at lower speeds where it is harder to maintain. Severn says that to select the appropriate motor, the designers must know the low point that the motor is going to be operated, as well as the speed and torque that the motor will be operated and for how long. Electric motors are inefficient when they are lightly loaded, when performing less work than they are designed to handle.

Las Vegas-based Power Efficiency Corp. is developing technology to improve low-speed motor efficiency. It has developed microprocessor-based technology for reducing the amount of electricity used by single-phase AC induction motors.

The technology uses an algorithm run by software on a microprocessor to reduce inefficiency. Internal testing looked at an electric motor at no load that resulted in more than 30 percent energy savings, and an electric motor at 40 percent of full load that resulted in more than 20 percent savings.

Sometimes it is not breakaway torque or peak torque that matters. Maxon faced a stall-torque challenge on a recent project. The company provided 10 motors that are used in the Horizon Multi-Media Medical Imager from Codonics, Middleburg Heights, Ohio. Although the motor gearboxes incorporate metal parts, some of the other mechanisms in the imager are plastic. The higher stall torques of the Maxon motors occasionally caused component breakages to occur. The continuous torque is used to size the motors, so the stall torque couldn’t be changed. To solve the problem, an encoder monitored when voltage needed to be reduced and the system would automatically reduce the voltage at the near-stall point, so that the movement could be more gentle.

EMI and more

The EC motor from Maxon is a brushless sterilizable motor. Steam sterilization is specified at 134 DegC (273 DegF) at up to 2.3 bar, for 20 minutes. Motor can be sterilized at least 100 times in an autoclave.

Motors for medical equipment have some unique and not so unique problems. One of the most important issues deals with electronic noise or EMI/RFI. If a motor generates electrical noise it could interfere with the electronics instrumentation and measurement equipment situated nearby.

Experts say the way to get around this is to specify low-inductance motors. A brushless motor works best in mitigating EMI noise; a brush motor is much more electrically noisy than a brushless motor. Whichever motor is chosen, EMI shielding may still need to be incorporated into designs.

In some cases, such as an MRI environment, Severn does not recommend the use of motors with a magnet because it would interfere with the machine’s operation. (Traditionally, motors had iron-cores that had wire wrapped around them and electricity was used to turn the wire and iron mass. In coreless motors, a hollow armature, typically made of copper, is situated over a stationary magnet system.)

High and dry

This cut away of Maxon’s EC-16 brushless motor shows that by assembling the motor such that the coils are outside the rotor, good heat dissipation and high overload capability is attained.

The ability to seal the motor may also be an issue, when it comes to devices that are exposed to water or other fluids or that need to be washed or sterilized. Some medical equipment such as infusion pumps, dialysis equipment, dermatological and dental tools may require sterilization. Many medical motor suppliers offer sterilizable versions of their models. Some examples include Maxon’s EC16 and EC 22, a 40 W and 50 W brushless motor, as well as MicroMo’s Series 2057B. These motors can be sterilized in autoclaves — considered one of the harshest environments that a motor would ever face. Maxon’s product has been tested out in the autoclave to at least 100 times at 20-min. cycles without needing to dismantle the motor.

Severn says that customers often design sealing into their devices. “For example,” he says, “for medical hand piece manufacturers, there are seals that are made at the end of the hand piece that can survive an autoclave environment.”

In some cases, a motor in a piece of medical equipment is subjected to radiation. Unless the motor encounters extreme levels of radiation it shouldn’t fail, but its sensors may. For example, electronically commutated motors require a sensor such as an encoder that can sense its position. Sensors that go into this environment must be robust enough to withstand the radiation levels. While some do, Rogen says that resolvers are the more robust method when dealing with radiation because there are no electronics present inside them, only coils.

While the set of motor requirements for a specific medical applications may be unique, some of the general issues are of concern to other product segments, also. Office machines have similar need for precise motion control steppers, and motors in foodservice appliances may also require reliable operation in hot environments similar to a sterilization, and sealed operation is just as essential in a soft drink machine or French fryer as it is in a blood pump. As always, design engineers can learn from colleagues in different business segments on how to solve some of their key design challenges.