Improvements in hybrid stepper motors foster better medical-equipment designs.
Hybrid stepper motors make for a good choice when applications call for low-cost yet fine resolution of shaft movement, as in medical syringe and peristaltic pumps, according to distributor M Rutty. The motors provide precise speed and motion control for analysers, ultrasound scanning, X-ray equipment, and pick-and-place machines, but the focus here is on how improvements to hybrids help design better medical pumps.
It’s useful to start with a discussion about stepper motors in general and hybrids in particular.
Other stepper types besides hybrids are called ‘can stack’ and ‘variable-reluctance (VR)’ motors. All steppers work best in applications needing less than 3,000 rpm.
Can-stack steppers are made with claw-toothed, stamped parts and have permanent magnets in their rotors. These motors normally have 3.6 to 18° step angles. VR steppers, on the other hand, lack a permanent magnet, relying instead on an induced magnetic field in the serrated (notched) rotor.
As the name implies, hybrid steppers combine the two technologies with a permanent magnet and ‘reluctance’ serrations in the rotor and stator. (Magnetic reluctance is a material’s capability to oppose the flow of magnetic fields through it.) These motors provide fine resolutions, usually with 0.9 or 1.8° step angles.
The number of incoming pulses and the rate at which steppers are fed precisely control motion because stepper motors are inherently digital. Thus, a pulse applied to the drive electronics results in a shaft movement of one step. Steppers are commonly used ‘open loop’ or without feedback because when properly sized, the motors produce the same number of steps every time.
How hybrids work
The operation of any electric motor relies on the interaction between its stator (stationary part) and rotor. In a hybrid stepper, electrical current in the coils around each stator slot creates electromagnetic poles in the stator. The serrated teeth in the rotor, which also has a permanent magnet for reinforcement, line up with the serrated teeth in the stator. The force with which this alignment takes place produces the torque to turn the rotor shaft.
Switching electronics energise the next coil and the rotor moves (steps) again to align itself to the new position of the magnetic pole in the stator. Energizing the coils sequentially produces a smooth rotating movement. When more torque is needed, it’s necessary to strengthen either the stator’s magnetic pole (more coils, more current, or larger diameter) or the rotor’s magnetic pole (stronger magnets or larger-diameter rotor).
Parameters of hybrid-stepper designs include the number of coils, the number of wire turns in each coil, the relative number of teeth in the stator and rotor, and the diameter and flux density of the magnet. Of course, already built motors have a fixed step-angle per step. However, there is a lot of flexibility with winding arrangements that allow trading-off speed versus torque for a given power output (a product of speed and torque).
The basic operation of a hybrid is to drive a full mechanical step. To improve position resolution, microstepping is used to create electrical steps between the mechanical steps of the motor. In this scenario, current levels are increased sequentially in the windings in small increments. In fact, it is not hard to find drivers that deliver 1/4th-step per step, 1/8th-step per step, 1/16th-step per step, and so on. The limit is about 1/256th step because anything finer surpasses the mechanical accuracies of the motor. Declining microstepping drive costs and smoother operation of hybrids warrant their consideration even for cost-sensitive applications.
Improvements in hybrids
One method that allows decreasing overall pump-package size is to overdrive the hybrid stepper to increase its output torque. While the practice is acceptable as long as the duty cycle is considered, problems can arise from the heat generated by the motor as well as heat-induced torque degradation. In a typical hybrid, aluminum end bells dissipate heat. But this practice requires additional design considerations for airflow or heatsinks.
Improved steppers have an aluminum housing that encloses the stator laminations. The housing improves the design by providing a conduit to dissipate the heat along the entire length of the stepper, making it more efficient. Torque loss due to temperature rise is decreased. This lets engineers increase the duty cycle of the pump for more efficient patient dosages. Also, by generating less heat when working, the pump stays cool to the touch, especially useful when patients or nurses might inadvertently touch the pump while adjusting medication.
Because hospital pumps that are near patients’ beds must work quietly to keep from disturbing the patient, motor noise is another design consideration. Noise can be generated by even slight movement from the bearings. “Capturing” the bearings provide a significant noise reduction. A snap ring for the front bearing and an O-ring for the rear do just that, preventing bearing movement during operation. This design almost eliminates shaft endplay and significantly reduces the risk of mechanical failure.
Some hybrids failed when their bearings wore out. So another improvement comes from increasing the size of the ball bearings, which allows a higher axial and radial load. The increased load capability gives the motor a longer lifetime.
Another key requirement of any pump application is pullout torque, the torque generated at a given speed. A motor produces a defined amount of torque at certain drive conditions. This dictates the motor size needed. Increasing a motor’s torque allows reducing the size of the overall pump package. Here, advances in magnet design give engineers added design flexibility. For example, using high-energy Neodymium magnets in place of more conventional magnet materials produces a higher magnetic flux, which translates to higher torque. Also, stator magnets have been added between each stator tooth to block the magnetic field from flowing around the stator teeth. This forces more of the magnetic field to flow through each tooth, increasing the torque output by up to 30 per cent. These improvements let engineers specify smaller motors, again reducing overall package size and weight.
Naturally, a drug pump will run at a speed that depends on the fluid or drug being dispensed. In many applications, the amount of fluid can be quite small, which requires the hybrid to be microstepped. Typical hybrids have stator laminations indexed and stacked, secured between the two end bells by way of four screws. The aluminum housing aligns the laminations during manufacturing better than the typical arrangement for a more uniform air gap between stator and rotor teeth. During microstepping, this uniform gap improves torque linearity. That is, the torque is more consistent for each microstep.
Torque linearity also improves with stator enhanced magnets. Infusion and syringe pumps in hospitals allow the use of finer dosages of medicine and ensure that precise amounts are dispensed each and every time.
Typically, a standard motor is specified in the initial stages of development and a variety of tests ensure it meets requirements. Several iterations of the motor are common because mechanical designs usually change over the course of a project. But improved hybrids are optimised for performance versus size and therefore shorten this design cycle.
And should there be a change in motor requirements, it affects motor selection less because the same size motor can have its torque increased up to 30 per cent using the stator enhanced magnets.
Because speed-to-market is a key performance indicator, exceeding initial projections can result in increased revenue for the company.
This article was brought to you by M Rutty on behalf of Portescap.