Better hybrid stepper motors mean better equipment

HYBRID stepper motors make for a good choice when applications call for low-cost yet fine resolution of shaft movement.

HYBRID stepper motors make for a good choice when applications call for low-cost yet fine resolution of shaft movement. The motors provide precise speed and motion control for analysers, ultrasound scanning, X-ray equipment, and pick-and-place machines.

But how can improvements to hybrids help design better medical pumps? To understand this question you first need to understand more about stepper motors in general and their hybrid types.

How stepper motors work

Regular stepper types besides hybrids are called can-stack or 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 degree 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 degree 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.

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. Energising 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).

Improvements to hybrid steppers

Improved steppers have an aluminium 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. Because hospital pumps that are near patients’ beds must work quietly, motor noise is another design consideration. “Capturing” the bearings provides 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.

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. Increasing a motor’s torque allows reducing the size of the overall pump package. Here, advances in magnet design give engineers added design flexibility.

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.

Improved hybrids are optimised for performance versus size and therefore shorten the 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.

Declining microstepping drive costs and smoother operation of hybrids warrant their consideration even for cost-sensitive applications.

* Simon Pata is product line manager at Portescap, a Danaher Motion Co. Story submitted by M Rutty & Co, 02 9457 2222.

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