Designers of medical pumps often have to deal with the challenge of implementing precise, yet low-cost motion control. For most medical pumps, there are three basic technology alternatives for implementing such electronic motion control: permanent magnet brush DC motors, brushless DC motors, or step motors. Step motors (sometimes called stepping motors, stepper motors, or simply steppers) are a solid choice for position or speed control. Steppers are inherently digital — a pulse applied to the drive electronics results in a shaft movement of one step. They are commonly used “open loop,” meaning without feedback, due to their ability to achieve the desired number of steps every time (if sized properly). The number of incoming pulses and the rate at which they are fed can be used to implement very precise, yet very simple motion (position, speed, and acceleration) control. As long as the speeds required are not too high (less than 3000 RPM, typically), steppers often offer a far simpler, lower-cost, and maintenance- free alternative.
There are three types of rotary step motors: can stack, VR (variable reluctance), and hybrid step motors. Can stack or PM (permanent magnet) steppers are made with “claw toothed” (stamped) parts and permanent magnets (radially magnetized) in the rotor. Unlike can stack steppers, VR steppers do not have any permanent magnets in the rotor, and they rely on an induced magnetic field in their serrated (notched) rotor for their operation. A hybrid of the two technologies (permanent magnet as well as “reluctance” serrations in the rotor and stator) has resulted in hybrid stepper motors.
Hybrid steppers are generally made with precise machined parts and offer finer resolutions (usually 1.8° or 0.9° step angles) when compared to can stack steppers, which normally have coarser resolutions (3.6° to 18° step angles). The can stack design, however, is less expensive, based on its stamped metal parts versus the machined parts for a hybrid design. Hybrid steppers are an excellent choice where low-cost, yet precise shaft position control with fine resolution is required, such as hospital infusion, syringe, and peristaltic pumps. In each of these types, the delivery rate will vary based on the medicine and status of the patient.
Hybrid Step Motor Construction
The operation of any electric motor can be viewed as the interaction between its stator and its rotor. In a hybrid stepper (Figure 1), 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 ring in it for reinforcement, line up with the serrated teeth in the stator. The force with which this alignment takes place produces the torque (or rotating moment) in the rotor shaft. With switching electronics, the next coil is energized and the rotor moves (steps) again to align itself to the new position of the magnetic pole in the stator. As the coils are energized sequentially, smooth rotating movement is achieved. If more torque is required, it can be seen intuitively that either the stator’s magnetic pole has to be strengthened (more coils, more current, or larger diameter), or the rotor’s magnetic pole has to be strengthened (stronger magnets or larger-diameter rotor).
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 are all of the parameters used in the design considerations of the motor. From an application standpoint, it is sufficient to say that the geometry of the motor and therefore its step angle per step are all fixed when the motor is chosen. There is generally a great deal of flexibility in the windings, however, for trading off speed versus the torque produced for a given power output, which is a product of speed and torque.
The method of driving the hybrid stepper can be full step, as described above, moving from one mechanical step to the next. Microstepping is an extension of the concept of half-stepping — the creation of an electrical step between the mechanical steps of the motor. The current levels are increased sequentially in the windings in smaller increments, further improving the position resolution. It is common to see drivers that can deliver 1/4 step per step, 1/8 step per step, 1/16 step per step, 1/64 step per step, and so on. Beyond 1/256 step, such finer resolutions are beyond the mechanical accuracies of the motor. With the declining costs of microstepping and the benefits to be had in terms of smoothness of operation, it is always a good idea to consider microstepping as an option, even in cost-sensitive applications.