For many years, stepper motors have been the most popular type of electric motor designed into instrumentation for a wide variety of reasons. Stepper motors have become increasingly commoditized, and can be sourced easily. In addition, the growing “maker movement” has simultaneously made them more popular and reduced their cost. Unlike servo motors, stepper motors don’t require tuning to optimize their performance. What’s more, scaling and motion commands are typically quick and simple to execute using stepper motors. Servo motors often require a bit more expertise in executing complicated (torque, velocity, or position) loop closures. Finally, micro-stepping allows most modern drive electronics to step or increment a stepper motor to a resolution of 50,800 steps per revolution or higher.
However, macro trends in laboratory automation are driving instrument designers to seek alternatives to stepper motors. Foremost among them is the pressure for higher throughput. Reducing the time needed to perform a test is one of the quickest ways to reduce its cost. Smaller footprints are another important cost-saving technique. More compact instruments are crucial to conserving floor and bench-top space. In life sciences instruments, reagents make up a big percentage of the incremental cost of each test. Reducing costs demands reducing both the size of the sample and the amount of reagent used. To accommodate this reduced sample size, an instrument’s precision of motion is increasingly critical. The desire for multipurpose design represents another important trend as consumers demand instrumentation with greater integrated functionality. Modern instrument buyers also want products that are increasingly intelligent and aware. One particularly popular attribute is “determinism,” which allows the instrument not only to maintain knowledge of all positions, but also the status in motor operating parameters, such as temperature or different fault conditions.
In response to these trends, instrument designers are increasingly switching to servo motors and direct drive technology. For the designer, it may be useful to explore how each of these technologies compares to stepper motors.
Servo Motors vs. Stepper Motors
Much like stepper motors, servo motors are available as conventional rotary motors — as well as linear servo motors (Figure 1). Several advantages are leading growing numbers of instrument builders to favor servo motors to boost their instruments’ performance:
More torque: For applications that only need short bursts of torque or force in linear servo equivalents (typically for accelerating an object), servo motors can be supplied with more current for short periods than they could handle continuously.
Higher speed: A servo motor will typically allow for 2 – 3 times the speed of a typical stepper motor. This potentially allows the designer to build in additional reduction, for more torque in a smaller package. For example, compare two equivalent stack length NEMA frame motors: one stepper (HV233), and one servo (SM233). Performance curves for both of these motors were generated under the same operating conditions under ambient temperature, and each was supplied the same 120 Volts AC to the drive electronics. If we were to compare the performance of the stepper motor to that of the servo’s continuance performance, there is a slight torque advantage in favor of the stepper motor at low RPMs (less than 1000); however, once we cross into higher RPMs, the torque performance of the stepper motor drops off drastically, and speed-torque performance of the servo becomes underscored. Now consider the peak performance of the servo motor relative to that of the stepper. Granted, the servo can only operate in this peak torque range for a short interval and must allow generated heat to be dissipated, but the extreme advantage of the servo in this range of operation is obvious (Figure 2).
Greater precision: Servo motors use feedback devices that use encoders to sense error in a system and correct for this error to meet a desired performance of a mechanism, through control loops around motors, torque, velocity, or position. A wide variety of encoder options are available. In rotary motors, options include typical glass disk or new low-cost magnetic, quadrature, extremely robust resolver, analog sine/cosine, and multi-turn absolute formats that eliminate the need for homing. Linear encoder technologies offer technologies similar to those available on rotary devices:
- Lower audible noise: Properly tuned servo motors are ninja-quiet. The biggest source of noise in servo-driven applications is often either the drive train or bearing. Stepper motors will typically emit around 68 dB during operation.
- Flexible winding technology: Modern design software allows simulating servo motor performance very closely. Servo motors can often be specially wound in a very controlled, almost CNC-like fashion to match their performance curve to an application’s speed and torque requirements.
Direct Drive Technology vs. Stepper Motors
Direct drive technology, as the name implies, creates direct coupling between the motor and payload without the use of any mechanical reduction or drivetrain. Its major drawback is its lack of mechanical advantage. Any force or torque required by the application must be directly generated by the motor, which might require using larger motors and larger overall current draw. However, direct drive technology offers several advantages over steppers in terms of instrument design:
- Integrated rotary kit motor: In many cases, a rotary kit motor can reduce the size of a given application. In rotary applications, kit motors eliminate gearing or belting. In linear applications, they can also be integrated directly to a screw to eliminate a coupling, which adds to the actuator’s overall length and compliance. A rotary kit motor often includes:
- Stator or coil – This is the wound, copper portion of the motor that uses current to generate a magnetic field to drive the motor.
- Rotor – The stator applies a magnetic field to a fixed set of magnets commonly referred to as the rotor.
- Hall effect sensors – Small Hall effect sensors are positioned at various offsets relative to one another to establish commutation angles within the drive electronics to indicate which phase should receive power at given rotational positions.
- Integrated linear motor: Essentially, a linear servo motor is just like a rotary motor that’s rolled out flat. The motor is coupled directly to the load, so there’s no mechanical advantage, and all the force the application requires must be created directly by the motor.