How to achieve high precision motion?

The most widely used technology for precision motion are the various variants of electric motors. Based on the power source, we differentiate between DC (direct current) and AC (alternating current) motors. Another main differentiation factor is the operating principle, which can be magnetism, electrostatics, or piezoelectricity.

Magnetism based DC and AC motors

Magnetism based DC and AC motors are the most commonly known as they are present in all fields of everyday life from electric cars to household appliances. Without detailing all variants of magnetism-based motors, let’s take a closer look on the types which are typically used in high precision motion applications:

  • DC Servo Motors: These motors are highly precise and offer very accurate positioning control due to closed loop control with encoder-based position feedback. They are often used in applications such as robotics, CNC machines, and other automated systems that require precise control.
  • Stepper Motors: These motors were originally the low cost alternative of servo motors, because of their inherent ability to control position, due to their built-in output steps, which allows them to be used as an open-loop position control, without any feedback encoder. This operation mode requires an initiation step at start up and its performance is limited, because over loading can lead to missed steps and positioning errors. To overcome these issues, nowadays it is common that stepper motors have encoder feedback and operate in closed-loop mode, therefore offering a viable alternative to servo motors.
  • Linear Motors: “Unrolling” an electric motors stator and rotor results in a linear motor. They are available in multiple types, like brushless, brushed, synchronous or induction. Besides heavy industry applications like maglev trains, they are also found in precision applications such as semiconductor manufacturing or precision stages, where they provide advantages over leadscrew and ballscrew driven positioning stages, in terms of acceleration, maximum speed, accuracy, constancy of velocity, and avoidance of vibration.

Piezoelectricity-based solutions

Piezoelectricity-based solutions are the main competitors against the above listed solutions for high precision movements. Here we distinguish two major groups. Piezo actuators and piezo motors.

  • Piezo actuators are composed of the stack of piezoceramic material that can contract or extend based on the input electrical signal. These can also have other mechanical components (e.g. guides) around them, which can help translate or convert the generated motion.
  • Piezo motors on the other hand are direct drive motor solutions, where one or multiple actuators inside the motor work in a precisely controlled manner to generate motion on the moving part (e.g. rod or axis) of the motor. Here we distinguish 3 types of motors: resonance drives (i.e. ultrasonic motors), inertia drives (i.e. stick slip motors) and walking drives.

How do the various electric motors generate high precision linear movement?

Magnetism-based electromotors typically create a rotative movement (except the linear motors). In order to generate linear motion from these motors, a transition mechanism is needed. The most common solutions to achieve this is the integration of lead screws and ball screws.

However, for direct drive piezo motors, these components are not needed as the generated motion is already linear due to their construction. Furthermore, applying linear motion along the circumference of a circle results in direct drive rotative piezo motors.

Why is it important to consider the full motion system and not just the motor?

As we saw above, high precision linear motion often cannot be achieved solemnly by the motor. Therefore, it is important to consider the full motion system, during the design and integration process. As an example, we can imagine a very cheap DC motor that would be suitable for a given task, but during the integration one would face that it needs gearbox, brake system and lead screw or ball screw solutions to fulfil all the operational requirements. On the other hand, a piezo motor might be more expensive when compared motor price to motor price, but due to its inherit features, it does not need additional components, which makes sourcing, assembly and maintenance much easier. This example shows, why it is worth to think in motion systems instead of only component solutions. The following table provides a non-exhaustive comparison of various solutions for high

TechnologyDC servo & stepper motorsLinear / voice coil motorPiezo motor
Linear motionCan be convertedYesYes
Rotative motionYesCan be convertedYes
Auto-hold / brake without power consumptionWhen extra mechanism addedWhen extra mechanism added or power is onYes, high precision position holding
Backlash at movement start or endPresent, due to the play of added mechanical components – can be reduced close to zero with special gearsDepends on the force of the magnets and coilsNo backlash
Trade-offs at designForce vs Speed:
Depending on the gearbox attached
Resolution vs price:
for high precision good encoder and controller is needed. Furthermore, for linear motion a high quality spindle is needed
Force vs price:
for higher force more magnetic material is needed
Stroke vs weight:
Longer stroke needs more magnets that increase the weight
Resolution vs stroke:
High resolution is more challenging with longer stroke
Repeatability vs price:
Despite high resolution, the movement is not absolute, which results in low repeatability 		Force vs price: for higher force more ceramic material is needed
Force vs stroke:
for higher force shorter stroke lengths are available
Force vs speed:
for higher force motor speeds typically decrease