How to achieve high precision linear motion!

Which motor technology is right for your application

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.

Piezo actuators and piezo motors – is there a difference?

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.

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:

Technology	DC servo & stepper motors	Linear / voice coil motor	Piezo motor
Linear motion	Can be converted	Yes	Yes
Rotative motion	Yes	Can be converted	Yes
Auto-hold / brake without power consumption	When extra mechanism added	When extra mechanism added or power is on	Yes, high precision position holding
Backlash at movement start or end	Present, due to the play of added mechanical components – can be reduced close to zero with special gears	Depends on the force of the magnets and coils	No backlash
Trade-offs at design	Force 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

The precision of a DC motor combined with gears and a spindle depends on several factors:

Gear Ratio: The gear ratio determines how much the motor’s rotation is translated into the rotation of the spindle. Higher gear ratios can provide finer control and precision but may sacrifice speed.
Motor Control: The control system for the motor plays a significant role. Precise control systems, such as PID (Proportional-Integral-Derivative) controllers, can help achieve more precise positioning and speed control.
Mechanical Design: The mechanical design of the gears and spindle system is crucial. Any backlash or play in the gears can introduce imprecision.
Quality of Components: The quality of the motor, gears, and spindle components also affects precision. Higher quality components typically result in better precision and reliability.
Feedback Systems: Adding feedback systems like encoders or sensors can enhance precision by providing real-time information about the position and speed of the system, allowing for adjustments and corrections.
Environmental Factors: Environmental factors such as temperature, humidity, and vibration can also affect precision.
Tolerance and Calibration: Proper calibration and maintenance are essential for maintaining precision over time.

In general, with the right combination of these factors, a DC motor combined with gears and a spindle can achieve high levels of precision, suitable for various applications such as robotics, CNC machines, and automation systems. However, the achievable precision will ultimately depend on the specific requirements of the application and the engineering trade-offs made during design and implementation.

Motor sizing is just the beginning…

This paper focused on properly sizing a motor for a relatively simple single axis linear motion application. Although the principles are identical for a more complex system such as an X-Y table or a multi-axis precision pick-and-place mechanism, each axis will need to be analyzed for load independently. Another consideration outside the scope of this article is how to choose an appropriate safety factor in order to meet the desired life of the system (number of cycles). System life isn’t just a function of the motor size, but also the other mechanical elements in the system such as the gearbox and lead screw assembly. Other factors such as positioning accuracy, resolution, repeatability, maximum roll, pitch, and yaw, etc. are all important considerations to ensure the linear motion system meets or exceeds the application goals.

Achieving precision in a DC motor combined with gears and a spindle involves considering various factors. These include the gear ratio, motor control system like PID controllers, mechanical design quality, components’ quality, feedback systems, environmental factors, and calibration. The right combination of these factors can result in high precision suitable for robotics, CNC machines, and automation systems.

Motor sizing is crucial and involves analyzing each axis independently for load. Different motor types such as DC stepper, DC brush servo, and DC brushless servo have their strengths and weaknesses, impacting factors like torque, speed, cost, and complexity.

Comparison of different DC motors with Linear Piezomotor

DC Stepper Motor

Strengths:

Open-loop positioning – No encoder required.
Simple “pulse and direction” signal needed for rotation.
High torque density at low speeds.
Can be in a “stall” position without exceeding the temperature rating.
Lowest cost solution.

Weaknesses:

No position correction if the load exceeds output torque.
Low power density – torque drops off dramatically at higher speeds.
Draws continuous current, even at standstill.
High iron losses at above 3000 RPM.
Noticeable cogging at low speeds (can be improved with a micro-stepping drive).
Ringing (resonance) at low speeds.

DC Brush Servo Motor

Strengths:

Linear speed/torque curve compared with a stepper.
Low-cost drive electronics (4 power switching devices).
Many different configurations available.
Very smooth operation possible at low speeds.
High power density – flatter torque at higher speeds compared with a stepper.

Weaknesses

Motor draws high current in overload condition.
Encoder needed for closed-loop positioning.
Limited in speed due to mechanical commutation.
Brush wear.
High thermal resistance (copper is in the armature circuit).

DC Brushless Servo Motor

Strengths:

High power density – flatter torque at higher speeds compared with a stepper.
Linear speed/torque curve compared with a stepper.
Electronic commutation – no mechanical brushes.
Low thermal resistance (copper is in the stator circuit).
Highest speeds possible compared with stepper or brush DC motors.

Weaknesses:

Highest cost among the three motor technologies.
Motor draws high current in overload condition.
Encoder needed for closed-loop positioning.
Higher drive complexity and cost (6 power switching devices).
Rotor position sensors required for electronic commutation

Linear Piezo Motor – Strength and weaknesses

Strengts:

Extremely precise positioning at sub-micrometer levels.
High response rates and acceleration.
No backlash or mechanical play.
Compact size and lightweight.
No electromagnetic interference.
Can operate in vacuum or cleanroom environments.

Weaknesses:

Limited force output.
Limited stroke length.
Relatively higher cost.
Sensitive to temperature fluctuations.
Requires complex drive electronics and control algorithms.
Susceptible to wear and aging over time.

Power conversion in the linear motion system starts with understanding load requirements and translates into motor power supply analysis to ensure efficient movement. Each motor type offers distinct advantages and disadvantages, and the choice depends on specific application requirements