Comparing motor technologies for linear motion

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 and trade-offs.