How to choose servo motors and drives

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Choosing suitable servo components for a linear motion machine is straightforward process. This article gives a method for choosing suitable servo motors and drives for a machine with linear axis, such as CNC, 3D printing or pick'n'place systems.

A typical lead screw driven linear axis comprising linear guides, lead screw and a motor. To drive this, a motor driver and power supply is needed.

Defining goals[edit | edit source]

The first step is to define goals of the system. Find numbers to the following questions:

  1. How much is the required linear force?
  2. How much is the maximum desired speed?
  3. What is the required accuracy (resolution, precision, repeatability)?
  4. What is the required stiffness / nature of mechanical load (stationary/vibrating)?

Choosing electromechanical parts[edit | edit source]

Choosing linear guides[edit | edit source]

Selection of linear guides have different choices to match desired speed range, stiffness, precision and cost. The following table summarizes most common choices with pros and cons:

Type Cost Precision Stiffness Speed capability
Round linear bearing (sliding) Low Low Low/moderate Medium/high
Round linear bearing (ball) Low/medium Medium Medium (Supported only at end points) Medium
Track linear guide Medium/high High High (Supported from whole length) Medium
Dovetail Low/medium Medium/high Medium/High Low
Roller Low/medium Medium Medium High

Choosing rotary-to-linear transmission[edit | edit source]

A typical solution to produce linear motion from rotary motor is to use a lead screw. There are mainly two kinds of lead screws: trapezoidal (sliding) and ball screw. Another common solution is to use belt or rack and pinion system.

Type Cost Precision Stiffness Speed capability
Trapezoidal screw Low Low Low/medium Medium
Ball screw (rolled) Medium Medium High Medium
Ball screw (ground) High High High Medium
Timing belt Low Low Low High
Rack and pinion Low Low Low High

Transmission ratio and it's effect to performance[edit | edit source]

The resolution of transmission comes mainly from the transmission ratio: how much linear motion is produced per one revolution of motor (i.e. pitch of lead screw). Typical screw transmissions are between 2 mm/turn to 10 mm/turn while typical timing belt or rack & pinion systems have ratio of 20 to 100 mm per turn.

Higher ratio leads to higher speed capability but same time reduces stiffness, linear force and precision and vice versa.

Choosing motor[edit | edit source]

Choosing motor drive[edit | edit source]

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