How to choose servo motors and drives
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.
Defining goals[edit | edit source]
The first step is to define goals of the system. Find numbers to the following questions:
- How much is the required linear force?
- How much is the maximum desired speed?
- What is the required accuracy (resolution, precision, repeatability)?
- 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:
|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|
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.
|Ball screw (rolled)||Medium||Medium||High||Medium|
|Ball screw (ground)||High||High||High||Medium|
|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.