Resolution
This can be defined as one of two things. The theoretical resolution of a motion system is the smallest step that can be measured by the encoder. This does not necessarily equate to the smallest step size the system can make. Where the resolution does refer to the smallest mechanical step this can also be defined as the minimum incremental motion. Make sure that a specification sheet properly distinguishes between the two uses of the term.
Minimum Incremental Motion
The smallest step size that a positioning stage can actually make (see resolution above). With a mechanical stage this will usually be limited by stiction and friction in the bearings and screw thread. With a piezo driven actuator or flexure stage it is the encoder or electronic noise in the drive circuit that is often the limiting factor.
Repeatability
The accuracy with which a motion system returns to a particular position. This is often confused with accuracy when defining system requirements. There are two types of repeatability.
Uni-directional Repeatability
How accurately a system returns to a defined position when always approaching it from the same direction. In applications where positioning speed is not critical, but repeatability is important, overshooting the target and approaching it always from the same direction can significantly improve the performance. This is especially the case with mechanical micropositioning systems where there is backlash.
Bi-directional Repeatbility
Also known as backlash this is how accurately a positioning system returns to a particular position when approaching it from any direction. With closed loop piezo actuators and flexure stages the uni and bi-directional repeatabilities are the same as there is no backlash.
Backlash
This is the mechanical slack in a positioning system that has to be overcome when changing motion direction. One way to minimise the affect of backlash is to use an encoder that directly measures the motion of the moving platform.
Hysteresis
The effect of historical movement on the position of the object in question. In a mechanical system this can be equated to backlash. In an open loop piezo system hysteresis is caused by charge stored in the actuator.
Linearity
How closely a positioning system follows a linear response over a defined portion of its travel range. Can also be defined as accuracy. In a piezo system the type of metrology used and the sophistication of the controller will impact upon what can be achieved. PI can supply piezo systems with a linearity of better than 0.05%. In a motorised mechnical system the best linearity is achieved by using a linear encoder directly measuring the moving platform. If remote encoding is used the linearity is then affected by inaccuraciesin the screw thread.
Accuracy
When moving a defined step, how precise (within what tolerance) is the movement. Often mistaken for resolution or repeatability.