The demand for higher accuracy in manufacturing is driving the development of precision machinery and measuring tools. Analytical instruments and manipulators in the medical sector are advancing and getting more exact. The semiconductor industry is continuously working with higher precision instruments to scale down devices on the silicon wafer. Wherever there’s a demand for the highest precision, there’s usually a need for a Piezo LEGS® motor.
Whenever you want micrometer, nanometer, or even sub-nanometer precision and the conventional electrical motor is just not good enough, we have the solution. Our motors are precise down to the nanometer range, have instant response time and do not suffer from backlash problems. Direct drive eliminates the need for gears or lead screws to achieve high-precision movement. Small size and high-force output are further advantages.
Our patented Piezo LEGS® technology has been proven to give customers new possibilities.
Notes on Accuracy
Comparing specifications between different motion systems can be a tedious task since different vendors use different terms for the same specification. What do they really mean? In the following we try to sort this out and hold to the vocabulary given in ISO 5725-1.
Resolution
For a mechanical drive system resolution is the smallest change in position that can be output on an axis. For an encoder, resolution is the smallest increment that the encoder can detect.
Accuracy
According to ISO 5725 two terms, trueness and precision, are used to describe accuracy. How to interpret this is clarified with the figure given below.
Trueness
Is the closeness of agreement between the average of an infinite number of repeated measurements and a reference value (a true value or an accepted reference value). Trueness depends of systematic errors that in repeated measurements remains constant or varies in a predictable manner. The measure of trueness is often expressed in terms of bias. Bias is the difference between the expectation of test result and the accepted reference value.
Precision
Is the closeness between agreement of repeated measurements. Measurement of precision is always made at constant conditions during a short duration of time. In other word precision is how much a measurement result vary under constant conditions. Precision only depends on the distribution of random errors and do not relate to the true or specified value. Precision is normally expressed in terms of imprecision and calculated as a standard deviation of the test results. A large standard deviation reflects less precision.
Repeatability Is precision under repeatability conditions. This mean that independent test results are obtained keeping the measurement conditions constant, i.e. using the same method for identical test items, the same operator within using the same test set-up within a short period of time.
Accuracy can be explained by the figure below. The combination of trueness and precision is shown and how they interact on accuracy. For a motion system the ideal is to be at the upper right corner since then accuracy will be high. At the lower right accuracy can be improved by calibration. The key is to have high repeatability.
Interaction between trueness and precision and the effect on accuracy.
In a drive system accuracy depends on several factors such as backlash, friction, wear, hysteresis, drift, nonlinearity of drive or measurement system etc. Some of these factors can be handled by measuring directly at the point of interest. For example, a linear stage can have a built-in encoder, thus measuring directly on the moving part of the stage. Another method is to measure directly on the object to be moved with a laser interferometer.
Some advantages given in terms of accuracy when using a PiezoLEGS motor is worth mentioning. The motor relies on friction contact between a piezomaterial and a moving rod which is attached to the object to be moved. The PiezoLEGS motor has a true direct drive since the piezomaterial always is in direct contact with the drive rod. This reduces the number of motion components to a minimum, to the motor itself without relying on any further mechanical components. The inherent stiffness of the piezomaterial can thus be fully utilized to give a stiff motion system with no backlash, low friction and wear etc. Further, wear of the piezomotor system is minute and give a stable performance over time with repeatable performance. Tuning of such a system is often easier since the overall system is stable and predictable. Since the resolution of PiezoLEGS motors is at the subnanometer level, the limiting component in a PiezoLEGS driven system is often the encoder. PiezoLEGS motors thus provide means for reaching high accuracy in motion systems.
Interaction between trueness and precision and the effect on accuracy.