Linear Encoders For Linear Motors, Part II

Position deviations within a signal period
Position deviations over relatively large measuring lengths of, for example, one meter, and the position deviations within one signal period have a definitive influence on the accuracy of linear encoders. The position deviations within one signal period are determined by the quality of the scanning and the signal period of the linear encoder. They influence the control behavior of the drive-in as much as the commutation influences of the linear motor or other effects are not dominant.

For HEIDENHAIN encoders, the relative position deviations within one signal period over the entire measuring length lie at a maximum ±1% of the signal period for exposed linear encoders, and ±2% of the signal period for sealed NC linear encoders. This high quality output signal is attained partly through optical filtering.

The position deviations of the linear encoder within one signal period have an influence on:

	The attainable range of the motor
	The speed constancy of the motor
	The power loss of the motor

Mounting mode
The linear encoder should always be installed so that the scanning head or scanning unit moves. This minimizes the moving mass and provides the greatest possible rigidity in measuring direction. All following values for compliance and acceleration apply for this mounting mode.

Compliance of the connection to the machine
Frequently, the encoder is fixed too flexibly to the machine. On linear motors, this can limit the frequency range of the position control loop. Often it is falsely assumed that the fault lies with the linear encoder itself. For machine design, this means that the connection of the scale and scanning head of exposed linear encoders, or the scale housing and mounting block of sealed linear encoders, must have a natural frequency significantly above that of the linear encoder.

Moreover, it must be ensured that the linear encoder is not fixed to the machine at locations of vibration. In particular, the points of maximum vibration are to be avoided.

Compliance of the internal coupling in sealed linear encoders
In order to attain relatively high mounting tolerances, the scanning carriage of linear encoders is elastically connected to the mounting block by a spring. If this connection is relatively flexible, it results in feedback with low natural frequencies that can limit the frequency range.

The sealed linear encoders recommended by HEIDENHAIN for linear motors usually have natural frequencies in measuring direction from 2 kHz to 4 kHz and therefore, for most applications, lie above the first mechanical natural frequency of the machine by a factor of 5 to 20, and also by whole-number multiples above the range of the velocity control loop. The natural frequency of the linear encoder in measuring direction can therefore be ignored in comparison to this mechanical natural frequency, which determines the realizable range of the control loops and specifically of the velocity control loop.

Maximum permissible traversing speed
Relatively high rapid traverse speeds are desired for machine tools with linear motors in order to reduce the times between cycles. The sealing lips of the linear encoders recommended for linear motors are presently approved for traversing speeds up to 120m/min. The same applies to the guide for the scanning carriage. Lower values for traversing speeds sometimes result for the -3 dB cutoff frequency of the output signals or the maximum input frequency of the subsequent electronics.

Photo 1: HEIDENHAIN LIDA 101 exposed linear encoder for
measurement inspection applications.

Maximum permissible acceleration in measuring direction
Exposed linear encoders:
Maximum acceleration is limited only by the mass and stiffness of the internal components. Accelerations up to 200 m/s² are permissible for these encoders (Photo 1).

Sealed linear encoders:
Due to the relatively low mass of the scanning carriage, the pulling force of the spring allows a maximum permissible acceleration in measuring direction of 50 m/s² to 100 m/s², depending on the encoder.