Linear encoders are used in many motion control systems as direct positional feedback for the control loop. Conventional linear encoders can measure only one degree of freedom, rendering them blind to unavoidable errors in other directions. In multi-axis systems, an error in one axis carries over into the other axes, thereby affecting the entire motion system. Error in the first axis changes the actual position of the second encoder, and so on, yet these errors are not normally measured.
HEIDENHAIN is now developing MULTI-DOF (Degree Of Freedom) encoders allowing measurement in more than one degree of freedom, enabling higher accuracy and greater dynamic performance – key requirements for improving production processes.
Dplus encoders measure multiple degrees of freedom on a single machine axis, thereby directly and precisely measuring errors and the machine deviations they cause. Dplus encoders provide exceptional possibilities for optimizing your motion system, particularly when high dynamic performance and accuracy are called for.
The interferential measuring principle generates signals by utilizing the refraction and interference of light on finely divided gratings. The measuring standard consists of a flat surface with 0.2 μm-high reflecting lines. These lines are read by a scanning reticule featuring a light-permeable phase grating with an identical graduation period.
Interferential encoders use signal periods of 4 μm or less, and these largely harmonics free scanning signals can be highly interpolated. Consequently, these encoders are ideal when high resolution and accuracy are required.
The Dplus encoders, such as the LIP 6000 Dplus, have a carrier with two separate graduation tracks featuring diagonal graduations (±45°), thus permitting direct, high accuracy measurement of the primary and secondary directions along the entire measuring length.
A body in space can move along six possible axes. These are divided into translational degrees of freedom (X, Y, Z) and rotational degrees of freedom (RX, RY, RZ).
Normally, measuring multiple degrees of freedom requires numerous components.
Standard encoders require one scanning head and one measuring standard for each degree of freedom. The Dplus encoders from HEIDENHAIN, however, can significantly reduce the number of components required.
A Dplus scale with two separate graduation tracks and three scanning heads on the same scale, for example, can measure up to three degrees of freedom. This technology makes it possible to implement complex measuring tasks in a simple and compact design.
Out of Plane Gap Measurement
Conventional encoders can measure only one degree of freedom at a time. Dplus encoders, however, can measure up to three degrees of freedom in the encoder plane, such as X, Y and RZ. Additional measurements in a different plane would require additional encoders and a more complex system design.
The GAP 1081 gap encoder performs vertical measurement, enabling highly convenient and space-saving system expansion for additional directions. Because its components are mounted in the encoder’s main plane, the GAP 1081 delivers rapid measurement directly at the machine.
This encoder can be used for straightforward vertical positioning tasks and continuous vertical measurement along a linear plane. Two scanning heads deployed on a mirror can even measure the pitch or yaw of the given axis, thus greatly simplifying the metrology system design and reducing the required installation work.
The use of multiple encoders increases overall system cabling, making installation and downstream processing more complex. With the EnDat3 interface, HEIDENHAIN offers the optimal solution for transmitting a wide range of data on just one cable. Two position values, for example, are calculated in the interface PCB of a Dplus encoder and transmitted over a single cable. For single-cable transmission, the multi-head processing electronics process the position signals of multiple encoders. This permits convenient implementation of complex metrology system designs without extensive cabling or separate position value processing.
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