Optical Encoder

Any motion system in which position feedback is required could make use of an encoder. In certain cases, other instruments such as interferometers are used, but an encoder is the easiest, cheapest, and most common method of generating feedback. This goes for rotation stages, linear stages, and goniometer stages, all of which are supported by the Metirio.

The integrated light source illuminates a scale comprising an incremental grating with 20 µm pitch and a track with reference marks for absolute positioning. The reflected light generates an interference pattern which is detected by the diode array within the readhead. A relative movement between the scale and the readhead leads to change of the light hitting these photo diodes. During a continous relative movement the output voltage of these photo-diodes are analog sine and cosine signals. From these signals a relative position can be calculated using the formula given on Key Features of the METIRIO® encoder - SmarAct.

In general, it is important to keep the scale of an optical encoder as clean as possible, and they are less suitable for environments where significant contamination is likely. As the encoder relies on the reflected signal, debris will lessen the amount of power delivered and can alter the detected interference pattern. Despite this, the Metirio is engineered to maintain signal though a contaminated scale. Algorithms can also be applied to the raw quadrature signals to further increase resistance to changing signal quality.

The encoder itself will measure the reference track and output the signals it receives in analog - the Metirio will output the sine, cosine, and TTL reference signals from the read head. Each packaging option will output these same signals, according to the pinout which can be found in its respective datasheet.

Most applications for encoders involve integrating them into a motion system, which will have a controller requiring a certain data protocol. If the system can read the analog signals, such as is the case with SmarAct Motion’s MCS2 controller, only a cable with the proper pinout is necessary. In other cases, the motion system will be compatible with a certain protocol such as BISS-C, or ABZ. These are digital protocols created from the analog signals by a device such as the SmarAct Interface Module ENCODER EVALUATION KIT - SmarAct and will have resolution defined by the number of bits used to interpolate.

In general, the position can be calculated from the analog output  signals by using the formula given in Key Features of the METIRIO® encoder - SmarAct. More specific, the Metirio Evaluation Module (EEM) connects to the encoder, and a PC via USB, and displays the analog encoder signals and resulting position directly. Its corresponding Encoder Evaluation Program can be used to view them on a GUI. Here, the analog sine, cosine, and reference signals are visualized in raw and corrected form. Position data is also calculated, and can be seen in real-time, along with any errors that may be read. This program is intended to be used for first-time setup of an encoder, and can be used to change the optical power of the Metirio before saving those settings to its onboard memory. Please note that the module is limited to 50 kHz and is not intended to be used as part of a motion system.

To view the signals without the EEM, any method of digitization can be used, such as an oscilloscope or USB digitizer, provided a cable or PCB is made to power the encoder and split out the data channels. These signals will then need to be processed by the user to convert them to a position stream.

An optical encoder can achieve higher resolutions than a capacitive or magnetic encoder due to the unique properties of coherence, and the producibility of ultra-fine scale gratings.

The Metirio is compatible with any reflective scale with a 20um pitch. Our scale catalog is available upon request. All packaging variants contain the same read head and are therefore compatible with the same scales – uniquely, it can be used interchangeably with linear, rotary, and goniometer scales.

The Metirio can operate in temperatures from 0° to +90° C and is resistant to mechanical vibration. Additionally, the read head, and A1 package are UHV (10^-11 mbar) compatible.