By Tom Wyatt, HEIDENHAIN Corporation, Product Manager, Automation
Controlled servo drives function in many areas of automation technology, from robotics and handling systems to the drive technology of production machines and machine tools. The requirements of speed stability and rigidity necessitate ever increasing gain factors in the control loops. Your choice of position encoders influences such important motor properties as positioning accuracy, speed stability, and bandwidth for drive command-signal response and disturbance rejection, as well as power loss and size.
The techniques encoders use to determine position resolution and position error within one signal period determine its capabilities. Typically, these techniques focus on scanning and signal processing.
One widely used operating principle for encoders is that of photoelectrically scanning a regularly structured measuring standard. In this imaging scanning procedure, as used by absolute rotary encoders for example, a structured graduated disk rotates relative to a scanning point. Photovoltaic cells convert the incident light into electrical signals. The absolute position information is determined from the graduation on the graduated disk.
A new scanning technique for absolute rotary encoders uses a serial-coded absolute track to offer high quality signals and reduces the encoders’ sensitivity to contamination. The scanning and evaluation unit permits both high interpolation and optimal control loop performance. The rotary encoder transmits reliable digital interpolation and position value data without burdening the higher-level electronics. In addition, because these encoders comply with IEC 61 508 safety-related functions, they are suitable for use in SIL-2 applications. These changes have no effect on the dimensions of the encoders; and no mechanical changes of the motor dimensions are required.
The new scanning technique developed by Heidenhain uses a large photosensor that sends light through two tracks, one which is a pseudo random code and the other, incremental. This technique reduces the number of tracks of typical Gray-codescanning from 13 to 2.
A closer look at the new disks. The left photo shows a typical Gray-code disk. The right photo shows the graduated disk with the serial code track and the incremental track.
A closer look
The new scanning technique reduces the amount of tracks required when using the gray code method. It generates the absolute position value from two graduation tracks. Absolute information is encoded on one track of a newly developed graduated disk. The pattern of this serial code structure is unique over one revolution. The incremental track is located next to it. It is interpolated for the position value and at the same time is used to generate an optional incremental signal.
The information in the two tracks is processed in integrated electronics to form an absolute position value with a resolution of about 33 million measuring steps per revolution.
An optical filter in the scanning of the incremental track produces homogenous signals. Instead of the usual individual photovoltaic cells, one large-area specially structured photosensor generates scanning signals with a good sinusoidal shape and high consistency, even at different speeds, for each graduation track.
The consistently good quality of the sinusoidal scanning signals from this scanning technique significantly reduces the position errors within one signal period. This is why reliable 14-bit interpolation is now possible in a rotary encoder. Such high resolution allows the fine measuring steps necessary for high control quality and finely tuned velocity control.
The serial code structure, located in just one track, combined with the scanning technique, enables the encoder to offer high accuracy independent of the speed. Thus, whether you are operating at the maximum speed or nearly at a standstill, the positioning accuracy is always consistently high.
Theoretically, contamination is not an issue with sealed rotary encoders integrated in electrical motors. In practice, however, traces of oil from bearings, dust from the brakes and out-gassing from the motor coil can intrude into the rotary encoder. Because of the large scanning area over the entire width of the scale grating and the special arrangement of several scanning fields, encoders with enhanced scanning are not sensitive to contamination.
The new technique “optimizes” the scanning signals, as shown by the Lissajous pattern.
Position error is greatly reduced using the new scanning technique.
The new technique also improves encoder accuracy, which enables tighter position control so needed in today’s equipment.
Better “smarts”
The reduction of individual components improves reliability during manufacture and use. The most important factor here is the reduction of the scanning tracks from 13 to just 2, as well as the high degree of interpolation of the new scanning and evaluation electronics. These electronics also make it possible to increase the range for the power supply voltage, from 3.6 V to 14 V. This means that you do not need to monitor the encoder’s supply voltage, and therefore do not need sensor lines and a controllable power supply in the subsequent electronics.
For applications with safety considerations, engineers can use a system composed of an encoder, a data transmission line, and the controller for the communication link.
The combination of the new scanning and evaluation electronics with the EnDat interface offers important advantages. Since interpolation takes place within the rotary encoder, it is possible to have purely digital data transmission, which is less sensitive to noise. Because lines for the analog signals are no longer needed, the connection cables are less complex, enabling the use of simpler and thinner cables, as well as smaller connecting elements.
In addition, the entire position value is immediately available to the subsequent electronics, avoiding time-critical interrogations and calculation operations. The EnDat interface makes extensive monitoring and diagnosis of an encoder possible without an additional line. The diagnostic system automatically generates error messages and warnings. For encoders with purely serial interfaces, the valuation numbers provide the current state of the encoder and ascertain the encoder’s “functional reserves.” The valuation numbers can be read from the encoder cyclically via the EnDat 22 interface without influencing the quality of the axis control.
Changes in the scanning electronics reduces the number of components on the printed circuit board.
Safety first
Safety considerations are becoming increasingly important in applications. That is why the new scanning and evaluation electronics were developed based on the safety recommendations of IEC 61 508. Scanning in the encoder simultaneously produces two independent position values that are transmitted using the EnDat 2.2 protocol to the EnDat master in the subsequent electronics. The EnDat master assumes various monitoring tasks, and provides both position values through two independent processor interfaces to the safe control. Safety-related position measuring systems are permitted as single-encoder systems in applications with control category SIL-2 (in accordance with IEC 61 508) or performance level “d” (ISO 13 849).
Original article supplied by Mr. Ludwig Schmidt, DR. JOHANNES HEIDENHAIN, GMBH Heidenhain Corp., [email protected]
Filed Under: Motion control • motor controls, SENSORS