How do Absolute Encoders work?
To solve positioning problems in automation, it is often necessary to measure lengths and angles as exactly as possible. In general there are two different measuring systems:
Incremental Measuring Systems
The principle of the incremental measuring system is the scanning of a line pattern on a glass or plastic disc (see Image 1).
Image 1: Incremental Disc
Using this measuring system, every position of the measurement range/angle is identified by a definite code on a glass or plastic disc. This code is represented on the disc in the form of light and dark regions within different tracks. This combination relates to an absolute numerical value. Thus, the position value is always directly available, counters are not necessary. In addition it is not possible to get continuously invalid values caused by interferences or loss of the supply voltage. Movements which are done while the system is turned off are immediately measured after the system is powered up.
Image 2: Code disc with Gray-Code
The measuring system consists of a light source, a code disc pivoted in a precision ball bearing and an opto-electronic scanning device (see Image 3). A LED is used as a light source which shines through the code disc and onto the screen behind. The tracks on the code disk are evaluated by an opto-array behind the reticle. With every position another combination of slashes in the reticle is covered by the dark spots on the code disk and the light beam on the photo transistor is interrupted. That way the code on the disc is transformed into electronic signals.Fluctuations in the intensity of the light source are measured by an additional photo transistor and another electronic circuit compensates for these. After the electronic signals are amplified and converted they are then available for evaluation.
Image 3: Construction Absolute Encoder
Image 4: Principle of the Multi Turn
The total number of steps in this example is 16 x 16 x 16 x 8192 steps = 33.554.432 steps Binary.
All bits of a position are transferred simultaneously using one line for each bit. Data transmission is done by two transistors in push pull circuit. For example the signals can be evaluated via digital entries of an PLC. The conversion from gray to binary code has to take place in the control system, since the code of this method is transmitted directly.
The bit parallel interface is a very fast and for low resolutions cheap possibility of data transmission. For high resolutions or machines of bigger size installation costs can rise rapidly so that other methods of data transmission are more favorable.
For machines where several axis have to be automated (e.g. robots), the cabling of rotary encoders with bit-parallel interface can become a problem, especially when high resolutions are necessary.
Image 11 Relationship between transmission length and transmission rate
Image 12: Transmission
A multiple transmission of a position value is possible with doubling or multiplying the clock sequence. It is very important that a clock sequence includes n + 1 = 26 clocks for multi turn and n + 1 = 14 clocks for single turn.
After the last Low-to-High transition of a 26-clock sequence, a "L" signal appears on data output. The double (or multiple) successive position values are separated from another with this information (see Image 13).
Image 13 Multiple Transmission Multi Turn
CAN stands for Controller Area Network and was developed by the company Bosch for applications within the automobile area.
FRABA rotary encoders support two CAN protocols: CANopen and DeviceNet.
The COB-Identifier, which determines the priority of the message, is made from the function code and the node number.
PDOs (Process Data Objects) are needed for real time data exchange. Since this messages possess a high priority, the function code and therefore the identifier are low. SDOs (service data objects) are necessary for the bus node configuration (e.g. transfer of device parameters). Because these message telegrams are tranferred acyclicly (usually only while powering up the network), the priority is low.
FRABA-Encoder with CANopen-Interface
Further functions (direction of rotation, resolution,etc..) can be parameterized. FRABA rotary encoders correspond with the class 2 profile for encoder (DSP 406), whereby the characteristics of rotary encoders with CANopen interface are defined. The link to the bus is made by terminal blocks in the connection cap. In additon, the node number and Baud rate are set with turn switches. For configuration and parameterization various software tools are available from different providers. With the help of the provided EDS file (electronic datasheet) simple line-up and programming are possible.
The DeviceNet protocol is based on the system of connections. In order to exchange information with a device, a connection must first be established. The CAN identifier is used for the characterisation of this connection.
Additional parameters are also programmable such as direction of rotation, resolution and preset value. The adjustment of the node number and the Baud rate takes place in the connection cap using the turn switches. Easy programming and configuring is possible using the provided EDS file (electronics data sheet) with popular configuration tools
The data transfer is done using the “shift register with sum framework protocol” (in a data cycle all data is shifted through the ring).
FRABA Encoder with INTERBUS-Interface
Profile K1: not programmable 16 Bit process data
FRABA rotary encoders can be delivered in K1, K2 and K3.
9.1 INTERBUS Loop 2
9.2. INTERBUS LWL
For applications demanding for high noise immunity or high data transmission rates, fiber-optic cables are available as an alternative to the conventional transmitting media. The SUPI 3 OPC (Optical Protocol Chip) is used for these demands. It enables a distance diagnosis and optical power adjustment for LWL transmitters. The fibre optical cable can be easily connected to an existing INTERBUS network with a bus clamp. Advantages of this system are high noise immunity and also data transmission rates of up to 2 MBit/s.
Software limit switches function:
Zero point displacement