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Siemens offers both incremental and absolute encoders. SIMODRIVE sensors are built-on optoelectronic encoders for the recording of paths, angles, or rotary speeds of machines. They can be used in conjunction with:
- SIMOTION - motion control systems
- SIMATIC - programmable logic controllers
- SIMODRIVE - drive systems
- SIMOVERT MASTERDRIVES MC - drive systems
A distinction is made between incremental and absolute measuring procedures.
In the case of incremental encoders, the machine must travel to a reference point after each power-off state, as the position is not usually stored in the controller, and movements of the machine while the power is off are not recorded.
Absolute value encoders, on the other hand, also record these movements while the power is off and return the actual position at power-on. Travel to a reference point is not necessary.
All encoders are available in synchro flange and clamp flange versions. Encoders with a synchro flange can be attached to the machine by means of three clamps. Mounting with axial screws is also possible. The encoder is driven by means of a push-on coupling or a spring disk coupling. Alternatively, pulleys can also be used. The power supply for the encoder is 5 V DC, or optionally between 10 V and 30 V DC. The versions between 10 V and 30 V DC allow longer cables to be used. Most control systems supply the voltage direct from the measurement circuit.
Absolute value encoders (absolute shaft encoders)
Absolute value encoders (absolute shaft encoders) are designed on the same scanning principle as incremental encoders, but have a greater number of tracks. For example, if there are 13 tracks, then 213 = 8192 steps are included in the case of single turn encoders. The code used is a one-step code (Gray code), which prevents any scanning errors from occurring. After switching on the machine, the position value is transmitted immediately to the controller. There is no need to travel to a reference point. The data are transmitted between encoder and controller either via the synchronous serial interface (SSI), via EnDat or via PROFIBUS-DP 1). SSI and EnDat have advantages in time-critical applications.
In plants with a large number of encoders, PROFIBUS DP is more of an advantage due to the reduced wiring expense. The encoders with PROFIBUS DP are programmable.Single turn encoders resolve one rotation (360 degrees mechanical) into a specific number of steps, i.e 8192. A unique code word is assigned to each position. After 360 degrees the position values are repeated.Multiturn encoders also record the number of revolutions, in addition to the absolute position within one revolution. To do this, further code discs which are coupled via gear stages with the encoder shaft are scanned. When evaluating 12 additional tracks, this means that 212 = 4096 revolutions can be included.
Incremental encoders
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These encoders deliver a defined number of electrical pulses for each rotation, which represent the measurement of the distance or angle moved. Incremental encoders operate on the principle of optoelectronic scanning of index disks operating on the transmitted light principle. The light source is a light emitting diode (LED). The light-dark modulation generated as the encoder shaft rotates is picked up by optoelectronic elements. With an appropriate arrangement of the line pattern on the index disks connected to the shaft and the fixed aperture, the photoelectronic elements provide two trace signals A and B at 90° to one another, as well as a reference signal. The encoder electronics amplify these signals and convert them into different output levels.
The following output levels are available:
- RS 422 difference signals (TTL)
- Analog signals sine/cosine with level one Vpp
- HTL (high voltage transistor logic)
In the case of RS 422 encoders (TTL) the resolution can be improved by a factor of four by means of edge evaluation. In order to obtain an even finer resolution, in the case of encoders with sinusoidal signals, these signals are interpolated in the higher-level controller. Encoders with HTL interfaces (High Voltage Transistor Logic) are well suited for applications with counter modules. |
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2008-04-29 | 