Position determination
A rotary encoder typically outputs 100..10,000 sine/cosine periods per revolution. The EL5021 can continuously measure, interpolate and convert these two differential analog signals into a position value consisting of
- the number of periods --> period counter
- and the current position within the period --> period portion 0..360°
According to the customer's request, the maximum period portion can be determined with a resolution of 8..13 bits, equivalent to 256.. 8192 steps per 360° sine period. This is set in the CoE, index 0x8001:11 "Analog resolution". This is the theoretical maximum resolution desired by the user, which is also achieved at a standstill or during slow movement.
The actual current resolution depends, however, on the current input frequency of the sine/cosine signal.
- 1. The resolution automatically decreases as the frequency increases; the bits are frozen starting from the least significant bit (LSB). If the maximum period resolution is set to 12 bits, 6-bit period resolution will still be achieved at the maximum input frequency of typically 80 KHz.
- 2. If the frequency slows down significantly, the resolution actually achieved increases again.
- There is no online message about the currently applied frozen bits. See also the sample program.
The conversion to real mechanical revolutions depends on the number of periods per revolution of the encoder employed and is calculated in the PLC or NC.
Preset period resolution [bits] | fmax [kHz] | Typical frequency (±15%) at which the automatic reduction of the preset period resolution begins | Min. period resolution at fmax |
|---|---|---|---|
8 | 250 | 9000 Hz |
|
9 | 4500 Hz |
| |
10 (default) | 2500 Hz |
| |
11 | 170 | 1500 Hz |
|
12 | 80 | 750 Hz | 6 bits |
13 | 40 | 650 Hz |
|
If fmax is exceeded, a frequency error is displayed (index 0x6001:04 "Frequency error"),
amplitude errors (level too low, input below limit) are displayed in index 0x6001:05 "Amplitude error".
The position is indicated in 32 bits as a composite process data. Depending on the desired max. resolution, these 32 bits are composed, starting from the right (LSB), of:
- Analog part: 8 ... 13 bits according to setting in index 0x8001:11 "Analog resolution"
- 24 ... 19 bits for the number of periods, extent given by the number of bits available.
Hence, a sequential 32-bit position value is available that can be linked directly to the NC, for example. The conversion factor in bits/mm must then be set in the NC, taking into account
- the no. of sine periods per mechanical revolution, e.g. 5000
- the selected interpolation resolution, e.g. 10 bits
 | Resetting of the counter value on change of period resolution When changing the period resolution in CoE (index 0x8001:11"Analog resolution") and "Enable C reset" (index 0x8000:01) the current counter value is reset. |
| Display "Period resolution 8-bit" In the setting “Period resolution 8-bit”, the display of the 32-bit process data corresponds to the EL5101 Encoder terminal with activated micro-increments. |