Position or time delay in signals is common to all encoders. It can be corrected if needed in the control system by putting in a factor derived from the calculated delay.
At high rotation speed a position delay between magnet position and the electrical output appears because of filtering. Filtering is carried out with an RC filter. Position delay can be calculated with formula:
Δφ = -Arctan{ƒ/ƒ0} (ƒ = frequency, ƒ0 = (2πRC)-1)
For the AM256 and AM512B (8, 9 bit encoder ICs) and up to 9 bit RM/RE, the value of the resistor is 10 kΩ and the recommended value of capacitor is 10 nF.
|
Parameter |
Symbol |
Typ. |
|
Position delay @ 10 Hz (600 rpm), |
Δφpos |
0.36° |
|
Position delay @ 100 Hz (6000 rpm), |
Δφpos |
3.6° |
For 10+ bit encoders the resistor value is 10 kΩ and the value of capacitor is 22 nF.
|
Parameter |
Symbol |
Typ. |
|
Position delay @ 10 Hz (600 rpm), |
Δφpos |
0.8° |
|
Position delay @ 100 Hz (6000 rpm), |
Δφpos |
7.87° |
Using the above equation, we can therefore calculate the position delay in both degrees and time. In the following example we have a 10 bit encoder rotating at 1000 rpm.
|
1000 rpm |
= 16.7 Hz |
|
|
ƒ0 |
= (2πRC)-1 |
|
|
|
= (2 x π x 10 x 103 x 22 x 10-9)-1 |
|
|
|
= (1.38 x 10-3)-1 = 723.43 |
|
|
Δφ |
= tan-1{ƒ⁄ƒ0} |
|
|
|
= tan-1{16.7⁄723.48} = tan-1{0.02308} = 1.3° |
|
|
@ 1000 rpm - 1 rotation = 0.06 s |
||
|
|
(0.06⁄360) x 1.3 = 2.16 x 10-4 = 0.216 ms |
|
