Relationship Between Maximum Movement Speed, Resolution And Edge Separation; Important - Balluff BML Información Básica

Interfaces for BML Magnetic Encoder System
2
Interfaces (continued)
2.8.2 Relationship between maximum movement
speed, resolution and edge separation

Important!

– The controller must be able to count the
specified minimum chronological edge
separations indicated in the tables for the
sensor head (see e.g. Tab. 2-4 and Tab. 2-5)
(note counting frequency of the controller!).
– The min. edge separation can even occur at
a standstill due to the internal interpolation
process.
– Always select the next-higher travel speed or
the next-largest min. edge separation,
otherwise position detection errors may be
created by the controller during measured
value evaluation.
V corresponding to edge separation and
max
Min. edge
separation
1 µm
0.11 µs 5 m/s
0.26 µs 2 m/s
0.42 µs 1 m/s
0.94 µs 0.6 m/s
1.8 µs 0.3 m/s
3.5 µs 0.15 m/s
7 µs 0.079 m/s
14 µs 0.039 m/s 0.079 m/s
21 µs 0.026 m/s 0.052 m/s
Tab. 2-4: Example of a selection table for max. travel speed,
resolution and min. edge separation
The min. edge separation determines the min. count
frequency which the processor must be capable of:
Minimum count frequency = 1/ minimum edge separation
Accordingly the sampling frequency must be double the
value of the count frequency.
The signal frequency of the A- and B-signals is ¼ of the
minimum count frequency. The relationships are shown in
Tab. 2-5.
Min. edge Min. count
separation frequency
[µs] [kHz]
0.11 9091
0.26 3846
0.42 2381
0.94 1064
1.80 556
3.50 286
7.00 143
14.00 71
21.00 48
Tab. 2-5: Example of a table for minimum edge separation
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resolution
Resolution
2 µm 5 µm 10 µm
10 m/s 20 m/s 20 m/s
4 m/s 10 m/s 20 m/s
2 m/s 6 m/s 12 m/s
1.2 m/s 3 m/s 6 m/s
0.6 m/s 1.6 m/s 3.2 m/s
0.3 m/s 0.79 m/s 1.5 m/s
0.15 m/s 0.39 m/s 0.79 m/s
0.19 m/s 0.38 m/s
0.13 m/s 0.26 m/s
Min. sampling Signal
frequency frequency [kHz]
[kHz] Fundamental
18182 2273
7692 962
4762 595
2128 266
1111 139
571 71
286 36
143 18
95 12
Determination of a suitable sensor for the available
controller:
Example (see Tab. 2-4):
Assumptions:
– Your controller can detect a min. edge separation of
0.5 μs. If there is no sensor with this min. edge
separation, select a sensor with a larger edge
separation.
– The max. movement speed of the system should be
1 m/s.
Determination of a suitable sensor:
– You need a sensor with min. edge separation 0.94 μs
– To be able to travel at max. 1 m/s, select the type with
a resolution of 2 μm
Determination of a suitable controller for the existing
sensor
What max. counting frequency is required of the controller?
The period of the input signal is four times the edge
separation.
The max. frequency of the input signal is then 1/( 4 × edge
separation).
Example:
For an edge separation of 0.94 μs the max. frequency of
the input signal is 1/(4 × 0.94 μs) = 266 kHz.
The max. counter frequency for a 4x evaluation = 1/edge
separation = 1/0.94 μs = 1.064 MHz.
The minimum count frequency which the controller must
be capable of at minimum count frequency = 1/minimum
edge separation is determined from the minimum edge
separation.
Accordingly the sampling frequency must be double the
value of the count frequency.
The signal frequency of the A- and B-signals is ¼ of the
minimum count frequency. The relationships are shown in
Tab. 2-2.