Geo Brick Drive User Manual
Appendix D
292
APPENDIX D
Absolute Serial Encoders Limitation With Turbo PMAC
The following is a summary of certain limitations which could be encountered with higher resolution
absolute serial encoders, and a description of related registers with respect to the proposed setup
techniques. Note that techniques 1 and 3 are processed in the Encoder Conversion Table (ECT) using the
standard 5-bit shift, whereas technique 2 is processed with no shift.
Quick Comparison
Parameter/Description
Technique 1/3
Technique 2
Units
Resolution
Scale Factor (SF)
Rotary
SF =2
ST
SF =2
ST-5
counts/revolution
Linear
SF =1/RES
SF =1/(32*RES)
counts/user unit
Maximum open-loop velocity
2
18
*
ServoClk
counts/msec
Maximum closed-loop velocity
2
23
*3/(Ixx08*32)
counts/msec
Maximum travel
before rollover
Rotary
2
47
/SF =2
47-ST
2
47
/SF =2
47-(ST-5)
revolutions
Linear
2
47
/SF
user units
Where ST:
is the rotary encoder Singleturn resolution in bits
RES:
is the linear encoder resolution in user units (e.g. mm)
ServoClk:
is the PMAC servo update rate in KHz
Ixx08:
is Motor xx’s position scale factor
Resolution Scale Factor (SF)
Turbo PMAC expects the motor count Least Significant Bit LSB to be left-shifted (5 bits), per techniques
1 or 3. The only difference then with technique 2, when unshifted, is that the motor position loop will
now consider 1 LSB of the source to be 1/32 of a motor count, instead of 1.
Example:
Take a 37-bit absolute serial rotary encoder (25-bit single turn, 12-bit multi-turn) and its
equivalent linear scale (e.g.10 nm resolution):
Technique 1/3
(5-bit shift)
Rotary
2
ST
2
25
= 33,554,432
counts/revolution
Linear
1/RES
1/0.00001= 100,000
counts/mm
Technique 2
(no shift)
Rotary
2
ST-5
2
20
= 1,048,576
counts/revolution
Linear
1/(32*RES)
1/32*0.00001= 3,125
counts/mm
Note
Regardless of the processing technique, the servo algorithm utilizes
“internally” the entire data bits stream (i.e. 25 bits) for its calculation.
The performance is not compromised.
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