Delta Tau GEO BRICK LV User Manual
Geo Brick LV User Manual
Appendix D
269
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.
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):
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.
“internally” the entire data bits stream (i.e. 25 bits) for its calculation.
The performance is not compromised.