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EZMotion User/Programming Guide 81
Revision A8 www.controltechniques.com
= 2279 - 1375 + 0
= 904 revs
Note that this value is NOT correct. This is because after 5000 revs, we have exceeded the absolute resolution of the encoder
in a single direction.
So you can see that when using One-Sided Mode, as long as the motor does not move more than 4096 revolutions in the
positive direction from the home position, EZMotion can correctly calculate the absolute machine position on power up.
Two-Sided Mode
Two-Sided Mode implies that once the absolute home position has been defined, the user wishes to distribute the full multi-
turn resolution of the encoder evenly in BOTH DIRECTIONS from the home point. Using the example of a 12-bit multi-turn
absolute encoder, the motor could then travel 2048 revs in either the positive or negative direction without experiencing any
problems due to absolute rollover. IMPORTANT NOTE: In Two-Sided mode, if the motor moves in excess of 2048 revs in
either direction from the home position, EZMotion cannot detect this condition, and therefore the absolute position would be
incorrectly calculated on the next power up.
Example:
The user homes the motor to a sensor. When the sensor activates, the motor is at the machine home position P
M
= 0 revs
(where P
M
is the Position feedback of the Machine). When the motor is at the home position of 0 revs, the absolute encoder
reads P
A
= 1375 revs (where P
A
is the Position feedback of the Absolute Encoder). In this example, 1375 is just a randomly
chosen number for demonstration purposes. Therefore, EZMotion now uses this relationship of P
M
and P
A
at the home
position to calculate the machine position (P
M
) on power up.
Position Feedback of Machine when at Home = P
MH
= 0
Position Feedback of Absolute Encoder when at Home = P
AH
= 1375
We then need to calculate the value equal to half the resolution of the encoder (E
half
), and the position that distributes the
encoder resolution in half. This is the position feedback of the absolute encoder at the rollover point (or P
AR
).
E
half
= Max Resolution / 2
= 4096 / 2
= 2048
P
AR
= P
AH
+ E
half
= 1375 + 2048
= 3423
P
AR
must be within the max resolution of the encoder, so if P
AR
calculated above is > Max Encoder Resolution we need to
subtract the max resolution:
P
AR
= P
AR
- Max Resolution
If the motor then moves 1000 revs in the positive direction, P
M
will be P
MH
+ 1000 or P
M
= 1000,and P
A
is P
AH
+ 1000 or P
A
=
2375. If the machine is powered-down and then back up, EZMotion would read the position from the absolute encoder P
A
,
subtract the position of the absolute encoder at the home position P
AH
, and then add the position feedback of the machine at
the home position P
MH
, to get P
M
.
Calculation #1: Used when P
AR
> E
half
AND P
A
< P
AR
P
M
= P
A
- P
AH
+ P
MH
= 2375 - 1375 + 0
= 1000 revs
If the motor is again at the home position and then moves 1000 revs in the negative direction, P
M
will be P
MH
- 1000 or P
M
=
-1000. If we power down, and then back up, P
A
would be P
AH
- 1000 or P
A
= 375. Once again, the conditions for Calculation #1
are met, so we will use it again to find P
M
.
P
M
= P
A
- P
AH
+ P
MH
= 375 - 1375 + 0
= -1000 revs
If we now start from the home position again, and move the motor 2000 revs in the negative direction, P
M
will be P
MH
- 2000
or P
M
= -2000. If we power down and then back up, P
A
should be P
AH
- 2000, or P
A
= -625. However, we know that since this
is a 12-bit multi-turn absolute encoder, when traveling in the negative direction, the absolute encoder rev counter will reach 0
and rollover to 4096. Therefore, we will never get a value of -625 on the absolute encoder. In this case, the absolute encoder
would read P
A
= (P
AH
- 2000) + Max Encoder Resolution = 3471. Since the user has defined the application type to utilize
Two-Sided Mode, EZMotion can interpret this position from the absolute encoder to calculate the correct machine position. If
the machine was powered-down, and then back up, the P
M
would be calculated as follows:
Calculation #2: Used when P
AR
> E
half
AND P
A
> P
AR
P
M
= P
A
- P
AH
- Max Enc Resolution + P
MH
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