BH8-Series – User’s Manual
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© 2007 Barrett Technology®, Inc.
Document: D3000, Version: AF.00
Page 39 of 82
In addition to the motor feedback data, there is a single global feedback datum, which is sent if its
corresponding global parameter is non-zero:
LFT ("Loop Feedback Temperature"): Signed 2-byte word
The feedback data are sent in a specific order: first all data for motor 1, then all for motor 2, then
motor 3, then motor 4, then any global datum. Note that if a given motor was not specified in the
initiating LOOP command, or if a specific value isn't enabled by the corresponding finger or
global parameter, then the corresponding datum is not transmitted.
If the LFV datum is included, then the hand will divide it by the parameter Loop LFVC before
sending it to the host. Note: the hand treats an unscaled LFV datum as all integer; this is different
from an unscaled LCV datum, which is treated as 4 bits of integer and 4 bits of fraction.
6.2.3
Loop Feedback Delta Position
The LFDP ("Loop Feedback Delta Position") datum is a special case. Each time a motor's position
is queried using "FGET P", the reported position is remembered. In loop mode, if the LFDP
parameter is non-zero then the present position is read and compared to the previously reported
position. The difference is divided by the LFDPC ("Loop Feedback Delta Position Coefficient")
parameter, clipped to a single signed byte, and then sent to the host. The host should then multiply
the received value by LFDPC and then add it to the reported position.
The problem with using delta position is that the reported position can change at most by +127/-
128 in each cycle. If the motor position changes more than this in a single cycle then the reported
position will lag behind the actual position.
Example: say LFDPC is 2, the last reported position was 1500, and the position suddenly jumps to
2000. The first feedback block will include the delta position datum 127, which actually means
254; the hand will internally update the reported position to 1754. The next feedback block will
include the delta position 123, which actually means 246; the reported position will be updated to
2000. Subsequent feedback blocks will include the delta position value 0 (until the next position
change).
If desired, any unreported position change can be discarded by setting the LFDPD ("Loop
Feedback Delta Position Discarded") global parameter to true. With this set, the above example
would result in the single value 127 being sent to the host, followed by 0s.
6.3 Parameter Summary
This is a summary of the different motor and global parameters, which affect RealTime mode.
Most of the parameters are flags, specifying whether a specific datum is to be present in a control
or feedback block. The four remaining parameters are coefficients or flags, which affect how the
firmware interprets or generates a datum.
