18th annual conference on manual control.pdf - Acgsc.org

esulting angular position and stretch-evoked electromvograohic
activity was monitored. All experiments were performed under
the control of a real time COmDutinq system.
Sinusoidal Oscillation: The methods used and the oarameter
estimation techniques have been described by Agarwal and
Gottlieb (1977a, 1977b) and Gottlieb and Agarwal (1978).
Briefly, sinusoidal torques were applied to the wrist or ankle,
and the resulting joint angles were measured. The ratio of
the peak-to-peak joint angle to peak-to-peak torque amplitudes
are computed for each of several frequencies. The second
order fit is computed to minimize the followin_ error
criterion:
Error = _/ [ log (\Measured Model compliance_12
complianco/.a
In the wrist experiments a sliqht tapering of inout amolitudes
was introduced to prevent excessive excursion of the 'wrist at
the lowest frequencies. Data collected in this way is
qualitatively similar to data collected without the taoer,
except that the wrist appears slightly stiffer at low
frequencies than it ac_ually is.
Step Tor_lue Perturbation: The motor apnlied one-second
duration steps of torque to flex or extend the joint. These
steps were typically randomized in amplitude and interstep
interval. Subjects were instructed to either do nothinq when
the step was oercieved, or react to restore the joint to its'
starting position. Responses were grouoed by perturbation
amplitude and averaged.
RESULTS
Normal Compliance Measurements: Compliance measurements were
made using sinusoidal torque inputs over a given frequency
range (@.5 to i_.@ hz for the wrist, 3.Z to 12._ hz for the
ankle). These measurements typically showed an underdamped
second order response. Compliance curves for the wrist were
constructed at three different input amplitudes using five
different bias torques, and are given xn fiqure 2. The
parameters J, B, and K for these curves are shown in table i.
The compliance measurement was very sensitive to input
amplitude, and showed large decreases in K, the stiffness, as
larger amplitude sinusoidal torques were used. The compliance
measurements were also sensitive to bias :torque, showinq
increases in K with increasing bias torques. Less pronounced
but consistent changes were seen in B, the viscosity term. The
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