We make several experimentally based observations that enhance the understanding of the dynamics of geared robot drives. We show that: (1) at low velocities, a gear transmission can behave as a nonminimum-phase system; (2) this nonminimum phase behavior is intimately related to the dynamics of friction at gear-tooth contact; and (3) by virtue of compliance in bearings and supports, gear drives do not have a constant transmission ratio over all frequencies, but in stead show step changes in transmission ratio. These findings guide the development of a new dynamic model of a gear transmission that captures these salient characteristics and makes explicit the different torque-transmission modes. We remark on the implications of these findings for robot force control.
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