This article studies the ways that the performance of direct drive (DD) serial robots changes as system size is changed. We are particularly interested in the physical laws for scaling down direct drive arms to small sizes. Using theoretical scaling analysis, we show that there is a net physical performance advantage to small direct drive arms. A key factor for direct drive robot performance is the torque-to-mass ratio of the actuators, U. We show how U varies with the scale of DD actuators, and we also calculate how the dynamic performance varies with scale and U. We compare our calculations with experimental measurements of actuators of various sizes taken from small hard disk drives and compare them with published data for larger motors. Finally, we describe a prototype, five- axis, direct drive serial arm having a reach of 10 cm and a work volume of about 136 cm3. Some potential applications are briefly discussed.
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