Passive Robotics: An Exploration of Mechanical Computation

We invite the reader to think of a passive wrist as a mechanical computer. The wrist computes a particular motion in response to every applied force, and this defines its control law. Suppose that the design of a wrist (the geometric layout of mechanical elements -- springs, hydraulic cylinders, dampers, and so on -- which compose it) is held fixed. We can "program" the wrist by changing the parameters (for example, the spring stiffnesses) of some of these elements. What is the range of control laws such a wrist can execute? The thesis of this paper is that a passive wrist, of fixed design, can be programmed to execute a wide range of useful control laws. We consider in particular wrists whose actuators are unpowered hydraulic cylinders, the ports of which are coupled to one another via variable-conductance constrictions. By selection of these conductances the wrist is progammed, much as an analog computer is programmed. We characterize mathematically the range of control laws such a device can compute.