The Role of Dynamic Models in Cartesian Force Control of Manipulators

Abstract

Dynamic models are as important in Cartesian force control as they are in position control. A variety of Cartesian force control schemes are examined, comprising some that do incorporate a dynamic model into the control loop (resolved acceleration force control, operational space method, and impedance control) and some that do not (hybrid control and stiffness control). Stability analyses and experimental implementations are presented that demonstrate not only that using a dynamic model leads to more accurate control, but also that not using a model can in certain cases make force control unstable. Experiments on the MIT Serial Link Direct Drive Arm show that resolved acceleration force control is stable and accurate in producing force steps and sinusoidal force responses and in complying to sinusoidal position disturbances.

Automatic Synthesis of Robot Compliant Motions in Dynamic Environments

“…Although most fine-motion planning schemes neglect the problem, the dynamic effects of robot/environment contact are well known and have been widely addressed in the literature (for example, in An and Hollerbach 1989;Mills 1990;Eppinger and Seering 1992).…”

Time-varying force tracking in impedance control a case study for automatic cell manipulation

“…Besides, this hybrid method ignores the dynamic manipulator-environment coupling, which may be sometimes beneficial to the system. As a result, the hybrid position/force control is not possible to control the commanded position or force accurately and may also cause some unstable response [7]. However, the impedance control method not only establishes a user-specified dynamical relationship between the end-effector position and force, but also provides a unified framework for controlling a manipulator both in free space motion (without environmental contact) and in compliant motion (with environmental contact) [8].…”

The cause of kinematic instability in hybrid position/force control: contact compliance

“…It is further shown in Appendix A of [5] that this control can be kine matically unstable when the constraint is modelled as a spring (F,z = k,zp). It is concluded in [5], based on those results, that kinematic instability is "independent of contact". However, it is now clear from Theorem 1 (more specifically, Remark 2) that contact compliance (including the case of k, = 0) is the cause of' kinematic instability.…”

“…Note that the constraint shown in Figure 1 is actually of the form a P ( p ) 5 0, since it permits 'loss of contact'. However, we will use the 'bilateral' form ( l ) , since we wish to compare our stability results to those reported in [5] for bilateral elastic constraints.…”

“…The term J-'-control is chosen because the inverse of the manipulator Jacobian is used in the position feedback path, rather than the Jacobian transpose which is used in stiffness control. An and Hollerbach investigated the stability of these three hybrid controls for the case of unconstrained planar robots [4,51 and the case of planar robots in contact with compliant constraints [5]. Instability was reported for J-'-control, but not for stiffness control or resolved acceleration control.…”