On control of flexible mobile manipulators:: Positioning and vibration reduction using an eye-in-hand range camera

In this paper the positioning of a long flexible manipulator on a moving platform is investigated. The problem is to position the gripper at a requested relative distance in front of an object with an unknown location. For this purpose the gripper is equipped with a range camera giving the distance to surrounding objects within, similar to 1% and with a sampling rate above 1kHz. The range measurements are used in combination with internal angle measurements from joint encoders to estimate both the flexibility in the mechanical construction and the relative distance from gripper to object. For the motion control of the manipulator, a time scaled feedback controller is suggested. A fast inner loop is used to damp out oscillations and reject disturbances both from the platform and the manipulator. An outer control loop, with a lower closed loop bandwidth, then steers the gripper, based on the range measurements, to the requested final position in front of the object. This loop assumes a stationary rigid platform, and a rigid manipulator. At this moment only simulations of a flexible manipulator on a rigid platform have been studied. However, the results show that the flexibility can be estimated from indirect measurements of the range to the object and the joint angles. Also, good damping and disturbance rejection is achieved as long as the bandwidth of the actuators is sufficiently high, compared to the oscillation. The use of range measurements of the surrounding objects makes the positioning task very robust with respect to an uncertain platform position. Copyright (C) 1998 IFAC.