Experiments in force control of robotic manipulators

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B. Loop Closure
The second classification of algorithms requires examining the method in which the signal from the force sensor is processed through the servo loop. If we put aside the issue of coordiC. Matrix of Control Types nate transformations for the moment, each axis of a force controlled arm can be viewed as a single Given the two independent algorithm classifiinput (the motor), dual output (position sensor and cations, it is possible to form a matrix of possiforce sensor)system. The method by whichthe sigble servo types, Figure 3 . Also indicated here are nals from the two sensors are processed and fed some of the previous force servo investigations. back to the motor determines the performance characteristics of the servo loop. Although it is imCartesian joint possible to make an unequivocal classification for based based all force servos, it is possible to group most algorithms into three broad categories: torque based,velocitybased,orpositionbased.This coslyos Salisbury classification is based upon the concept of succestorque sive loop closure, that is, closing an inner loop based CraigAdept 1 I on one sensor and then closing an outer loop using Raibert another sensor. The general situation is illustrated in Figure velocity Whitney 1, showing the sensor signals being processed, based along with command inputs, to form a corrective torque command for the manipulator's motors. This model is appropriate in a general sense for all electric arms where the basic control variable is position Adept P3 Adept # 2 motortorque.Forhydraulicarms,thisdiagram based would be modified to show actuator velocity as the control variable. Although in some sense Figure 1 models all electric arms, is itpossible to further Fig. 3 . Matrix of force servo classifications. classify some control loops into an inner/outer loop arrangement. An exampleis shown in Figure 2. Here, an inner loop closed is on theposition senMuch of the previous work in force control has sor with an outer loop closed around this on the dealt with direct computation force sensor (along with possible input from the of control torques to position sensor). It is also possible to have a the motors. Examples of this are Khatib's COSMOS system at Stanford similar arrangement with an inner loop closed on [ 3 1 , the Craig-Raibert hybrid control method 1 2 1 , and Salisbury's active stiffvelocity instead of position. In this case, the ness control [ll. On the other hand, Whitney's imcommand to the inner loop would be velocity. (This generating velocity is similar to the case of a hydraulic actuator). plementation [ 4 1 involved commands to feed either a hydraulic arm or an electric arm with a velocity servo. This corresponds to a velocity based inner loop classification. Not COMMANDS shown in the matrix is a Cartesian-based force conPOSITION c COMPENSATION trol system with an inner loop on position which L FORCE was developedby one of the authors while at FORCE I Hewlett-Packard Laboratories. This implementation POSITION proved that such position based inner loop systems are feasible. Fig. 1. Generalized force feedback serv6iIelN~fp699 AS indicated in the figure, the present authors have implemented three different force conINNER LOOP trol systemsat Adept, two joint based controllers I ----- --- - --- - - 1 on a PUMA 560 manipulator, and a Cartesian based I I system controllingan AdeptOne robot. Both servos COMMANDS 1 on the PUMA used the joint stiffness matrix suggested by Salisbury to translate user requirements intojointspace.TheCartesianschemeonthe AdeptOne is of our own design, and is documented in this paper. Our experience with these implementaFila 5.1701 tions and our familiarity with the previous implementations have led us to a number of conclusions Fig. 2. Force servo with inner position loop. regarding the different approaches to force cont . rol Summarizing, it is possible to have three different situations: 1 ) the force sensor signal is processed directly to become a torque command, 2 )
these
techniques.
A new classification scheme for robotic force 11. Classification of Force Control Algorithms control algorithms is presented. It is shown that algorithms can be categorized according to whether Much work has been done by many researchers on they are joint-based or Cartesian-based. Further, the subject of force contro