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Circle 6 groups the computations required to check initial and terminal conditions (e.g. contact established and destination reached, respectively, for RUB). Checks, however, are performed in circle 1. Circle 9 computes the rotation matrix and the Jacobian of the compliant frame with respect to the base frame (J(q)). Circle 8 executes the force-control algorithm to obtain u, yF and ~F. Info on current action_command is the command identifier (e.g. RUB). A decomposition of the functions of circle 8 is shown in Fig. 6, whose clear rationale is given by the force algorithm scheme of Fig. 4. Thanks to this modular organization, changing (for instance) the force-control algorithm is reflected in changes to just one or few circle functions, i.e. a small part with sharp boundaries. The same is true for changing the force sensor, and so on. Force measttremellts £1t SRF fofee_meas ~ " &5p ~ P mitimt Rotation_matrixillfo Jacobialt_iltfo ( Position ilet pc~lt~ coll'ectiOlt correction ~l. Fig. 6 Decomposition of circle 8 functions (hybrid control algorithm) 5. DESIGN OF THE IMPLEMENTATION ARCHITECTURE As the final part of the case study, this section shows how the fimctional architecture proposed above may be implemented by the extension of a commercial controller. The specific case of the Comau C3G 9000 controller is considered. The new controller is named here C4G. The hardware modifications will be considered first, and then a software architecture will be proposed. The illustrative example will then focus on the AA relevant to the RUB action. The proposed solution is, however, also suitable for implementing the other actions defined in Section 3, provided that the relevant additional software modules are developed. In the choice of a solution, economic constraints are of major concern when mass-produced systems like industrial controllers are considered. Moreover, taking into account that applications requiring sensor-based control will likely remain in a minority for many years, a modular solution featuring exteroceptive sensor-based controls as an add-on item to the standard controller seems the most interesting, and perhaps the only viable, one. Impedance control and on-line path-planning algorithms, needed to implement actions like INSERT and FOLLOW, may be implemented using the same circles, introducing the proper functions in each circle. The overall controller AA therefore has the same structure, even if the fimctions of each circle are actually the logic sum of the functions relevant to each action. 5.1 Hardware architecture Figure 7 shows that the architecture of the control unit of the C4G is derived from the standard C3G control unit by integrating additional computing and interface boards. The C3G control unit is based on the VME-bus and comprises two processing boards, the robot CPU (RBC) and the servo CPU (SCC). The RBC board is equipped with Motorola 68020/68882 CPUs, and with a shared memory area that can be accessed from the RBC itself, as well as from other boards on the VME-bus. Tasks nmning on the RBC accomplish user interface functions, and the translation and interpretation of the user's programs. They also drive the Operator Control panel and the programming terminal. The programming language is PDL2, a Pascal-like language endowed with powerful motion-control instructions. PDL2 supports multi-tasking, communication via LAN and serial links, and analog and digital I/O, so that the flow of program execution can be easily controlled and modified in real time, according to external events. The SCC is a multiprocessor board equipped with Motorola 68020/68882 CPUs and with Texas 320C25 DSP. The Motorola CPUs perform trajectory generation, in either joint or Cartesian space, and kinematic inversion, both every 10 msec; the DSP executes position set-point micro-interpolation and joint position control, at a sampling interval of 1 msec. The additional boards are the sensor-based control CPU (SBC) and I/O boards to interface exteroceptive sensors. The SBC is in charge of supplying the additional computing power required to execute the new control functions. It gets sensor data from dedicated I/O boards plugged in the VME bus, and exchanges data with the RBC and SCC through the shared memory on the RBC, and it has to compute the corrections to position set points for hybrid control. The SBC hardware design has not been camed out; a second RBC or SCC would, rather, be used to eliminate the need for the board design and reduce spare-part costs. 5.2 Abftware architecture The software modules that implement the control functions relevant to the RUB action on the SBC board are shown in the structural chart in Fig. 8. Finding the mapping between the functions of the AA for hybrid control and the modules of the software architecture (numbers in italics point to circles in Figs 5 and 6) is straightforward; thus the traceability between the essential functions of the controller and the design solutions is evident. The main program "Compute_position_corrections" successively activates the modules