2。3 Integrated quick-coupling connectors

To make a modular manipulator be reconfigurable, it is necessary that the modules can be easily connected with each other。 We have developed a quick-coupling mechanism with which a secure mechanical connection between modules can be achieved by simply turning a ring handtight; no tools are required。  Keyed flanges provide precise registration of the two modules。 Turning of the locking collar on the male end produces two distinct motions: first the fingers of the locking ring rotate (with the collar) about 22。5 degrees and capture the fingers on the flanges; second, the collar rotates relative to the locking ring, while a cam mechanism forces the fingers inward to securely grip the mating flanges。 A ball- transfer mechanism between the collar and locking ring automatically produces this sequence of motions。

At the same time the mechanical connection is made,pneumatic and electronic connections are also established。 Inside the locking ring is a modular connector that has 30 male electrical pins plus a pneumatic coupler in the middle。 These correspond to matching female components on the mating connector。 Sets of pins are wired in parallel to carry the 72V-25A power for motors and brakes, and 48V–6A power for the electronics。 Additional pins carry signals for two RS-485 serial communication busses and four video busses。 A plastic guide collar plus six alignment pins prevent damage to the connector pins and assure proper alignment。 The plastic block holding the female pins can rotate in the housing to accommodate the eight different possible connection orientations 。 The relative orientation is automatically registered by means of an infrared LED in the female connector and eight photodetectors in the male connector。

2。4 ARMbus communication system

Each of the modules of the RMMS communicates with a VME-based host interface over a local area network called the ARMbus; each module is a node of the network。 The communication is done in a serial fashion over an RS-485 bus which runs through the length of the manipulator。 We use the ARCNET protocol implemented on a dedicated IC (SMC COM20020)。 ARCNET is a deterministic token-passing network scheme which avoids network collisions and guarantees each node its time to access the network。 Blocks of information called packets may be sent from any node on the network to any one of the other nodes, or to all nodes simultaneously (broadcast)。 Each node may send one packet each time it gets the token。 The maximum network throughput is 5Mb/s。

The first node of the network resides on the host interface card。 In addition to a VME address decoder, this card contains essentially the same hardware one can find on a module motherboard。 The communication between the VME side of the card and the ARCNET side occurs through dual-port RAM。

There are two kinds of data passed over the local area network。 During the manipulator initialization phase, the modules connect to the network one by one, starting at the base and ending at the end-effector。 On joining the network, each module sends a data-packet to the host interface containing its serial number and its relative orientation with respect to the previous module。 This information allows us to automatically determine the current manipulator configuration。

During the operation phase, the host interface communicates with each of the nodes at 400Hz。 The data that is exchanged depends on the control mode—centralized or distributed。 In centralized control mode, the torques for all the joints are computed on the VME-based real-time processing unit (RTPU), assembled into a data-packet by the microcontroller on the host interface card and broadcast over the ARMbus to all the nodes of the network。 Each node extracts its torque value from the packet and replies by sending a data-packet containing the resolver and tachometer readings。 In distributed control mode, on the other hand, the host computer broadcasts the desired joint values and feed-forward torques。 Locally, in each module, the control loop can then be closed at a frequency much higher than 400Hz。 The modules still send sensor readings back to the host interface to be used in the computation of the subsequent feed-forward torque。