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03月

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A Rapidly Deployable Manipulator System
Christiaan J.J. Paredis，H. Benjamin Brown，Pradeep K. Khosla

Abstract：
A rapidly deployable manipulator system combines the flexibility of reconfigurable modular hardware with modular programming tools，allowing the user to rapidly create a manipulator which is custom-tailored for a given task. This article describes two main aspects of such a system，namely，the Reconfigurable Modular Manipulator System (RMMS)hardware and the corresponding control software.

1 Introduction
Robot manipulators can be easily reprogrammed to perform different tasks，yet the range of tasks that can be performed by a manipulator is limited by mechanicalstructure.Forexample，a manipulator well-suited for precise movement across the top of a table would probably no be capable of lifting heavy objects in the vertical direction. Therefore，to perform a given task,one needs to choose a manipulator with an appropriate mechanical structure.
We propose the concept of a rapidly deployable manipulator system to address the above mentioned shortcomings of fixed configuration manipulators. As is illustrated in Figure 1，a rapidly deployable manipulator system consists of software and hardware that allow the user to rapidly build and program a manipulator which is customtailored for a given task.
The central building block of a rapidly deployable system is a Reconfigurable Modular Manipulator System (RMMS). The RMMS utilizes a stock of interchangeable link and joint modules of various sizes and performance specifications. One such module is shown in Figure 2. By combining these general purpose modules，a wide range of special purpose manipulators can be assembled. Recently，there has been considerable interest in the idea of modular manipulators [2，4，5，7，9，10，14]，for research applications as well as for industrial applications. However，most of these systems lack the property of reconfigurability，which is key to the concept of rapidly deployable systems. The RMMS is particularly easy to reconfigure thanks to its integrated quick-coupling connectors described in Section 3.
Effective use of the RMMS requires，Task Based Design software. This software takes as input descriptions of the task and of the available manipulator modules；it generates as output a modular assembly configuration optimally suited to perform the given task. Several different approaches have been used successfully to solve simpli-fied instances of this complicated problem.
A third important building block of a rapidly deployable manipulator system is a framework for the generation of control software. To reduce the complexity of softwaregeneration for real-time sensor-based control systems，a software paradigm called software assembly has been proposed in the Advanced Manipulators Laboratory at CMU.This paradigm combines the concept of reusable and reconfigurable software components，as is supported by the Chimera real-time operating system [15]，with a graphical user interface and a visual programming language，implemented in Onika
Although the software assembly paradigm provides thesoftware infrastructure for rapidly programming manipulator systems，it does not solve the programming problem itself. Explicit programming of sensor-based manipulator systems is cumbersome due to the extensive amount of detail which must be specified for the robot to perform the task. The software synthesis problem for sensor-based robots can be simplified dramatically，by providing robust robotic skills，that is，encapsulated strategies for accomplishing common tasks in the robots task domain [11]. Such robotic skills can then be used at the task level planning stage without having to consider any of the low-level details
As an example of the use of a rapidly deployable system,consider a manipulator in a nuclear environment where it must inspect material and space for radioactive contamination，or assemble and repair equipment. In such an environment，widely varied kinematic (e.g.，workspace) and dynamic (e.g.，speed，payload) performance is required，and these requirements may not be known a priori. Instead of preparing a large set of different manipulators to accomplish these tasks—an expensive solution—one can use a rapidly deployable manipulator system. Consider the following scenario：as soon as a specific task is identified，the task based design software determinesthe task. This optimal configuration is thenassembled from the RMMS modules by a human or，in the future，possibly by another manipulator. The resulting manipulator is rapidly programmed by using the software assembly paradigm and our library of robotic skills. Finally,the manipulator is deployed to perform its task.
Although such a scenario is still futuristic，the development of the reconfigurable modular manipulator system，described in this paper，is a major step forward towards our goal of a rapidly deployable manipulator system.
Our approach could form the basis for the next generation of autonomous manipulators，in which the traditional notion of sensor-based autonomy is extended to configuration-based autonomy. Indeed，although a deployed system can have all the sensory and planning information it needs，it may still not be able to accomplish its task because the task is beyond the system’s physical capabilities. A rapidly deployable system，on the other hand，could adapt its physical capabilities based on task specifications and，with advanced sensing，control，and planning strategies，accomplish the task autonomously.

2 Design of self-contained hardware modules
In most industrial manipulators，the controller is a separate unit housing the sensor interfaces，power amplifiers，and control processors for all the joints of the manipulator.A large number of wires is necessary to connect this control unit with the sensors，actuators and brakes located in each of the joints of the manipulator. The large number of electrical connections and the non-extensible nature of such a system layout make it infeasible for modular manipulators. The solution we propose is to distribute the control hardware to each individual module of the manipulator. These modules then become self-contained units which include sensors，an actuator，a brake，a transmission，a sensor interface，a motor amplifier，and a communication interface，as is illustrated in Figure 3. As a result，only six wires are required for power distribution and data communication.
2.1 Mechanical design
The goal of the RMMS project is to have a wide variety of hardware modules available. So far，we have built four kinds of modules：the manipulator base，a link module，three pivot joint modules (one of which is shown in Figure 2)，and one rotate joint module. The base module and the link module have no degrees-of-freedom；the joint modules have one degree-of-freedom each. The mechanical design of the joint modules compactly fits a DC-motor，a fail-safe brake，a tachometer，a harmonic drive and a resolver.
The pivot and rotate joint modules use different outside housings to provide the right-angle or in-line configuration respectively，but are identical internally. Figure 4 shows in cross-section the internal structure of a pivot joint. Each joint module includes a DC torque motor and 100:1 harmonic-drive speed reducer，and is rated at a maximum speed of 1.5rad/s and maximum torque of 270Nm. Each module has a mass of approximately 10.7kg. A single，compact，X-type bearing connects the two joint halves and provides the needed overturning rigidity. A hollow motor shaft passes through all the rotary components，and provides a channel for passage of cabling with minimal flexing.
2.2 Electronic design
The custom-designed on-board electronics are also designed according to the principle of modularity. Each RMMS module contains a motherboard which provides the basic functionality and onto which daughtercards can be stacked to add module specific functionality.
The motherboard consists of a Siemens 80C166 microcontroller，64K of ROM，64K of RAM，an SMC COM0 universal local area network controller with an RS-485 driver，and an RS-232 driver. The function of the motherboard is to establish communication with the host interface via an RS-485 bus and to perform the lowlevel control of the module，as is explained in more detail in Section 4. The RS-232 serial bus driver allows for simple diagnostics and software prototyping.
A stacking connector permits the addition of an indefinite number of daughtercards with various functions，such as sensor interfaces，motor controllers，RAM expansion etc. In our current implementation，only modules with actuators include a daughtercard. This card contains a 16 bit resolver to digital converter，a 12 bit A/D converter to interface with the tachometer，and a 12 bit D/A converter to control the motor amplifier；we have used an ofthe-shelf motor amplifier (Galil Motion Control model SSA-8/80) to drive the DC-motor. For modules with more than one degree-of-freedom，for instance a wrist module，more than one such daughtercard can be stacked onto the same motherboard.

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. As shown in Figure 5，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 (8@45 degrees). The relative orientation is automatically registered by means of an infrared LED in the female connector and eight photodetectors in the male connector.

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 [1] implemented on a dedicated IC (SMC COM0). 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，as is depicted in Figure 6. 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.

5 Modular and reconfigurable control software
The control software for the RMMS has been developed using the Chimera real-time operating system，which supports reconfigurable and reusable software components [15]. The software components used to control the RMMS are listed in Table 1. The trjjline，dls，and grav_comp components require the knowledge of certain configuration dependent parameters of the RMMS，such as the number of degrees-of-freedom，the Denavit-Hartenberg parameters etc. During the initialization phase，the RMMS interface establishes contact with each of the hardware modules to determine automatically which modules are being used and in which order and orientation they have been assembled. For each module，a data file with a parametric model is read. By combining this information for all the modules，kinematic and dynamic models of the entire manipulator are built.
After the initialization，the rmms software component operates in a distributed control mode in which the microcontrollers of each of the RMMS modules perform PID control locally at 1900Hz. The communication between the modules and the host interface is at 400Hz，which can differ from the cycle frequency of the rmms software component. Since we use a