Introduction
Industrial units that handle hazardous items often feel the need for robotic devices to minimize risk to their employees. Chemical units, bio-waste disposal agencies and nuclear power plants are a few such entities where robotic arms help reduce the risk of injury and infection to humans. An effort in this direction was made by the development of a dual-arm robotic system as a collaborative project between Bhabha Atomic Research Centre (BARC) and IIT Bombay.

In contrast to automation systems, where the task and the product is well defined and routine, a dual-arm robotic system must have reasonable flexibility in the task definition. The ideal system should have the dexterity of a pair of human hands and the intelligence to judge the easiest way to finish a task. A single robotic arm may perform well on routine pickplace jobs but may not be competent to handle complex manipulation like transferring a liquid from one container to another or handling a flexible sheet.

Arm Configuration, Electronics and Interfacing
Each of the pair of robotic arms, (Figure 1) has an anthropomorphic (human type) elbow configurations with six joints. There are three joints at the wrist that support the gripper (the end-effector) and the arm itself has three more joints to position the wrist at the desired location. A parallelogram mechanism is added to the links to provide dynamic balancing and decoupling of the link Coriolis forces. The arms are capable of handling upto 15 Kg weight. The link lengths are 425 mm and 400 mm respectively, giving a wrist reachable distance of approximately 800 mm. The end-effector is presently a forktype pneumatically actuated gripper. This can be replaced by an instrumented gripper, having a force-torque embedded sensor and controlled gripping movement to handle delicate objects. The end-effector can also be a tool or a sensor to cut and inspect the object, if required. The weight of each robot is about 50 Kg. The complete system with its control hardware requires about 3 meters × 3 meters space.

The control system (Figure 2) is designed so as to facilitate continuous upgradation and experiments in the hardware. The robots are powered by DC servomotors. The motor control is achieved by a feedback control system based on a PC. These motors are coupled to encoders to generate feedback information. There are limit switches at the joint ends to trip the power supply in the event of malfunctioning. The motors are connected to their respective amplifiers which use Pulse Width Modulated (PWM) signals generated using the output from the PC Interface Card. Feedback from the sensors and limit switches are connected to a PC Interface Card. This card is also like an independent computer. It has one DSP processor with memory modules, D/A converter and EPROM to store data and code. It can communicate with the host computer using an ISA bus. The card has its own program structure that can be used to process encoder data and set different parameters to establish a closed loop control. For simple applications this card has sufficient memory and programming capabilities to be used without any interfacing with a PC.

The PC is used to compute the set points or the desired trajectories of the robots. It is also used to program the interface card’s internal code module and set its parameters. The software is developed inhouse and the source code therefore, is readily available for improvement and analysis. The pair of robots is easy to operate. A graphical user interface is available to command the robots using the mouse. Alternatively, a pre-planned trajectory can be fed in the form of a data file using the facility in the software.

Research Implications
Tasks such as pouring a liquid from one container to another, pushing a piston into a cylinder or turning the tap of a valve, are few such examples that need two arms to work in cooperation and in synchronism. These tasks, though appear ordinary, call for extraordinary dexterity properties in the robots’ workspace. In the case of single arm robots, to avoid the dexterity constraints, the workspace is pre-planned and structured, with pick-up points and placement point positioned according to the task’s requirement of desired orientations. However, in the case of dual-arm manipulators, the robotic arms need to maintain the orientation of their end-effectors with reference to each other, either constant or at some desired magnitude throughout their trajectories spanning a large area
of their workspace. An example can be given as pushing a piston into a cylinder. In this task the two end-effectors must maintain the same orientation while moving towards each other in order to push the piston. The successful execution of this apparently simple task is surprisingly dependent on many factors such as the length of the travel, the distance between the two robots, initial orientation of the cylinder and joint limits. It is, therefore, essential that the task is carefully planned in the section of the workspace where task specific orientation and location demands are satisfied. The studies on this aspects have resulted in the development of algorithms that makes the planning of the tasks more systematic.

The dual-arm system, developed from scratch, also gives an opportunity to students of various disciplines such as Electrical, Electronics, Mechanical, Computer Science and Industrial Design, to modify, improve, experiment and prove their skills on a working system.

Contact: banavar@iitb.ac.in.