2.1) General presentation of the PUMA robot.

    The robot we have used for this project is an UNIMATION PUMA 500 robot.
The PUMA 500 is a 6-Degrees Of Freedom industrial robot and is most commonly used in automated spot welding applications.
Pumas are probably the most common robot in university laboratories and one of the most common assembly robots.
Originally designed by Vic Schienman and financed by GM at MIT in the mid-70's, the Puma (Programmable Universal Machine for Assembly) was produced for many years by Unimation (later purchased by Westinghouse and sold at a loss later to Staubli, a Swiss company).

The system is composed of two separated parts: The robot and its control computer.

The robot:

The robot arm is the mechanical component of the system incorporating 6 degrees of freedom, each controlled by a DC servomotor. It is sufficiently flexible to be taught a wide variety of tasks.
Each member of the robot arm is connected to another member at a joint. Through each joint passes one or more axes around which the members of the arm rotate.
The members of the robot arm are shown in the following figure. The robot arm members contain the various servomotors and gear trains.

The robot arm

To achieve maximum strenght with minimum weight, the upper arm and forearm are of monocoque construction.

Each member of the arm assembly is driven by a permanent-magnet DC servomotor through its associated gear train. Each motor in the PUMA robot arm contains an incremental encoder and a potentiometer driven through a 116 to 1 gear reduction. The proper functioning of the PUMA requires control of the position and the velocity of each joint of the robot arm.

For a servo-controlled robot system, position must be measured relative to a relative known initial, absolute position.
The potentiometers, incorporated in the motor, are used to initialize the PUMA, to establish its absolute position.
The incremental encoders are mounted on the shaft of each motor and provide position change and velocity signals for the servo system. Position change signals are read from the encoders, and velocity signals are calculated. Approximately 32 times during each 28-millisecond window of the digital servo system, the signals from the encoders are compared to the calculated position and any necessary correction signals are generated.

The servomotors for the major axes (joints 1, 2 and 3) are equipped with electromagnetic brakes. These brakes are activated when power is removed from the motors, thereby locking the robot arm in a fixed position. This safety feature removes the risk of injury or damage that could result from the arm collapsing if power is accidentally removed.

Power for the motors is supplied through the cable connecting the robot arm and the controller. Feedback signals from the incremental encoders and potentiometers are also carried by this cable.

The robot we have used was equipped with a pneumatic-controlled hand. This hand has two stable status: open or close.
The compressed air necessary to open and close the gripper was supplied by a stand-alone air compressor.

The gripper
 

The computer controller:

The controller is the master component of the electrical system. All signals to and from the robot pass through the controller and are used by it to perform real-time calculations to control arm movement and position.
Operating controls and indicators are located on the front and top panel of the controller. Connections for the robot arm, terminal, floppy disk drive and accessories are located on the controller rear panel. Software is stored in the computer memory, located in the controller. The software interprets the operating instructions for the robot arm, and the controller transmits these instructions from the computer memory to the arm. From incremental encoders and potentiometers in the robot arm, the controller/computer receives data about arm position. This provides a closed loop control of arm motions.
A floppy disk drive is available to record the programs on diskettes.

The computer controller

The controller is equipped with a teach pendant (see picture above) which can be used to position the robot arm by manipulating the joints. Several teach modes are available to provide complete teaching flexibility. Arm positions may be recorded and stored in the memory by use of a RECORD pushbutton, on the teach pendant. Arm velocity is controlled by three position toggle switch. The pneumatic gripper can be set by the clamp switch.

By the teach pendant (and by programming too), two coordinates systems can be selected.

The World Mode:

The reference coordinates for the World Mode are fixed in the robot arm base shown in the following figure.

The World coordinate system.

This mode can be chosen when the gripper is to move parallel to any of the World axes, or to rotate about those axes.

The Tool Mode:

The reference coordinates for the Tool Mode are fixed in the gripper, with their origin at the center of its mounting flange. See the following figure.

The Tool coordinate system

This mode can be chosen when the gripper is to move parallel to or rotate about the tool coordinate axes.

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