Archive for July, 2009

Cincinnati Milacron T3 Robotic Arm

July 27, 2009

CINCINNATI MILACRON T3 ROBOTIC ARM

OPP

An industrial robot is officially defined by ISO as an automatically controlled, reprogrammable, multipurpose manipulator programmable in three or more axes.
At Cincinnati Milacron Corporation, Richard Hohn developed the robot called The Tomorrow Tool or T3. Released in 1973, the T3 was the first commercially available industrial robot controlled by a microcomputer as well as the first U.S. robot to use the revolute configuration.
This robot is a more classically designed industrial robot. Designed as a healthy compromise between dexterity and strength this robot was one of the ground breakers, in terms of success, in factory environments. However, while this robot was a success in industry its inflexible interfacing system makes it difficult to use in research.
The Cincinnati Milacron T3 robot is an example of jointed arm robot which most closely resembles the human arm. This type of arm consists of several rigid members connected by rotary joints. In some robots, these members are analogous to the human upper arm, forearm and hand; the joints are analogous to the human shoulder, elbow and wrist.
The T3 robot arm is mounted on a rotary joint whose major axis is perpendicular to the robot mounting plate. This axis is known as the base or waist. Three axes are required to emulate the movement of the wrist and they are called: pitch, yaw and roll.
CONTROL SYSTEM
The T3 robotic arms are controlled using a Hierarchical Control System. A Hierarchical control system is partitioned vertically into levels of control.
The basic command and control structure is a tree, configured such that each computational module has a single superior, and one or more subordinate modules. The top module is where the highest level decisions are made and the longest planning horizon exists. Goals and plans generated at this highest level are transmitted as commands to the next lower level where they are decomposed into sequences of sub goals. These sub goals are in then transmitted to the next lower control decision level as sequences of less complex but more frequent commands.
HIERARCHICAL CONTROL SYSTEM
The hierarchical control structure serves as an overall guideline for the architecture and partitioning of a sensory interactive robot control system.
The system is configured in the hierarchical manner and includes five major subsystems:(1) The Real-Time Control System (RCS)(2) The commercial. T3 Robot equipment(3) the End-Effector System(4) The Vision System(5) The Watchdog Safety System
The Real-Time Control System as shown in figure is composed of four levels:(1) The Task Level(2) The Elemental-Move Level(3) The Primitive Level(4) The T3 Level
The Task, Elemental-Move and Primitive levels of the controller are considered to be Generic Control Levels which would remain essentially the same regardless of the particular robot (commercial or otherwise) being used.
The T3 Level, however, uses information and parameters particular to the T3 Robot and is, therefore, unique to the T3 Robot. The Joystick shown provides an alternate source of commands to the Primitive Level for manual control of the robot and is not used in conjunction with the higher control levels .The T3 controller is subordinate to the T3 Level of the RCS and communicates through a special interface.
The End-Effector System consists of a two fingered gripper equipped with position and force sensing .The gripper is pneumatically actuated and servo controlled by a controller which is subordinate to the Primitive Level of the RCS.
There are three sensory systems on the robot:
1. The finger force and position sensors on the gripper which report data to the End Effector Controller2. The 3 point Angle Acquisition System which reports data to the T3 Controller, the T3 Level of the RCS and to the Watchdog Safety System3. The Vision System which reports data to the Elemental-Move Level of the RCS.Of the sensor systems, the vision system is obviously the most complex. It performssophisticated image processing which requires substantial computational time.
The Watchdog Safety System does not fit directly into the hierarchical control structure. It is an independent system which monitors robot motions and compares them to previously defined limits in position, velocity and acceleration. The Watchdog System has the power to stop the robot if any limits are exceeded and consequently monitors both the mechanical and control systems of the robot.
PARTS OF THE REAL TIME CONTROL SYSTEM(1) Task LevelThe Task Level interfaces with the Workstation Level above it and the Elemental-Move Level below it. The Task Level receives commands from the Workstation Level in terms of objects to be handled and named places in the workstation.For example, the task might be to find a certain part on the tray at the load/unload station, pick it up and put it in the fixture on the machine tool. This task could be issued as one command from the Workstation Level to the Task Level of the RCS.
(2)Elemental-Move LevelThe E-Move Level interfaces with the Task Level above it and the Primitive Level below it. In addition, the E-Move Level interfaces with the Vision System from which it acquires part position and orientation data. The E-Move Level receives commands from the Task Level which are elemental segments of the Task Level command under execution. These are generally single moves from one named location to another. If a part acquisition is involved, data from the Vision System is requested to determine the exact location of the next goal point. The E-Move Level then develops a trajectory between the new goal point and its current position.
(3)Primitive LevelThe Primitive Level interfaces with the E-Move Level above it and the T3 Level and End-Effector Controller below it. The Primitive Level is the lowest level in the RCSwhich is robot or device independent. Subsystems subordinate to the Primitive Level are considered to be at the device level in the control hierarchy. In this system, these subsystems or devices are the robot and the end-effector. The Primitive Level interfaces with the Joystick. The Joystick is a peripheral device which is used for manual operation of the robot. Using the Joystick, the operator can control robot motion in several coordinate systems (world, tool or individual joint motions).
(4) T3 LevelThe T3 Level interfaces with the Primitive Level above it and the commercial Cincinnati Milacron T3 Robot Controller below it. In addition there is a sensory interface which supplies the six individual joint angles. The T3 Level is so named because elements of it are peculiar to the T3 Robot. From a control hierarchy point of view the T3 Level does not constitute a logical control decision level but is infact a “gray box” necessary to transform command and feedback formats between the Primitive level and T3 controller.
APPLICATIONS
Hydraulically actuated, the T3 is used in applications such as welding automobile bodies, transferring automobile bumpers and loading machine tools. In 1975, the T3 was introduced for drilling operations and in the same year T3 became the first robot to be used in the aerospace industry.

ServoMechanisms

July 27, 2009

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A system for the automatic control of motion by means of feedback. The term servomechanism, or servo for short, is sometimes used interchangeably with feedback control system (servosystem). In a narrower sense, servomechanism refers to the feedback control of a single variable (feedback loop or servo loop). In the strictest sense, the term servomechanism is restricted to a feedback loop in which the controlled quantity or output is mechanical position or one of its derivatives (velocity and acceleration). See also Control systems.

The purpose of a servomechanism is to provide one or more of the following objectives: (1) ac­curate control of motion without the need for human attendants (automatic control); (2) maintenance of accuracy with mechanical load variations, changes in the environment, power supply fluctuations, and aging and deterioration of components (regulation and self-calibration); (3) control of a high-power load from a low-power command signal (power amplification); (4) control of an output from a remotely located input, without the use of mechanical linkages (remote control, shaft repeater).

The illustration shows the basic elements of a servomechanism and their interconnections; in this type of block diagram the connection between elements is such that only a unidirectional cause-and-effect action takes place in the direction shown by the arrows. The arrows form a closed path or loop; hence this is a single-loop servomechanism or, simply, a servo loop. More complex servomechanisms may have two or more loops (multiloop servo), and a complete control system may contain many servomechanisms. See also Block diagram.

Servo loop elements and their interconnections. Cause-and-effect action takes place in the directions of arrows. (After American National Standards Institute, Terminology for Automatic Control, ANSI C85.1)
Servo loop elements and their interconnections. Cause-and-effect action takes place in the directions of arrows. (After American National Standards Institute, Terminology for Automatic Control, ANSI C85.1)

Servomechanisms were first used in speed governing of engines, automatic steering of ships, automatic control of guns, and electromechanical analog computers. Today, servomechanisms are employed in almost every industrial field. Among the applications are cutting tools for discrete parts manufacturing, rollers in sheet and web processes, elevators, automobile and aircraft engines, robots, remote manipulators and teleoperators, telescopes, antennas, space vehicles, mechanical knee and arm prostheses, and tape, disk, and film drives.

History

James Watt’s steam engine governor is generally considered the first powered feedback system. The windmill fantail is an earlier example of automatic control, but since it does not have an amplifier or gain, it is not usually considered a servomechanism.

The first feedback position control device was the ship steering engine, used to position the rudder of large ships based on the position of ship’s wheel. This technology was first used on the SS Great Eastern in 1866. Steam steering engines had the characteristics of a modern servomechanism: an input, an output, an error signal, and a means for amplifying the error signal used for negative feedback to drive the error towards zero.

Electrical servomechanisms require a power amplifier. World War II saw the development of electrical fire-control servomechanisms, using an amplidyne as the power amplifier. Vacuum tube amplifiers were used in the UNISERVO tape drive for the UNIVAC I computer.

Modern servomechanisms use solid state power amplifiers, usually built from MOSFET or thyristor devices. Small servos may use power transistors.

The origin of the word is believed to come from the French “Le Servomoteur” or the slavemotor, first used by J. J. L. Farcot in 1868 to describe hydraulic and steam engines for use in ship steering.

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July 19, 2009

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