Compared to coded pattern encoders, incremental encoders have four main advantages:

1. They are simpler and less expensive
2. They need no decoding circuits, only a counter.
3. Their range is only limited by the counter capacity. Additional encoder with step down gearing is used to increase the range that can be covered.
4. The measurement origin can be chosen at any point by resetting the counter (floating zero).

Drawback:

1. Incremental encoders do not measure absolute position, only incremental changes. Therefore, any mistake in the count is carried along to all subsequent counts.

Applications

• Precise angle measurement
• RPM or direction of rotation measurement
• X/Y positioning tables
• Positioning in handlings-systems

Reference:

ü        www.baumerelectric.com

ü        www.avagotech.com

ü        Industrial automation: circuit design and components, by David W Pessen-  1989

ü        www.cmcontrols.com

ü        www.directindustry.com

A transfer function is  defined as a ratio of two polynomials:

Where N(s) and D(s) are simple polynomials

These can be writtern as:

N(s)=(s − z1)(s − z2) . . . (s − zm−1)(s − zm)
D(s)=(s − p1)(s − p2) . . . (s − pn−1)(s − pn) ,

Zeros are the roots of N(s) (the numerator of the transfer function) obtained by setting N(s) = 0 and solving for s.

Poles are the roots of D(s) (the denominator of the transfer function), obtained by setting D(s) = 0 and solving for s.

All of the coefficients of polynomials N(s) and D(s) are real, therefore the poles and zeros must be either purely real, or appear in complex conjugate pairs.

Poles and Zeros of a transfer function are the frequencies for which the value of the transfer function becomes infinity or zero respectively.As s approaches a zero, the numerator of the transfer function (and therefore the transfer function itself) approaches the value 0. When s approaches a pole, the denominator of the transfer function approaches zero, and the value of the transfer function approaches infinity.The values of the poles and the zeros of a system determine whether the system is stable, and how well the system performs. Control systems, in the most simple sense, can be designed simply by assigning specific values to the poles and zeros of the system.

A system is characterized by its poles and zeros .Because the transfer function completely represents a system differential equation, its poles and
zeros effectively define the system response. In particular the system poles directly define the components in the homogeneous response.The locations of the poles, and the values of the real and imaginary parts of the pole helps to determine the response of the system.

The stability of a linear system may be determined directly from its transfer function. An nth order linear system is asymptotically stable only if all of the components in the homogeneous response from a finite set of initial conditions decay to zero as time increases.In order for a linear system to be stable, all of its poles must have negative real parts.

Reference:

http://en.wikibooks.org/wiki/Control_Systems/Poles_and_Zeros

web.mit.edu/2.14/www/Handouts/PoleZero.pdf

The poles are given by s = –p1 and s = –p2). When we add a zero at s = –z1 to the controller, the open-loop transfer function will change to:

We can put the zero at three different positions with respect to the pole.The effect of changing the gain K on the position of closed-loop pole and type of responses.

(a) The zero s = –z1 is not present.This means that we can choose K for the system to be overdamped,critically damped or underdamped.

(b) The zero s = –z1 is located to the right of both poles, s = – p2 and s = –p1.In this case, the system can have only real poles and hence we can only find a value for K to make the system overdamped. Hence  time response is low

(c) The zero s = –z1 is located between s = –p2 and s = –p1.This case provides a root locus on the real axis. The responses are therefore limited to overdamped responses. Fast response is possible.

(d) The zero s = –z1 is located to the left of s = –p2 .By placing the zero to the left of bothpoles, the vertical branches of case (a) are bent backward and one end approaches the zero and the other moves to infinity on the real axis. With this configuration, we can now change the damping ratio and the natural frequency (to some extent). The closed-loop pole locations can lie further to the left than s = –p2, which will providefaster time responses.

Since there is a relationship between the position of closed-loop poles and the system time domain performance, we can therefore modify the behaviour of closed-loop system by introducing appropriate zeros in the controller.

Effect of a closed loop zero on unit step response of asecond order system

C(t)is the response of the sysem witout adding zero

Cz(t)  be the response of the system with a zero at s=-z

Control Systems Engineering, Nagrath & Gopal

www.palgrave.com

Servomechanism

Servomechanism refers to an automatic device for the control of a large power output by means of a small power input or for maintaining correct operating conditions in a mechanism. It is a type of feedback arrangement for the automatic self-regulation of an electrical, mechanical, or biological system by returning part of its output as input. For example , the constant speed control system of a DC motor is a servomechanism that monitors any variations in the motor’s speed so that it can quickly and automatically return the speed to its correct value. Servomechanisms were first used in military and marine navigation equipment. Today they are used in automatic machine tools, satellite-tracking antennas, celestial-tracking systems on telescopes, automatic navigation systems, and antiaircraft-gun control systems. Other examples are fly by wire systems in aircraft which use servos to actuate the aircraft’s control surfaces, and radio controlled drivers which use RC servos for the same purpose. Many autofocus cameras also use a servomechanism to accurately move the lens, and thus adjust the focus. A modern hard disk drive has a magnetic servo system with sub-micrometre positioning accuracy.

The design of servomechanisms is considered to be a branch of both robotics and cybernetics.

To sum it all up 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).

All servomechanisms have at least these basic components: a controlled device, a command device, an error detector, an error-signal amplifier, and a device to perform any necessary error corrections (the servomotor). In the controlled device, that which is being regulated is usually position. This device must, therefore, have some means of generating a signal (such as a voltage), called the feedback signal, that represents its current position. This signal is sent to an error-detecting device. The command device receives information, usually from outside the system, that represents the desired position of the controlled device. This information is converted to a form usable by the system (such as a voltage) and is fed to the same error detector as is the signal from the controlled device. The error detector compares the feedback signal (representing actual position) with the command signal (representing desired position). Any discrepancy results in an error signal that represents the correction necessary to bring the controlled device to its desired position. The error-correction signal is sent to an amplifier, and the amplified voltage is used to drive the servomotor, which repositions the controlled device.

A typical system using a servomechanism is the communication satellite tracking antenna of a satellite earth station. The objective is to keep the antenna aimed directly at the communications satellite in order to receive and transmit the strongest possible signal. One method used to accomplish this is to compare the signals from the satellite as received by two or more closely positioned receiving elements on the antenna. Any difference in the strengths of the signals received by these elements results in a correction signal being sent to the antenna servomotor. This continuous feedback method allows a terrestrial antenna to be aimed at a satellite 37,007 km (23,000 miles) above the Earth to an accuracy measured in hundredths of a centimetre.

A servomechanism is unique from other control systems because it controls a parameter by commanding the time-based derivative of that parameter. For example a servomechanism controlling position must be capable of changing the velocity of the system because the time-based derivative (rate change) of position is velocity. A hydraulic actuator controlled by a spool valve and a position sensor is a good example because the velocity of the actuator is proportional to the error signal of the position sensor.

Servomechanism may or may not use a servomotor. For example a household furnace controlled by thermostat is a servomechanism, yet there is no motor being controlled directly by the servomechanism.

A common type of servo provides position control. Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force. Other types of servos use hydraulics, pneumatics, or magnetic principles. Usually, servos operate on the principle of negative feedback, where the control input is compared to the actual position of the mechanical system as measured by some sort of transducer at the output. Any difference between the actual and wanted values (an “error signal”) is amplified and used to drive the system in the direction necessary to reduce or eliminate the error. An entire science known as control theory has been developed on this type of system.

Typical servos give a rotary (angular) output. Linear types are common as well, using a screw thread or a linear motor to give linear motion.

Another device commonly referred to as a servo is used in automobiles to amplify the steering or braking force applied by the driver. However, these devices are not true servos, but rather mechanical amplifiers.

In industrial machines, servos are used to perform complex motion.

HISTORY

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.

James Watt’s steam engine governor is generally considered the first powered feedback system. The windmaill 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 steeing engine, used to position the rudder of large ships based on the position of ship’s wheel.  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 2 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 1 computer.

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

RC SERVOS

Small R/C servo mechanism
1. Electric motor
2. position feedback potentiometer
3. reduction gear
4. actuator arm

RC servos are hobbyist remote control devices servos typically employed in radio controlled models, where they are used to provide actuation for various mechanical systems such as the steering of a car, the flaps on a plane, or the rudder of a boat.

RC servos are composed of an electric motor mechanically linked to a potentiometer. PWM signals sent to the servo are translated into position commands by electronics inside the servo. When the servo is commanded to rotate, the motor is powered until the potentiometer reaches the value corresponding to the commanded position.

Due to their affordability, reliability, and simplicity of control by microprocessors, RC servos are often used in small-scale robotics applications.

The servo is controlled by three wires: ground (usually black/orange), power (red) and control (brown/other colour). This wiring sequence is not true for all servos, for example the S03NXF Std. Servo is wired as brown(negative), red (positive) and orange (signal). The servo will move based on the pulses sent over the control wire, which set the angle of the actuator arm. The servo expects a pulse every 20 ms in order to gain correct information about the angle. The width of the servo pulse dictates the range of the servo’s angular motion.

RC Servos are usually powered from either NiCd or NiMH packs common to most RC devices. More recently these systems are powered by Lithium Polymer (LiPo) packs. Voltage ratings vary from product to product, but most servos are operated at 4.8 V or 6 V DC from a 4 or 5 cell NiCd or NiMH battery, or a regulated LiPo pack.

References:Wikipedia,Brittanica,Google.

CINCINNATI MILACRON T3 ROBOT ARM

INTRODUCTION

Cincinnati Milacron built large industrial robots primarily for welding industry. It was one of the first companies to change from hydraulic to electric robots. Milacron pioneered the first computerized numerical control (CNC)  robot with improved wrists and the tool centre point (TCP) concepts. The first hydraulic machine, the T3 , was introduced in 1978. It closely resembled the General Electric Man-mate, ITT arm, and other predecessors (Sullivan 1971). Constructed of cast aluminium, it is available in two models of 6-axes revolute jointed arms. The largest, the T3-776, uses ballscrew electric drives to power the shoulder and elbow pitch. The ballscrews replaced the hydraulic cylinders originally used on the T3 robots. The elbow is a classical example of intermediate drive elbow. The same techniques, only upside down, appear in the shoulder. Shoulder yaw is provided by the standard bullgear on a base mounted motor drive. End users have discovered that ballscrews are not sufficiently reliable and are pressuring for an alternators. The eventual disappearance of ballscrews in industrial robots seems inevitable.

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.

CONTROL SYSTEM

The T3 robotic arms is controlled  using a Hierarchical Control System.A Hierarchical control system is partitioned vertically into levels of control. The basic comand 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 subgoals. These subgoals are in turn transmitted to the next lower control decision level as sequences of  less complex but more frequent commands. In general,the decisions and corresponding decompositions at each level take into account: (a) conrmands from the level above, (b) processed sensory feedback information appropriate to that control decision level, and (c) status reports from decision control modules at the next lower control level.

The hierarchical control structure serves as an overall guideline for the architecture and partitioning of a sensory interactive robot control system.

CMI T3-776 CONTROL SYSTEM BLOCK DIAGRAM

The figure shown above depicts  the schematic block diagram of the integrated control structure as configured on the Cincinnati Milacron T3 Robot. 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. That is, these levels 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 shown in figure  is part of the T3 Robot equipment as purchased from Cincinnati Milacron. This 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 Controller

(2)The 3 point Angle Acquisition System which reports data to the T3 Controller, the T3

Level of the RCS and to the Watchdog Safety System

(3)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 performs

sophisticated 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 Level

The Task Level interfaces with the Workstation Level above it and the Elemental-Move Level below it. In the current configuration, the Task Level has no direct interfaces with sensory systems. 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 Level

The 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. A trajectory maybe simply a straight line move to the goal point or a more complex move, involving departure, intermediate and approach trajectories. These trajectories can be constructed using pre-stored trajectory segments or data acquired from the Vision System. If no pre-stored segments are found for the desired move and the use of vision data is not appropriate, then a straight line path to the new goal point is calculated.

(3)Primitive Level

The 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 RCS

which 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. T3 The Level shown in figure  is not a true control decision level by itself and could be logically combined with the T3 Controller at the device level. The robot and end-effector are, therefore, at the same control decision level subordinate to the Primitive Level. Additionally, 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). Under Joystick control the human operator assumes the higher level planning and control duties normally handled by the E-Move and Task Levels when the robot is operating automatically. The actual Joystick unit has groups of small joysticks, rotory and rocker switches dedicated to each coordinate system. These are configured  such t hat  the  robot  will move basically the way the lever is pushed or the switch turned that the robot will move basically the way  the lever is pushed or the switch turned, giving the operator  a    relatively  feel  for  the  motion  produced ’The Primitive Level receives commands from the E-Move L e v e l  in terms of goal points in Cartesian space.These points differ  from those received by the E-Move Level from the Task Level in that they are not named locations and therefore  assume no knowledge of the Workstation layout. These points are typically more closely spaced than those at the higher Levels although this is not necessarily the case.

(4) T3 Level

The 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.

References:Wikipedia,Brittanica,IEEE,Google(Pictures)