Schematic of Synchros Transducer The complete circle represents the rotor. The solid bars represent the cores of the windings next to them. Power to the rotor is connected by slip rings and brushes, represented by the circles at the ends of the rotor winding. As shown, the rotor induces equal voltages in the 120° and 240° windings, and no voltage in the 0° winding. [Vex] does not necessarily need to be connected to the common lead of the stator star windings.

Closely related in design to three-phase AC synchronous motors are stepper motors, where an internal rotor containing permanent magnets or a magnetically-soft rotor with salient poles is controlled by a set of external magnets that are switched electronically. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. Unlike a synchronous motor, in its application, the stepper motor may not rotate continuously; instead, it “steps” — starts and then quickly stops again — from one position to the next as field windings are energized and de-energized in sequence. Depending on the sequence, the rotor may turn forwards or backwards and it may change direction, stop, speed up or slow down arbitrarily at any time.

The simplest way to think of a stepper motor is a bar magnet and four coils. When current flows through coil “A” the magnet is attracted and moves one step to the right. Then, coil “A” is turned off and coil “B” turned on. Now, the magnet moves another step to the right and so on.

Steppers are generally commutated open loop; the driver has no feedback on where the rotor actually is. A stepper motor can be a good choice whenever controlled movement is required. They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism. Some of these include printers, plotters, high-end office equipment, hard disk drives, medical equipment, fax machines, automotive and many more.

Reference:

ü       www.wikipedia.com

ü       www.madabout-kitcar.com

ü       www.codeproject.com

ü       www.kilowattclassroom.com

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

Example of the pole and zero distribution of a rational transfer function in the complex plane

Real axis zeros tend to spread the loci faster and stabilize the system. Real-axis poles, on the other hand, tend to make the loci spread more slowly and curve toward instability. The root locus design method involves three steps: the closed loop function is determined, and then the open loop transfer function, and finally a compensation network are synthesized.

 For stable systems, pole should be in right hand of S plane (negative real value) and if it is imaginary axis (non real), it indicates system is oscillatory.
If it is real and imaginary (a complex), then system response is damped oscillation. If poles are left in S plane then system is highly unstable.

Reference:

 ü       Web.mit.edu

ü       virtual.cvut.cz/dynlabmodules/ihtml

ü       www.yahooanswers.com

ü       www.datasheetcatalog.org

Consider the second-order system

Poles are s = –p1 & s = –p2. When we add zero at s = –z1 to the controller, the open-loop transfer function will change to:

We can put zero at 3 different positions with respect to the poles:

1. To the right of s = –p1 Fig (b)

2. Between s = –p2 & s = –p1 Fig(c)

3. To the left of s = –p2 Fig (d)

 The effect of changing the gain K on the position of closed-loop poles and type of responses:

 (a) The zero s = –z1 is not present.

Over, critically or under damped

  (b) The zero s = –z1 is located to the right of s = – p2 & s = –p1.

Over damped

 (c) The zero s = –z1 is located between s = –p2 & s = –p1.

Over damped

 (d) The zero s = –z1 is located to the left of s = –p2.

Flexible configuration

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

In short we can conclude that a peak overshoot results on adding zero to a control system.

Reference:

ü       Web.mit.edu

ü       www.wikipedia.com

ü       www.palgrave.com

ü       Applied control theory- James r Leigh-1987

High-performance airplanes need special servomechanisms called flight-control systems to compensate for performance instabilities that would otherwise compromise their safety. The aerodynamic designs that optimize a plane’s performance sometimes cause instabilities that are difficult for a pilot to manage.
A plane may have a tendency to pitch up and down uncontrollably, or yaw back and forth under certain conditions. These two instabilities may combine with a third problem where the plane tends to roll unpredictably. Sensors called accelerometers pick up these oscillations before the pilot is aware of them and servomechanisms introduce just the right amount of correction needed to stop the unwanted activity. The servos that perform this magic are called pitch dampers, yaw dampers, and roll dampers. Their effect is to smooth out the performance of a plane so that it does only what it should. Without servomechanism technology flight-control systems would be impossible and the large safe aircraft we take for granted would be impractical.
Various servomechanisms provide the enabling connection between data and mechanical actions. If all servomechanisms were to disappear from technology overnight, our world would be much less comfortable, much less safe, and certainly less convenient.

Servomechanisms are classified on the basis of whether they depend upon information sampled at the output of the system for comparison with the input instructions. The simplest servomechanisms are called open-loop servomechanisms and do not feed back the results of their output. Open-loop servomechanisms do not verify that input instructions have been satisfied and they do not automatically correct errors.
An example of an open-loop servomechanism is a simple motor used to rotate a television-antenna. The motor used to rotate the antenna in an open-loop configuration is energized for a measured time in the expectation that antenna will be repositioned correctly. There is no automatic check to verify that the desired action has been accomplished. An open-loop servomechanism design is very unsatisfactory as a basis for an antenna rotator, just as it is usually not the best choice for other applications.
When error feedback is included in the design the result is called a closed-loop servomechanism. The servo’s output result is sampled continuously and this information is continuously compared with the input instructions. Any important difference between the feedback and the input signal is interpreted as an error that must corrected automatically. Closed-loop servo systems automatically null, or cancel, disagreements between input instructions and output results.
The key to understanding a closed-loop servomechanism is to recognize that it is designed to minimize disagreements between the input instructions and the output results by forcing an action that reduces the error.
A more sophisticated antenna rotator system, compared to the open-loop version described earlier, will use the principles of the closed-loop servomechanism. When it is decided that the antenna is to be turned to a new direction the operator will introduce input information that creates a deliberate error in the servomech anism’s feedback loop. The servo’s electronic controller senses this purposely-introduced change and energizes the rotator’s motor. The antenna rotates in the direction that tends to null the error. When the error has been effectively canceled, the motor is turned off automatically leaving the antenna pointing in the desired direction. If a strong wind causes the antenna turn more slowly than usual the motor will continue to be energized until the error is canceled. If a strong wind repositions the antenna improperly the resulting error will cause the motor to be energized once again, bringing the antenna back into alignment.
Another example of a simple closed-loop servomechanism is a thermostatically-controlled gas furnace. A sensor called a thermostat determines that heat is required, closing a switch that actuates an electric circuit that turns on the furnace. When the building’s temperature reaches the set point the electric circuit is de-energized, turning off the fuel that supplies the flame. The feedback loop is completed when warmed air of the desired temperature is sensed by the thermostat

• wapedia.mobi/en
• encyclopedia.thefreedictionary.com
• www.britannica.com
• science.jrank.org
• www.cinnanitizoo.org

RC servos
RC servos are servos typically employed in industrial robotics, automation, and radio-controlled models. They are also 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. Robotics is the science and technology of robots, their design, manufacture, and application. … This article does not cite any references or sources. … 1:10 scale radio controlled car (Saab Sonett) A radio-controlled model (or RC model) is a model that is steerable with the use of radio control. …
RC servos are comprised of a DC motor mechanically linked to a potentiometer. Pulse-width modulation (PWM) signals sent to the servo are translated into position commands by electronics inside the servo. When the servo is commanded to rotate, the DC motor is powered until the potentiometer reaches the value corresponding to the commanded position that it’s ordered. Pulse-width modulation of a signal or power source involves the modulation of its duty cycle, to either convey information over a communications channel or control the amount of power sent to a load. …
Due to their affordability, reliability, and simplicity of control by modern microprocessors, servo motors are often used in small-scale robotics applications.
The servo is controlled by three wires: ground (black/orange), power (red) and control (brown/other colour) and will move based on the pulses sent over the control wire. 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 pulses sent over the control wire set the angle of the servo horn. 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.
A servo pulse of 1.5 ms width will set the servo to its “neutral” position, or 90°. For example a servo pulse of 1.25 ms could set the servo to 0° and a pulse of 1.75 ms could set the servo to 180°. The physical limits and timings of the servo hardware varies between brands and models, but a general servo’s angular motion will travel somewhere in the range of 180° – 210° and the neutral position is almost always at 1.5ms.
Servo motors are often powered from nickel-cadmium battery packs common to most RC devices. Voltage ratings vary from product to product, but most servos are operated at 4.8 V DC or 6.0 V DC (a 4 or 5 cell battery). The nickel-cadmium battery (commonly abbreviated NiCd and pronounced nye-cad) is a popular type of rechargeable battery for portable electronics and toys using the metals nickel (Ni) and cadmium (Cd) as the active chemicals. … Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. …

                                                        A typical industrial robotic controller consists of two controller levels, a system level controller and servo loop controllers. The system level controller calculates the set points for each servo loop controller. Various types of control algorithms can be used to generate the set points such as position, velocity or torque. The servo loop controllers perform low level control on each of the robot ’sactuators .  A 6 degree of freedom robot requires 6 servo controllers.

The robot used by the Robotics Research Group is a Cincinnati Milacron Inc. (CMI) T3-776 heavy duty industrial robot. The robot’s factory controller consists of a CMI ACRAMATIC version 4 system controller and CMI Silicon Controlled Rectifier (SCR) .T3-776 was not capable of executing complex control strategies. Most current industrial robot system controllers are designed for a particular machine. Each brand and, in some cases, model of robot has a different controller architecture. Thus control algorithms designed for one robot cannot necessarily be ported to another. At Cincinnati Milacron Corporation, Richard john 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.

The T3 robotic arms are controlled using a Hierarchical Control System