Feedback devices, control

The phasor /, and /, are separated and then controlled separately as discussed later. For more precise speed control a pulse encoder feedback device can also be employed. The characteristics now improve to Figure 6.10. The torque can now be maintained constant at any speed, even at zero speed. 

Tacho generator (TG) - This is an analogue voltage feedback device and can provide a speed input to the inverter control circuit. It provides only a low level of speed regulation, typically x 0.4% of the set speed. 

What will be given is examples of control loops for the common kinds of equipment and operations. Primarily these are feedback arrangements, but, as mentioned earlier, feedback devices usually are necessary supplements in primarily feedforward situations. 

A photoinduced discharge apparatus includes a corona charging unit, an exposure system, and a noncontacting voltmeter. A grid is usually positioned between the corona wires and the sample to control the corona current. The apparatus is normally provided with a beam splitter and radiometer. The surface potentials are determined by a capacitively coupled probe and FET amplifier. The time derivative of the voltage can be obtained electronically or by numerical differentiation on a computer. When a time resolution greater than 10 ms is adequate, commercial voltmeters, for example, Monroe Electronics Isoprobe or Trek Electrostatic voltmeters, can be used. These are feedback devices, based... 

A surveying robot is a programmable feedback controlled multifunctional device with several axes designed to both move and/or orientate measurement devices to perform specific measurement tasks along variable programmed paths. 

Developments in web tension control systems are providing increased capability and function to eliminate problems (Fig. 3-34). They include ultrasonic roll diameter sensors, pneumatic pressure gauge tension monitors, capston-Mt. Hope tension systems, and so on. As an example, replacement of a web-tensioning system s conventional electromechanical drive with an ordinary ac motor enables processors to lower system cost and improve web consistency, as has been done for many years. A vector control system uses a belt and pulley arrangement to remotely couple an encoder to the shaft of the ac induction motor. This approach provides closed-loop feedback, without requiring that one modify the relatively inexpensive motor by installing a special feedback device on it. 

Most servo feeds manufactured today utilize a trapezoidal move profile with its four distinct jerk points that can cause slippage but some are also available with controls that can execute s-curve move profiles. Systems that are electronically synchronized to press rotation are available as well. These units require a special controls package and feedback device, either a resolver or encoder, that are attached to the press crank to track press rotation. Their top speeds are still limited by the available drive power as opposed to press-driven feeds which can run as fast as the press and tooling are capable of handling. 

Servomechanism theory was further developed during World War II. The development of the transistor in 1951, and hence, digital electronics, enabled the development of electronic control and feedback devices. The field grew rapidly, especially following the development of the microcomputer in 1969. Digital logic and machine control can now be interfaced in an effective manner, such that today s automated systems function with an unprecedented degree of precision and dependability. 

Measurement of the moisture content of sinter feed with feedback to direct water additions at various mixing points is thus an essential part of good sinter machine control. Various devices are used for online moisture measurement, including infrared devices and electrical conductivity probes. The latter are most common and can simply be two trailing electrodes in contact with the feed material on a conveyor belt. These devices are not highly reliable and need careful and regular calibration since they are affected by the presence of soluble salts, which can change the electrical conductivity of the free interstitial water in the sinter bed. 

Utility functions (military, medical, safety, and pragmatic applications, such as charging and controlling digital devices, caller ID and other smart phone apphcations, and visual feedback... 

The electrical and electronic control system consists of the field devices feedback system, electric motor drive system and the safety and manual logic system. The commands from the micros ar checked in the safety logic system for safe operation before being sent to the fluid power system. The fluid power system operates the device in the correct direction, at the required speed and force, to position it accurately. The feedback device (say potentiometer) is read by the micro and appropriate commands initiated. The device is also connected to the computer independent display on the console. Operation and alarm messages are diplayed on the VDU and logged by the printer. 

A variety of AC and DC drives are available for extruders. DC motors with solid-state eontrollers and power supplies provide the most flexible seenario for control. A speed control feedback loop on the drive will minimize variation in the extruder output. The unwind roll should be equipped with a tension control device such as an electronic regulator. 

Control of parameters during an industrial chemical process leads to better quality products, chiefly because of variations in the inputs and fluctuations in the quality of intermediates. The earliest applications of automatic control to chemical production involved simple independent feedback devices such as thermostats. But, as the number of inputs and variables to control grows, there will be interactions and possible instability. 

For example, one type of smart system is used in helicopters to reduce aerodynamic cockpit noise created by the rotating rotor blades. Piezoelectric sensors inserted into the blades monitor blade stresses and deformations feedback signals from these sensors are fed into a computer-controlled adaptive device that generates noise-canceling antinoise. 

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