Torque feedback system

A torque feedback system has been developed to dampen the surface torque oscillations and consequently the stick-slip motion at the bit. The system consists of (see Figure 4-309)... 

Figure 4-309. Torque feedback system (a) block diagram (b) effect on drill string torque. (Courtesy Anadrill [113].)...

While all these paramclcrs are extremely essential for a process line, with the R D in the field these limitations have been overcome with the use of phasor controls. To implement the.se controls different manufacturers have tidopled different control and feedback systems to monitor and control the torque and field components. They have also given these controls different trade names. The basic technological concept remains the same but process implementation may vary from one manufacturer to another. Below we attempt to identify the more common phasor controls introduced by a few leading manufacturers. 

The most important component of the feed system in a flood-fed extruder is the feed hopper with possible stirrer and/or discharge screw. A mechanical malfunction of this system can be determined by visual Inspection. If the feed hopper is equipped with a discharge screw (crammer feed), constancy of the drive should be checked. For proper functioning of a crammer feeder, the drive of the crammer feeder should have a torque feedback control to ensure constant feeding and to avoid overfeeding. 

The master arms of the MIRO Lab system make use of a different architecture. They are composed of a Delta parallel stage for the translations and a serial wrist for the rotations. Two versions were successively used first an omega.7 haptic interface with a passive wrist, then a sigma.7 device with seven actuated axes (both from Force Dimension, Nyon, Switzerland). This hybrid architecture suffers from the same drawbacks as serial robots. While the Delta is very light and transparent, it has to carry a bulky and relatively heavy actuated wrist. In order to remain within an acceptable volume and weight, the torque feedback is limited and, even so, the two master arms cannot be placed side by side very close to each other. 

Saturable reactors, which are adjustable by a small dc signal, have also been used for both primary (stator) and secondary (rotor) control. In the primary they control motor voltage and therefore torque. In combination with fixed secondary resistors and feedback from a tachometer, this system can be used for precise speed and torque control of cranes, hoists, etc. Even reversing can be accomplished by using two saturable reactors in each of two (of three) phases. Other combinations of fixed or saturable reac tors in the primaiy and/or secondaiy, all combined with secondary resistors, provide a wide range of capabiUties and flexibihty for the wound-rotor motor. 

Thyristors have been replacing saturable reactors they are small, efficient, and easily controlled by a wide variety of control systems. A modern crane control drive uses fixed secondary resistors and two sets of primaiy thyristors (one set for hoist, one for lower). With tachometer feedback for speed sensing, the control for the motor provides speed regulation and torque hmiting in both directions, all with static-devices. A wide variety of control systems is possible the control should be designed for the specific application. 

The vision of braking and steering by wire will demand new, extremely reliable sensors. Even in early implementations of steer-by-wire systems, in which manual control can override any system failure, more than one sensor is normally used for the sake of redundancy. Many of the sensor principles required are already established in the market, including steering-angle sensors (e.g., for vehicle dynamics control) and pedal-position sensors. Mechanical action or feedback control, however, will drive the emergence of torque and force sensors. 

The torque produced by the motor is therefore reduced in proportion to the square of the rms value of the applied voltage. Circuits are available for both star and delta connected motors. Closed loop feedback control may be used to adjust the firing of the thyristors, thereby making accurate speed regulation possible. These systems are only used for small machines, e.g. up to 20 kW because they tend to produce many harmonic currents and voltages in the supply. 

Abstract A bond graph method is used to examine qualitative aspects of a class of unstable under-actuated mechanical systems. It is shown that torque actuation leads to an unstabilisable system, whereas velocity actuation gives a controllable system which has, however, a right-half plane zero. The fundamental limitations theory of feedback control when a system has a right-half plane zero and a right-half plane pole is used to evaluate the desirable physical properties of coaxially coupled inverted pendula. An experimental system which approximates such a system is used to illustrate and validate the approach. 

One solution to transducer deflection is to eliminate it with a feedback control servo see Figure 8.2.5 (Franck, 1985a). The transducer is essentially a dc motor in which torque is measured by the current needed to prevent any deflection. No servo system is instantaneous, and a combination of high frequency and torque can lead to transducer compliance (Mackay and Halley, 1991). The upper frequency is about 100 rad/s, whereas stiff torsion bar systems have resonant frequencies above 10 rad/s and piezoelectric sys-... 

Frequently, an interface application will require that the PC controls the motion of an object. A device that Is often used to power or move a shaft in precise Increments, directions and speeds Is a stepper motor. Here we consider two approaches to this problem. The first uses the stepper motor, a device whose angle of rotation is known reliably from the pulses which have been sent to it. Once calibrated and Initialized, no feedback of the rotor s position is necessary, unless the speed demanded is too high or the torque required Is too great. Running a motor this way without feedback is called OPEN LOOP. The second method uses feedback, is a CLOSED LOOP approach, and Is called a servo system. The servo system can respond more quickly and accurately than the open-loop stepper motor system and Is relatively insensitive to hardware variations. However, It requires position sensors as well as more complicated drive electronics to ensure stability. 

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