Fisher Fisher - Dead Band Adjustment for Electric Actuators

1. Definition of dead zone

In the field of instrumentation, it is also known as the insensitive area of the instrument. A finite interval in which a change in the input does not cause any perceptible change in the output of the instrument. The dead zone is mainly caused by friction and intermittent intermittent between the internal components of the instrument.

In the field of valves, the valve dead band is also a general phenomenon, which refers to the range or width of the controller output (CO) value that cannot cause the process variable (PV) to change when the input signal changes direction...... When a load disturbance occurs, the process variable (PV) deviates from the set point. This deviation passes through the controller and then through the process to produce a corrective action. However, an initial change in the controller output may not result in a corrective change in the corresponding process variable, but only if the controller output has a change large enough to overcome the dead zone.

2. Causes of dead zones

There are many causes of dead zones, but friction and drift of control valves (control valves), torsion of rotary valve shafts, and dead zones of amplifiers are a few common forms. Since most of the action of a regulated control is made up of small signal changes (1% or less), a control valve (control valve) with a very large dead zone may not even respond to so many small signal changes at all, a well-made valve should be able to respond to 1% or less of the signal to effectively reduce the degree of process deviation. However, it is not surprising that some valves exhibit a dead zone of 5% or more. In a recent factory audit, 30% of valves had more than 4% dead zones. More than 65% of audited loops have a dead zone greater than 2%. Figure 2-3 illustrates how big the combination of dead zones is. This diagram represents an open-loop test of three different control valves (control valves) under normal process conditions. These valves accept a range of step inputs ranging from 0.5% to 1%. Step testing must be performed under those critical flow conditions that allow the performance of the entire control valve (control valve) assembly to be evaluated, rather than just the performance of the valve actuator under most standard test conditions.

Friction is a major cause of dead zones. Rotary valves are very sensitive to friction caused by high seat loads. For some seal types, a high seat load is necessary to obtain a closing rating. Due to the high friction and low drive strain stiffness, the valve shaft twists and cannot transmit motion to the control elements. As a result, a poorly designed rotary valve can exhibit a large dead zone that can have a decisive impact on process deviations. Manufacturers typically lubricate the seal of the rotary valve during the manufacturing process, but after only a few hundred cycles, the lubrication layer wears off. In addition, pressure-induced loads can also cause seal wear. As a result, the friction of the valve may increase by 400% or more for some valve types. This means that it is misleading to draw conclusions about the performance of the valve by using standard types of data to evaluate the valve until the torque is stabilized. Gates B and C (Figs. 2-3) show that these higher frictional moment factors can have a devastating effect on the performance of a regulating valve (control valve). Packing friction is the main source of friction for linear control valves (control valves). In these types of valves, the measured friction can vary greatly depending on the valve form and packing structure.

The type of actuator also has a fundamental effect on the friction of the control valve (control valve) assembly. In general, a spring membrane actuator produces less friction on the control valve (control valve) assembly than a piston actuator. Another advantage of the spring film actuator is that its friction is relatively constant and does not change with time. The friction of the piston actuator increases significantly with the wear of the guide faces and O-rings, the loss of lubrication layers, and the deterioration of the elastomer's performance class. In this way, in order to ensure continuous performance, piston actuators require more frequent maintenance than spring film actuators. Without maintenance, process deviations can increase significantly without the operator being aware of it.

When the device changes direction, this pneumatic movement causes discontinuities in motion. Idle strokes typically occur in gear-driven units with a wide variety of configurations. Rack and pinion actuators are particularly susceptible to dead zones due to idle stroke. Some valve shaft connections also exhibit a dead zone effect. In general, spline connections have a smaller dead zone than keyway connected valve shafts or double D-shaped structures. Although friction can be greatly reduced by a good valve design, it is a difficult problem to eliminate it altogether. A well-designed and manufactured control valve (control valve) should be able to eliminate dead zones caused by idle travel and shaft torsion. In order to achieve the best results in reducing the degree of process deviation, the total dead zone of the entire valve assembly should be equal to or less than 1%, ideally as low as 0.25%.

3. Adjustment of dead zones

The dead zone is used to adjust the sensitivity of the valve. Take the electric actuator as an example: the input 4mA valve is fully closed, 20mA is fully open, when the dead zone is adjusted to *** hour, the sensitivity is *** large, and the input 4.01ma valve may follow the change of the signal and move to the corresponding opening. If the dead band is increased, perhaps the valve will still close when the input signal is 4.2. The potentiometer that regulates the dead zone is the part of the comparator's circuit. When the small disturbance signal enters the control system, the valve will oscillate near the opening when the valve sensitivity is high, the dead zone can effectively solve this problem, but at the same time sacrifice the sensitivity.

Fisher Fisher - Dead Band Adjustment for Electric Actuators

Fisher Fisher - Dead Band Adjustment for Electric Actuators

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