Back-drive control

It is now common to couple the centrate and back-drive controls to a PLC together with signals to and from other parts of the decanter plant, such... 

Regular power operation is attained by moving the reflector upward at a constant speed of Imm/day to compensate for the reactivity decrease due to the bum-up of the core. Since no feedback system or control system are used, the reflector speed remains constant and the electric output is adjusted by varying the feed water flow rate to control die core inlet temperature. The controllable range of the power level by the water flow is 10% at the rated power, which is limited by the steam generator heat balance. Beyond this range, a back-up control mechanism to adjust the reflector position is installed in the driving mechanism. 

Sometimes the frame is mounted on a sub-frame together with the drive motor, and where necessary a back-drive system, to control the gearbox pinion shaft, which will in turn control the conveyor-to-bowl differential speed. The back-drive system will be described later, but for the present it suffices to say that it is essentially a braking motor or similar device coupled to the gearbox pinion shaft. The main motor is offset from the bowl and drives the bowl by means of a set of V-belts. The back-drive can also be offset, in which case it would be connected with a timing belt. The timing belt is to facilitate more accurate speed control. However the back-drive system can also be mounted direct in line with the pinion of the gearbox. 

The back-drive system is a means of controlling the speed of the gearbox pinion shaft (and thereby the conveyor differential speed) using, for instance, a motor or a brake. This could be offset from the gearbox shaft, in the same manner as the main drive, and connected by a belt. This belt would be a timing belt because of the accurate control required. Normally the back-drive is connected directly and in line with the gearbox pinion. 

The control of the main drive is relatively simple and straightforward. The control of the back-drive system has required dedicated development. Today the back-drive systems can control the decanter at a fixed differential, at a fixed torque or cake dryness or with some hybrid control system. The hybrid system, say, controls at a fixed differential, until a pre-set torque limit is reached and then at that torque level until the differential changes to a pre-set level. 

With main drive and back-drive systems under control, it remains to control centrate quality. For this, a good centrate monitor is required, capable of assessing the level of suspended solids in the centrate. This has been difficult but there are a few reliable devices now available on the market [20]. The monitor is then coupled via a PID controller to the polymer pump speed control. 

For waterworks applications, decanters normally will be operated with polymer addition facilities, at alternative admission points. They will need full erosion protection for the flights, using tiles. They will have some kind of cake baffle, possibly a cone, and a variable speed back-drive, with good differential and torque control. They will be operated with deep neutral ponds, with axial flow (i.e. with flight windows, such that the liquid flows parallel to the axis, rather than around the helical space), and with provision for wash-out prevention at start-up. 

The decanter system itself hardly needs further description. The main motor and back-drive motors are the main control inputs. Larger decanters may have a separate oil lubrication system for the main bearings, in which oil flows, temperatures and pressures are monitored. 

The main motor controller is a separate controller, and depends upon the type of installation and motor. The motor could be AC, DC or inverter type. Rarely, it could be a hydraulic motor. The starter could be DOL (direct-online), particularly if a fluid coupling is fitted, it could be a soft-start inverter system, or a DC system. With an Inverter system the back-drive, also an inverter type, could be connected through the DC bus to allow power regeneration. The starter Itself could be actuated by a separate master system. Undoubtedly there will be interlocks with the starter, to cause it to de-energise with certain scenarios. 

The equipment used for the CIP feature is described in Section 2.4.14. A small PLC, or an adjunct to one of the standard controllers, is required to supervise the CIP operation. An operator giving the start command to the PLC or equivalent will initiate the CIP sequence of events. The feed will be stopped. Then the main motor and back-drive system will be de-energised, and allowed to coast down to the required CIP speed, when the main drive donkey motor will be energised and take over to rotate the bowl at such a speed as to generate slightly less than Ig (about 70% of Ig). The more sophisticated systems will also have a donkey motor to rotate the gearbox pinion. 

There are many types of sen.sors used to feed-back the process operating conditions to the switching logistics of an inverter unit. They can be in terms of temperature, pressure, volume, flow, time or any activity on which depends the accuracy and quality of the process. Direct sensing devices used commonly for the control of a drive and used frequently in the following text are speed. sensors, as noted below. 

A hardened-steel or hardwood shoe bearing against the back of the chain is another method of controlling chain tension. The method is satisfactory for small horsepower drives operating on fixed centers at slow or moderate speed with ample lubrication. 

Directional control valves are designed to direct the flow of fluid, at the desired time, to the point in a fluid power system where it will do work. The driving of a ram back and forth in its cylinder is an example of when a directional control valve is used. Various other terms are used to identify directional valves, such as selector valve, transfer valve, and control valve. This manual will use the term directional control valve to identify these valves. 

Controlled flow of H back across membrane drives ATP synthesis... 

Feedback control. The traditional way to control a process is to measure the variable that is to be controlled, compare its value with the desired value (the setpoint to the controller) and feed the difference (the error) into a feedback controller that will change a manipulated variable to drive the controlled variable back to the desired value. Information is thus fed back from the controlled variable to a manipulated variable, as sketched in Fig. 1.7. 

There has been a real revolution in instrumentation hardware in the last several decades. Twenty years ago, most control hardware was mechanical and pneumatic (using instrument air pressure to drive gadgets and for control signals). Tubing had to be run back and forth between the process equipment and the control room. Signals were recorded on strip-chart paper recorders. 

The basic purpose of integral action is to drive the process back to its setpoint when it has been disturbed. A proportional controller will not usually return the controlled variable to the setpoint when a load or setpoint disturbance occurs. This permanent error (SP — PM) is called steadystate error or offset. Integral action reduces the offset to zero. 

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