Rack Position

divisions. Whenever the operator engages the “Raise11 or “Lower” switch positions, the As-U-Roll rack moves at a nominally constant rate, i.e. the system is “bang-bang". For each As-U-Roll the operator's switch controls relays which, in turn, energize hydraulic solenoid valves Feeding a hydraulic motor. This motor is geared onto the As-U-Roll rack.

Clearly, closed-loop control of these actuators is desirable if they are to form part of an automatic scheme. The optimum means of providing such control would be to replace the bang-bang elements with a proportional

servo valve system, but this was not possible for

financial reasons. Therefore a simple closed-loop system around the existing plant has been incorporated in the system software. This takes the form of figure 4.2. The transfer function of the hydraulic valve (time constant) has been estimated from plant tests (see Chapter 5-)* The transfer function of the rack is an integrator whose gain is found from the rack velocity. Although this velocity is nominally fixed, the hydraulic

supplies to the hydraulic motors are fitted with variable restrictions in each direction, so that in practice each rack may raise and lower at different rates. Furthermore, the hydraulic supply to these motors is not rated to

the plant engineers slowed down its response at the

Author’s request, - the ’’actual” value was then obtained (see Chapter section 5«4.l). Some backlash is to be expected in the rack mechanism, and this is therefore also shown in figure 4.2. The "actual” magnitude of the backlash has also been estimated from plant tests. The controller simply takes the form of a small pro­

portional gain and an imposed dead-band to prevent system hunting (which would shorten the life expectancy of the mechanical components). The initial selections of con­ troller gain and dead-band were, made by digital and analogue dynamic simulations discussed in Chapter 7-

For the purposes of control system design and sim­ ulation (see Chapters 6 and 7) the non-linear As-U-Roll

system has been replaced by a second order system which gives a comparable response to the system of fig.4.2 under simulated conditions. The resultant system is described by (4.1) 2 2 s

+2%

(for "demonstration” system) (For "actual” system) and s = Laplace Operator

Figure 4.3 shows a step response as an example of the representation of the non-linear system by equation

(4.1). The reason for using the somewhat unreal values of the "demonstration" system is to illustrate where the lack of fit occurs at the origin of figure 4.3 • For the "actual" values, the fit is much better in this area as the effective dead-band width (d /K ) is much smaller._{a a}

(However, the fit at the MtopM of the characteristic is not quite so good).

Since the actuators are not mutually interactive, the block diagram matrix (G j is simply G = g (s ).I„_{a a a d} where g (s) is given by the "actual" values in equation_a

(4.1) and Ig is the identity matrix (8 square).

It is also worth reiterating at this point that constraints are imposed by the mill manufacturer upon the relative positioning of adjacent As-U-Roll actuators. This is designed to prevent attempts at excessive backing shaft bending gradients, or excessively sharp maxima or minima in the bending profile - any of which could

damage the mill. The means of ensuring compliance with these constraints are discussed in detail in Chapter 8

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4.3 First Intermediate Roll Lateral Adjustment Actuators The manual system for control of the first inter­ mediate roll lateral positions is very similar in concept to that previously described for the As-U-Roll systems. Again the operator is provided with three position

switches ("In-Of f-Out" ) which control "constant" speed hydraulic motors (about 590rpm ) via relays and hydraulic valves • The major differences are that the drive is not directly transmitted to the rolls, but is fed via quite long runs of chain drive and gear trains (overall

reduction ratio about l8.7:l) which drive an internally threaded thimble (see figure 4.4). This thimble is rotated by the chain drive (at about 31 .^pm) and en­ gages a non-rotating threaded section coupled to the end of the first intermediate roll, which is therefore moved into or out of the threaded thimble depending upon the thimble's direction of rotation. The pitch of the thread is about 6mm, giving a lateral velocity of 3»15mm/s.

In addition,the position indication device is more complex. A. selsyn transmitter is driven by one of the intermediate

shafts in the chain drive system. This is cabled to a matching selsyn receiver mounted at the front of the mill. The shaft of the receiver drives via a gear train onto a leadscrew arrangement, which linearly moves an indicating pointer visible to the mill operator. Apart from these differences (mechanical drive arrangement and trans­

Chain