La preocupación por tu figura, ¿te ha hecho pensar que

An

proporti onal control by os cillati ng the s et poi nt so that the average output from the controller is proporti onal to

the mean distance from the set point2 6 • _{Figure 2 .2} gives an example of this for an on/off controller with a dead space. A symmetrical sawtooth waveform is

applied in series with the input, the period of the sawtooth being less than the time of the fastest

fluctuation to be controlled . If F is some control

setting in the power supply, and is modified by the controller so that when the controller is on/off the rate of change of F is

dF _{(instantaneous) = ±K} dt

and if A is the amplitude of the waveform,

then for an excursion a from the set point it can be easily shown that

dF

dT (average over several cycles) = - 2- 9K A

This gives a control of the integral type2 7_-2 9 _whose

response can be adjusted to give optimum control by varying either K or A, subject to the condition that A must be greater than the dead space. A similar equation is obtained if the control is on-off, the average input power adjusting itself to the excursion from set point .

Improvement of the on-off control was first tried using this principle in conjunction with the original

controller circuitry. The controller, instead of reading the thermocouple voltage directly,

reads the unbalance current of a resistance-balanced Wheatstone bridge with the thermocouple in one arm

{Figure 2. 3) . In this configuration, an open circuit in the thermocouple due to aging gives an increase in galvanometer current, not a decrease to zero as with a simple circuit. This means that the controller turns the furnace off instead of possibly destroying it.

Insertion of a few ohms resistance in the thermo­ couple arm gave galvanometer movement equivalent to about 10°_{c temperature change, which was suitable for}

an oscillation amplitude. The oscillation was produced by a S n potentiometer in sawtooth motion, driven by two brass segments which al�ernately contacted a

rubber wheel on the potentiometer shaft. Apart from mechanical hirsuteness, instability resulted from

the small value of the resistance and consequent large effect of contact wear. Optimum control could not

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be obtained, since amplitude adjustment was not possible .

When this modification was tried it improved the control by a factor of two over the original performance. Since this was still insufficient, it was decided to

build a separate power control and incorporate an improved oscillator.

2.4 An Improved Control

This section covers the circuit used to provide the basic control. The power elements will be discussed later. A bridge circuit is desirable for the control portion , since it can be balanced , and larger currents and voltages can be used in the bridge itself than are tolerable in the galvanometer circuit. This means that the previous contact resistance problem can be avoided.

The set point of the controller is adjusted by a pointer unsuitable for motor drive , so a better way to obtain a temperature sweep is to add to the controller input a steadily increasing voltage aiding the thermo­ couple. For the controller to maintain set point under these conditions , it must sweep the temperature down.

This sweeping voltage and the oscillating voltage were introduced by the circuit shown in Figure 2.4. A 3 pole 4 position rotary switch had its A and B contacts connected to a millivolt potentiometer. In positions l to 4 it read

output millivolts sweep arm current (mA)

oscillating arm current (mA)

battery voltage (volts)

Lead-acid accumulators were used to give stable, noise-free voltages.

The components were as follows : Label Value , ohms

s

s

"

Develops output voltage Current control

Current limit Voltage sweep

Adjust start point of sweep

Vary current to give oscillating output .

Ad just amplitude of oscillation .

The potentiometer P was driven up and down over

half its range by a synchronous motor. This gave a

variation in one loop current , and hence provided the

varying voltage in the thermocouple circuit . If P ₁

was increased , the fractional change in resistance was reduced , and hence the A . C. amplitude was reduced .

Thus the amplitude of oscillation (one of the para­ meters in the control equation) could be adjusted.

Obviously , a low setting of the resistance P

gave a higher current and a greater amplitude . A

current of 4mA average gave an oscillation of 1

±

10

_{c ,}

arm containing S and S provided a balancing D .C . _l

voltage , which could be increased to provide the

temperature sweep.

The thermocouple break protection bridge can be balanced to allow for the S n resistance added by this circuit , and the control bridge balanced by adjusting the P and S arms to give the same galvanometer reading with the battery on and off.

The temperature sweep can be started by turning on the drive motor at a suitable time .

2 . 5 Motor Drive s

The sweep potentiometer S was driven by a Sangamo synchronous motor through a chain of four Meccano gears . The last of these was mounted concentric with the potent­

iometer shaft and spring loaded onto a neoprene washer which acted as a clutch (Figure 2 . 6) . The resistance could thus be reset by hand without unmeshing gears ,

and the potentiometer was protected if driven to the end of its travel . A range of motors were avai lable to allow di fferent sweep speeds . The most used con­ fi guration was a 1 revolution per hour motor , with a 1 : 9 gear reduction . This gave 56 ohms/hour resistance

drop , corresponding to about 15°_{c at 1100}°_{c , increasing}

as the temperature dropped .

For the operation o f the gearing for the oscillation

see Figures 2 . 5 and 2 . 7 . Gears 2 , 3 and 4 were mounted

to mesh , 2 and 3 on the same level , 4 placed half a gear thi ckness above them. The upper halves of some of the teeth of 2 and 3 were milled off so that 4 was in mesh with either 2 or 3 at any position . As 2 and 3 were driven round by 1 , they alternately drove 4 through a constant angle and back again . Modi fication of some teeth on 4 was ne cessary so that the segments meshed without jamming. As the re lative positions of 2 - 4 were critical , a further ge ar , 5 , was mounted on the

shaft of P so that it could easi ly be removed and replaced

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if necessary . Sangamo motors were found to have insufficient torque to drive the system reliably , so gear 1 was driven

by a Philips motor type AU 5050/2 2 and a gearbox of the

AU 5 3 0 0 series se lected to give suitable speed . At speeds