208 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



speed of the central conductor feeding into the extruder applying the 

 core insulation, as shown in the block diagram. Since the extruder de- 

 livers insulating material at a constant rate, an increase in central con- 

 ductor speed results in a thinner than normal wall of insulation and thus 

 causes an increase in capacitance. 



The sensing element for the control loop consists of a capacitance 

 monitor. This is a device capable of measuring the unit length coaxial 

 capacitance of the cable core continuously as it moves through the water 

 in the trough. Since the capacitance of a polyethylene insulated core is 

 temperature sensitive, the monitoring electrode must be located at a 

 point in the cooling trough where the temperature of the core is stable 

 and known to a degree commensurate with the overall accuracy objec- 

 tives. The distance from the extruder to the electrode corresponds to 

 about 10 minutes of cooling time; hence, a servo system based on this 

 loop would be necessarily slow, due to the 10-minute delay in detecting 

 a drift in capacitance. 



Analysis shows that fast capacitance information of only moderate 

 accuracy may be used in combination with the slow loop to speed up 

 the response of the overall system to a satisfactory degree, without 

 sacrifice of precision of the slow loop. The sensing element used for the 

 fast loop consisted of a light-ray diameter gauge, which measures the 

 diameter (changes in diameter are the approximate inverse of the ca- 

 pacitance) of the hot core close to the extruder. The slow and fast data 

 are combined to control the extruder, as shown in the block diagram. 



The servo constants were chosen to minimize the deviations in unit 

 length capacitance occurring in core lengths corresponding to less than 

 J wave length of the top operating frequency. Stated in other words, the 

 objective for choice of servo loop constants was to assure equality in the 

 capacitance of all I wave sections of core. Echo measurements indicated 

 that a highly satisfactory degree of control was achieved. Overall servo 

 system performance was such that the standard deviation of the ca- 

 pacitance of the core lengths manufactured for the two crossings was 

 ±0.1 per cent. The capacitance monitor electrode and the servo console 

 is illustrated in Fig. 8. 



ADJUSTMENT OF CONCENTRICITY 



Means for setup and adjustment of the extrusion process to achieve 

 relatively accurate centering of the conductor in its sheath of insulation 

 was provided by a device called a concentricity gauge. This device op- 

 erates on the principle that two small, plane electrodes on opposite 



