CA CR CA CR CA CR

AOVANCE NO RETARD CORRECTION CORRECTION CORRECTION

Figure 3. The zones of correction *Contacts closed

printing cylinder with respect to the web is. energized when both W and CA contacts are closed. The coil for retarding operates when both W and CR are closed. Under a con­dition of correct register at the printing cylinder, the arm should be so positioned that shoe S drops over the trailing edge of the cam at an instant exactly half way through the closed interval of contact W. Reference to Cockrell's paper will show that in the electronic equipment the trailing edge of the cylinder disk slits is the indicating point. Under this condition the correcting motor will receive first an impulse to ad­vance, followed immediately by an impulse of equal duration to retard. The net cor­rection will be zero.

Suppose the gearing slips for some reason, and the printing cylinder drops back slightly with respect to the web. Contact W now will be closed predominantly during the time that CA is closed, and the correcting motor will advance the cylinder till register is restored. Similar reasoning applies to retard corrections. Figure 3 of this dis­cussion illustrates this action. As Cockrell explains, a single correcting pulse lasts only a matter of microseconds. The electronic equipment therefore must integrate these to obtain a signal long enough to operate the correcting motor.

Starting a new run on the press can be visualized easily with this mechanical analogy. With the automatic equipment dis­connected by a switch in series with inter­lock W, the operator brings the various colors into register by manual control of the correcting motor. Inspection of the product is his guide. He then moves the arm until shoe S slips over the trailing edge of the cam at the same instant that the arm of interlock switch W reaches its maximum deflection. Register operation then is switched over to automatic.

W. D. Cockrell: In reply to S. L. Burgwin's question about the 0.003 inch register toler­ance which I mentioned, I can say only that it was a compromise, arrived at after discus­sion of the various types of reproductions to be registered. If the cut consists of fine lines such as the reproduction of an etching or engraving, a difference of even 0.001 inch is noticeable. However, for other reproduc­tions, such as that of a water color having broad areas which blend, 0.010 inch register will be permissible. However, for the usual Kodachrome reproduction, 0.003 inch to 0.005 inch register will do a satisfactory job. The equipment described should hold better than 0.003 inch under constant speed con­ditions with uniform paper and humidity, but it is believed that the guarantee of 0.005 inch will cover most conditions of constant

speed operation encountered in the average color printing plant.

The high sensitivity demanded requires that we shock-mount our electronic equip­ment. This is shown in Figure 7 in the paper. However, in order to obtain the greatest accuracy of register, the optical system is mounted as rigidly as possible, since it is the position of the optical system (which ends with a slit through which the light is transmitted to the phototube) that determines the accuracy.

Obtaining the proper shape of electric wave front requires careful circuit design and co-ordination with the optical system. However, present day television practice re­quires the passage of much shorter signals and a much wider range of frequencies. Hence, the circuit requirements, while diffi­cult, are not at all impossible. The pro­jected width of slit is less than one-fourth that of the register mark so that the round­ing of the signal resulting from the optical system is not too great a handicap and a reasonably square wave-form results. The control is of the proportional type with a dead zone determined by the low torque re­quired to break the correcting motor starting friction.

K. P. Grenfell's mechanical analogy of the timing and synchronizing principles is quite interesting, and is a reasonably accurate picture of the action taking place. Since most press operators have mechanical rather than electrical training, analogies of this type should be very useful in presenting to them the theory of operation of the register control.

Electrical Accuracy of Selsyn Generator-Control Transformer System

Discussion of paper 46-113 by Harold Chestnut, presented at the AIEE summer con­vention, Detroit, Mich. , June 2 4 - 2 8 , 1946, and published in AIEE T R A N S A C T I O N S , 1946, August-September section, pages 570-6 .

C. Concordia (General Electric Company, Schenectady, N. Y.) : A paper such as this is valuable first in showing

1. The deviations of the actual component of a control system, the Selsyn, from the ideal propor­tionality assumed in so many studies. 2. The smallness of these deviations.

I t is difficult to obtain in small machines the good wave form ordinarily obtained m large machines by proper winding distribu­tion and pole shaping, and it may not be worth while in view of the deviation magni­tudes shown.

Another point that should be mentioned is that it is not possible to make deviations of any magnitude completely negligible simply by gearing the Selsyn system in order to make it run at a sufficiently high speed, since a reasonable accuracy of the gears lias to be considered, and there is some optimum division of effort between work de­voted to improving Selsyn accuracy and that devoted to improving gear accuracy.

A New Approach to Probability Problems in Electrical Engineering

Discussion and author's closure of paper 46-128 by H . A . Adler and K. W . Miller, presented at the AIEE summer convention, Detroit, Mich. , June 2 4 - 2 8 , 1946, and pub­lished in AIEE T R A N S A C T I O N S , 1946, October section, pages 6 3 0 - 2 .

Philip Doane (Consolidated Edison Com­pany of New York, Inc., New York, N.Y.) : If we assume infinitely small time intervals so that no two events occur within the same time interval, then origination of a period with r events existing can occur only by an increase or decrease of one event from a state with (r — 1) or ( r + 1 ) events existing. The numer of periods with r or more events then will be equal to the number with r events existing which originate as increases from the status with (r — 1) events existing. For r > 0 this will be, provided the average length and the frequency of the successive occurrences of any one event are the same for all events,

n\ r~Y n~r n r N 1 (T-t) IT = — P r

r (r-l)\(n-r)\ K J ' pT The average length as a fraction of the total time T of periods with r or more events ex­isting is then

/ Α / Γ = ( Ρ „ - Κ Ρ „ - ι + . . . +Pr+i+Pr)P/rPr

I t should be noted that , as in all of the equa­tions of the paper, the ending of a period with r events existing followed in the next time interval by the beginning of another period with r events existing is considered a division between two different periods.

Equation 12 apparently requires quali­fication as neglecting those cases in which successive originations of an event occur in time intervals immediately following the end of prior occurrences thereby oc­cupying time intervals otherwise desig­nated as intervals originating periods with the next lower number of events existing. In most practical eases the error would be negligible.

H. A. Adler: In view of several questions and special requests, the following additional background information seems desirable for clarification of the formulas presented in the paper. As an example, if there is a certain probability that four out of n events occur simultaneously, then it is obvious that, in general, it will be much less prob­able that all four events start and end simultaneously. For events which can change position in time continuously or in small steps, most overlaps will be only par­tial ones. All degrees of overlaps from small ones to complete ones are possible, and each has its own probability. From all pos­sible overlaps and their probable frequen­cies the average overlap or average dura­tion of simultaneous occurrences can be cal­culated.

This average overlap may be of practical importance in engineering problems as can

1118 Discussions AIEE TRANSACTIONS