Patent GB-677002-A (GB)
Title: Improvements in or relating to servo systems
Inventor: PORTER ARTHUR
IPC: G05B11/26
Filed: 1949-06-25
Abstract: 677,002. Electric correspondence control systems. MINISTER OF SUPPLY. Aug. 24, 1950. [June 25, 1949] No. 16917/49. Class 40 (i). [Also in Group XIX] A servo follow-up-control system, wherein the error between input and output quantities is determined at successive, preferably equallyspaced,' intervals and utilized to cause the output quantity to follow the input quantity closely, comprises a plurality of integrators arranged in a chain, output to input, and a summing device which sums all previously determined values of the error and provides a plurality of outputs which are fed to respective inputs of the integrators, each output comprising the resultant summation multiplied by a factor appropriate to the respective-integrator and to the time interval between successive error determinations. In addition means are provided which apply a correction, equal to the contemporaneous error, to the output quantity at each error determination. Fig. 1 (Prov.) illustrates the invention in a general form. The input function of (t) is followed by the output function g (t). These. are fed to a comparison device 4 which determines the error et at successive equally spaced instants. This is added in device 5 to the sum of all previous error determinations. The apparatus comprises three integrators 1, 2, 3 arranged as shown in series and fed with respective outputs from the summing device 5. These outputs each comprise the'sum of the errors multiplied by an appropriate factor, the numerical part of which is determined by the number of integrators used and the position of the appropriate integrator and the other part of which (i.e. K1, K2 or K3) represents the product of the proportionality constant of the integrator and a factor due to the time T between successive error determinations. In addition to the integration operation already described the output function g (t) is corrected after each error determination by the addition of the instantaneous error (et) thereto. In order to obtain a smooth, instead of a discontinuous correction, a modified function is derived from the error et in device 6 and applied to integrator 3 so that integrating over the succeeding interval T a smooth correction amounting to et is obtained. The action of the system is to determine the cubic curve (since three integrators are used) passing through the four previous, values of f (t) at the measurement times and thence predict the future values thereof. At the next error measurement, the addition of the error to the output quantity then modifies g (t) to pass through the instantaneous value of f (t) while the addition of the error to the inputs to the integrators modifies the coefficients of the cubic function to maintain f (t) and g (t) in coincidence at the four previous measurement times. It will be seen that the system also provides, at the input to integrator 3, the derivative of the function g (t), and hence to a close approximation, that of f (t). Fig. 1 illustrates an electrical system embodying the invention and employing two integrators uz, a Miller integrator 9 and an integrating motor 10. The input is represented by the rotation of shaft 1 and the output by rotation of shaft 3. These are converted to electrical quantities by potentiometers 2 and 4 and their difference determined at successive instants by the interval switch 5 and passed to error holder 6 where the error is stored as the charge on a condenser. Error summator 7 is an electronic circuit which determines the sum of all the error signals and passes this in appropriate fashion to the integrators 9 and 10. The error signal ej also is applied to integrator 10 for effecting the successive substantially instantaneous corrections in the value of g (t).