Patent GB-1191785-A (GB)
Title: Control Systems
IPC: G05B13/02
Filed: 1968-05-08
Abstract: 1,191,785. Logarithmic and exponential computation for control systems; selective signalling. SIEMENS A.G. 8 May, 1968 [19 June, 1967], No. 21895/68. Headings G4G and G4H. [Also in Division G3] In a control system the effect of a perturbing element such that x = Vy where x = output, y = input, and V is an unknown variable factor is overcome by introducing into the input of the element an exponential device and into its output a logarithmic device, so that x<SP>1</SP>=y log b  a + log b  V where a, b are bases, x<SP>1</SP> = output of the logarithmic device (Fig. 2, not shown), so that the perturbation becomes an additive logarithmic factor which may be compensated. The control signal may be logarithmically modulated, compared with the logarithm of the controlled variable, and the resultant logarithm of the quotient may be utilized to control the system in exponential dependence on its output. Diode resistor circuits or digital circuits may be utilized to introduce the logarithmic and exponential functions. A control system 4 (Fig. 3) comprises an integrated perturbing element 1 of variable gain V 2  and a proportionate element of gain V 1 . The input is preceded by an exponential device 2 of output a<SP>y</SP>, and amplifier 7 of output y, and a differencing circuit of output #, while the output x is followed by a logarithmic device 3, whose output x<SP>1</SP> is fed back to the differencing circuit for comparison with a control signal log b  x w  derived from control signal x w  over logarithmic device 6. It is shown that x<SP>1</SP> = log b  x = y (log a/log b) + log V 1  V 2 . In a further control system (Fig. 4) a dividing device 10 receives control signal x w  and output signal x fed back from the control system 4, and its output is switchable to logarithmic device 11 producing a signal log (x w /x) = #, which is amplified at 7 and applied to the control system through exponential device 2. Alternatively 11 is switchably replaceable by a three-state trigger the relation between whose output A and input E are as shown, with reciprocal input values E 1 , E 2  lying on and at either side of the intersection of a fictitious logarithmic characteristic L with the abscissa, so that a logarithmic characteristic is reproduced at three points. Logarithmic and exponential functions may be introduced digitally (Fig. 5); an input signal E being applied through a differencing circuit to a symmetrical 3 stage trigger 16 producing a pulse at + output for positive input and a pulse at - input for negative input, to energize AND circuits 17, 18 together with the output of constant frequency pulse generator 19; such pulses being selectively transmitted to forward or backward inputs y, r of binary counter 20 having six stages weighted from 2-<SP>2</SP> to 2<SP>3</SP>, operating a digital/analogue converter 21 directly and a digital/analogue converter 22 through selector 23, whereby gate circuits step differing values of input impedance of D.C. feedback amplifiers 24, 25. In amplifier 24 the input impedances are stepped inversely to the weightings, and it is shown that the content of counter 20 is reproduced linearly, and with switch 60 closed the output A 1  is fed back to differencing circuit 15, so that counter 20 varies until input E and output A 1  are in agreement. In amplifier 25, the counter 20 outputs are connected to respective OR circuits 26 to 30 together with an input from the next subsequent OR circuit, and produce output pulse when one or more input pulse is received, so that a signal at any one counter output produces signal at the outputs of all OR circuits associated with coun - ter circuits of lesser significance. Adjacent OR circuits are connected in pairs to exclusive OR logic circuits 31 to 35 producing outputs only when differing logic signals appear at the inputs. Gate circuits 36 to 40 are thus operated to connect respective input resistors to the amplifier 35 stepped logarithmically and inversely to the powers of the places of counter 20. For realization of the logarithm of zero, a NOT circuit 41 is connected to output of OR circuit 26 to operate switch 42 inserting a resistance R -00  of value such that output A2 of amplifier 25 is at its limit, while for realization of log 2‹ = 1 the exclusive OR gate unconnected to the amplifier produces a signal, and may be omitted. Further AND gates 43 and OR gates 44 may be introduced to increase the number of steps, and A2 = U log b  (E/kU) with switch 60 horizontal. With switch 60 vertical, it is shown that for feedback of A 2  to 16, A 1  = Ub (E/U) so that logarithmic or exponential functions may be thereby selected. A modification utilizing a shift register controlling similar exponential or logarithmic digital to analogue converters is described (Fig. 6, not shown) and a further modification of Fig. 4 (Fig. 7, not shown) employs stepping AND gates operating a shift register 45 controlling exponential or logarithmic input amplifier resistances. A three state amplifier utilizing positive feedback amplifiers and potentiometrically set operating levels is described (Fig. 8, not shown).