Patent GB-1340813-A (GB)
Title: Interpolative readout apparatus
IPC: G01R17/00
Filed: 1972-01-07
Abstract: 1340813 Evaluating functions by interpolations DAMON CORP 7 Jan 1972 [12 Jan 1971 23 Aug 1971] 751/72 Headings G4G and G4H [Also in Division G1] A non-linear voltage measuring system paticularly for receiving a transducer voltage which varies non-linearly with a parameter being measured and for producing a corrected output voltage which varies linearly with the parameter being measured, has two modes of operation-a calibration mode in which transducer voltages A, B, C, D and E, Fig. 1, corresponding to known values of the parameter (assumed to be a concentration) being measured are entered and stored in the system together with voltages K, L, M, N and O, which are the desired corrected output voltages for the known values of the parameter, and a measurement mode in which firstly an unknown value of the parameter is sensed producing a transducer voltage P, say, secondly the two calibration transducer voltages C, D, which are respectively below and above the transducer voltage P are determined, thirdly the ratio in which the transducer voltage P divides the interval between the voltages C and D is determined, and fourthly a voltage Q which divides the corrected calibration output voltages M, N in the same ratio is produced. This voltage may be digitized or otherwise displayed as desired. As described the system comprises five analogue storage devices 15, 16, 17, 18, 19, Fig. 2, to which transducer signals A, B, C, D, E, Fig. 1, are applied sequentially during the calibration mode, and five potentiometers R1, R2, R3, R4, R5, on which the corresponding corrected output voltages K, L, M, N, O are set manually. Entry of the calibration transducer voltages is as follows. A shift register 27 is cleared and reset to mark its first lead A. This energizes reed-contact drive windings 31 and 41 closing, in particular, contacts 21 and 51. The first calibration transducer voltage A is then applied to an input differential amplifier 11 where it is compared with the exist. ing contents of store 15 via a loop including contact 51, a contact 57, an amplifier 58 and a filter 59. Any difference produces an error signal which progressively modifies the value in store 15 until it equals the value A from the transducer. A manual advance key 52 is then depressed causing the shift register to advance to its B stage and the above process is repeated until all the transducer calibration voltages A, B, C, D, E, have been stored. The measure. ment mode proceeds automatically once a switch S3 is set in its left-hand position and the shift register is reset. The first phase of the measurement mode, that is the determination of the pair of transducer calibration voltages that straddle the measurement signal, is effected by comparing the measurement signal P, Fig. 1, sequentially with the transducer calibration voltages starting with voltage B. Thus, with the shift register marking its first output lead A the measurement signal P, Fig. 1, on lead 12, Fig. 2, is compared in the differential amplifier 11 with the value B from store 16 via a contact 61, a resistor R7, amplifier 58 and filter 59. If the measurement value P is larger than the value B from store 16, as detected by a polarity threshold detector 93, a gate 92 is opened to allow a clock pulse from an oscillator 78 to advance the shift register to its second stage. This results in the value C from store 17 being compared with the measurement voltage P, and the process continues until the shift register reaches a stage at which the measurement value P is less than the value from the selected store. When this occurs, which in the example given is when the shift register marks its C outputs lead and the value D is compared with the measurement voltage P, the threshold circuit 93 closes gate 92 and energizes the drive winding 94 of a readcontact 76, which initiates the next phase of the measurement cycle. In this phase, with the measurement voltage P still present on lead 12, the stored transducer calibration voltages C and D which straddle the measurement voltage are effectively applied alternately via contacts 53 and 57, and contact 63 and resistor R7, to the amplifier 58 under the control of a duty cycle modulator 77 which, by repeatedly closing contact 57 at the clock frequency of 20 c./s., but with a variable mark/space ratio, ensures that the signal resulting from the filter 59 is a linear combination of the transducer calibration voltages C and D. This linear combination, e.g. the mark/space ratio, is adjusted by the output signal from the differential amplifier 11 until it equals the measurement voltage P, and simultaneously the same linear combination of the stored corrected output voltages M and N is formed by an amplifier 86 and a filter 87. Hence a voltage Q, Fig. 1, is produced by filter 87 which is the desired corrected output voltage. The system described has the intrinsic error that the measurement voltage P is located not on the curve extending between the transducer calibration voltages C and D, Fig. 1, but on the straight line. This error is however said to be small. A mark/space ratio modulation circuit is outlined in connection with a second embodiment (Figs. 3, 4A and 4B, not shown) in which only two calibration points can be preset, and a modification (Fig. 5, not shown) is described enabling extrapolation outside the range set by the calibration points to be effected.