DEVICE FOR COMPENSATION OF TRANSFER FUNCTIONS

Abstract: Device for compensation of electric transfer functions of electric or electronic sensors (G), detectors, instruments or four-poles. According to the invention, the signal from the unit (G) to be compensated is connected to a differential amplifier (D) with a feed-back network (Z) having a transfer function that is as similar as possible to the transfer function to be compensated. Hereby the compensating device has a transfer function that approximately is inverse to the transfer function of said sensor (G). The signal that the sensor senses and that is distorted by the nonlinear transfer function of the sensor is then restored. The feedback network can either be passive or consist of electrically controllable active filters.

Description: Device for compensation of transfer functions The present invention consists of a device for compensation of electrical tra sfer functions of electric or electronic sensors, instruments or fourpoles. '.Then mesuring for instance temperatures by the use of 5 therπo element or thermistor instruments or when measuring- optical radiation by the use of thermopile radiometers or some pyroelectric radiometers, the fact that the rise time of the instrument in many cases is very long compared to the duration of the variations of the signal being measured, often 10 constitutes a problem. ' The rise time of thermopile detector is often in- he order of a half to one minute. When for instance a radiation with fast variations (within seconds or parts of) is measured, 4 the signal available on the output terminal of the instrument, " i - 1.5 as a rule does not have the slightest similarity with the unit being measured. This is.due to fact that the modulation transfer function (MTF) of the detector is not linear within the actual fre uency range. In these cases the instrument or detector is often said not to be fast enough, .-.hen it is of no consequence 20 that the amplifier to which the dstector or instrument is connected is fast and has a linear transfer function. There are a Ij-Trge amount of instruments and devices where the άeτector or sensor is sensitive but too slow for measuring fast processes. In tliese cases a device in accordance 25 with the present invention would sa e the instrument suitable also for measuring signals with fast variations. Instruments, which are sensitive but slow are mace by the πost advanced instrument manufacturers in the worlds leading industrial nations and it is easy to point cut such, instruments. '0 As it unάov.btedly is ' of grea value to have a possibility %& BAD ORIGINAL, of getting instruments wi h shorter time constants, it is understood that so far simple methods of realizing this with instruments having slow sensors have oeer± unknov-n. On the other hand, the increasing of the time constant of a fast instrument has never been a problem. The present invention solves these problems in the way that is described in the distinguishing part of the attached (main)claim. It is anticipated, that the sensor, detector or four pole, of which the transfer function iε to be compensated, is connected to the non-inverting input of a high amplification differential amplifier the output of which is connected to an output terminal as well as, through a feed-back network, with a transfer function arranged to be as similar with the transfer function of the sensor, detector or four pole as possible, to the inverting input of the differential amplifier. Hereby it might be suitable to let the feed-back-network consist of a number of adjustable filters. Alternatively as a rule it is possible to use passive filters instead. The filter types referred to in these cases are proportional filters, deriving and integrating filters. Besides, it might be suitable to have a possibility of by¬ passing the compensating device, constituted y the present invention, without having to disconnect it electrically. Hereby it iε suitable to have a switch, that connects the signal from the sensor, detector or four pole directly to the output terminal of the device while the differential amplifier with its feed-back network is arranged to be disconnected. Finally it might be suitable to make the compensating device, constituted by the present invention, as a separate unit, that can be connected between the instrument of which the tranfer function is to be compensated, and a read out unit as a dial instrument or a writing oscillograph. An example of how the compensating device can be made in accordance with the present invention, is described in detail below under reference to the attached figures 1-5, thus fig. 1 shows the output from a typical sensor when the measured unit is a step function, fig. 2 shows a sensor, fig. 3 shows a four-pole^ fig. 4 shows the fundamental arrangement of a compensating device in accordance with the present invention and fig. 5 shows a more complete compensating device in accordance with the invention, made as an external accessory for an instrument. Figure 1 shows an example of what the output, U . , from an instrument with a sensor or from a four-pole, might look like it the input signal is a step function. 'The detector or sensor of the instrument can for instance consist of a photo diode of pyroelectric type that is directed towards a lamp, that is . switched on at the moment t or cf a ther o element, that at the moment t is dipped into hot water. To a four- pole a step voltage is connected. When the innut signal Um. is applied to the detector/ instrument, the curve Uut. shown in.fig. 1 is obtained from the instrument output. The relationship between U. and U . y - ■ m . • ut is derived from the instrument ' tranfer function K {<_■) . The difference between U. and ϋ can be considered as the m ut measurement error, caused by the instrument that has been used. If an accessory in accordance with the invention is connected to the instrument output terminal and if the feed¬ back network Z (fig. 5) has such a transfer function X' {_ so that the voltage Uzwould be the answer to a sten function of the type U. , the voltage ϋ' , is obtained on the outnut ~ ■ _._ & ut . - terminal of the accessory. The error, U. minus U 1 . iε now considerably smaller in ut " and the equipment is faster. The error is reduced not only for step functions on the input but for any arbitrary input function. The aim is to make the transfer function of the feed-back network as identical with ^ the transfer function of the instrument as possible. Exactly identical transfer functions does not give any measuring errors at all and we get which is shown below. It is also nossible to show that a difference between the transfer functions K (< ) BAD ORIGINAL, and £' (i ) that causes the voltages UUt and to differ with n f- also will cause a measuring error (Um. -ITUu. %y that is about n Jb provided n is not too large. his means that the device in accordance with, the invention even if the transfer 5 function K {L. ) " άoes not exactly resemble the transfer function K ((.. ) of the sensor, detector or four-pole used, will work as an error reducing device, which, is.of great importance. Fig. 2 shows a swnsor, detector cr measuring instrument with the transfer function Z(C ). The input unit ?._ gives the 10 voltage U . on the output terminal of the sensor, detector or instrument. Fig 3 shows the equivalent data fcr a general elec¬ tric four-pole. Fig. 4 shows a compensating device in accordance with the _ invention, for a sensor etc, as shown in fig 2 or fig 3, with 15 the transfer function K( '). The output of the detector is con¬ nected to the non-inverting input of a differential amplifier D. A feed-back network, also with, the transfer function *-(ι^) - the transfer functions of the feed-back network and the sensor are anticipated to be identical - is connected between the inverting 20 input and the output terminals of the differential amplifier. The. output voltage in point c is then the amplification f times the voltage difference between the two inputs of the differential amplifier i.e. f• (b-d)=c which means that b-d=c . Because f should be chosen as large as possible, the i# 25 voltage difference b-d will be negligibly small. As it is known, the aim of the device iε to make the voltage in the point c to be a true copy of the voltage in the point a. If this would be the case, the voltage b-d would be exactly null, as the transfer functions of the sensor and the feed-back network are anticipated 30 to be exactly equal. As the device does not work unless b-d=c , it is not possible for c to be an exact copy of a, but c=a(l-l/f), thus the difference between the voltages in the points a and c can be reduced by increasing the amplification f. In the practical case the expression c=a is very near the truth. If for some reason 35 f not can be chosen high, it is still possible to eliminate the factor (1-1/f) by introducing a voltage divider z. , z„, that divides the eed-back voltage in the relationship l/(l-l/f). B E>ORIGINAL Fig. 5 shows a practical way of making the device, constituted by the present invention. Hereby the feed-back network Z iε formed by a number of variable active or passive networks or filters and with a switch S that can disconnect the signal from the differetial amplifier and connect it directly to the output terminal. Thus it is possible to achieve the unaffected signal from the instrument ___ ■ without having to disconnect the transfer function compensating device, " that constitutes the invention. Thus the described device can be installed in existent instruments or systems or be connected as a separate unit, with its own power supply if that is disirable. Of course it can be integrated in a design already when it is planned and does not need to be considered a device only for completion of already existing devices. Eventually, the differential amplifier should be chosen fast enough as otherwise its own transfer function must be considered, but as a rule this is not a problem. The invention is founded on the fact that the transfer function of the sensor, detector or instrument in question is constant and unchangeable. As a rule, this is the case, but in some cases the transfer function might be subject to changeε due to e.g. the temperature' or some other parameter. Then is it often advantageous to feel the parameter and let it control one or several of the filters in the feed-back network. Hereby it iε posεible to keep the transfer function compensated even under extreme conditions.

Claims: Claims 1. Device for compensation of the electric transfer function of a sensor (G) detector, measuring instrument or four-pole (F) c h a r a c t e r i z e d in that said sensor (G) or equivalent, is connected to the non-inverting input (I ) of a differential amplifier (D), the output of which, is connected to an output terminal (0 ) as well as, through a feed-back network (2) with a transfer function (K') > to the inverting input (I.) of the differential amplifier (D), where said transfer function (_ ' ) is arranged to be as similar as possible to the transfer function of the sensor (G), detector or four-pole. 2. Device according to claim 1, c h a. r a c t e r i z e d in that said feed-back network (Z) consists of one or several adjustable active or passive filters or networks. 3. Device according to claim 2, c h a r a c t e r i z e d in that one or more of said, adjustable filters are electrically controlled. 4. Device according to any one of claims 1 to.3 c h a r a c t e r i z e d in that said differential amplifier (D) is parailell to a by-pass in the form of a switch (S), transferring the signal directly from the sensor (G), detector or four-pole (F) , to the output terminal (0 ). 5. Device according to any one of the preceding claims, c h a r a c t e r i z e d in that said device (D, Z-, S) is deigned as a separate unit, to be connected between a sensor (&), detector, measuring instrument or a four-pole to be compensated and a read out unit.

Inventors: LUNDBAECK A
Applicants: LUNDBAECK A
Application year: 1980
Main classification: G01D3
IPC codes: G01D3, G01K7