AU601644B2 - Improvements in or relating to electronic circuitry and methods of operating the same - Google Patents

Description

601644 S F Ref: 101972 FORM COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int Class Complete Specification Lodged: Accepted: Published: Priority: This document contains th anclmentls made und 'n -49 and is correct fiu Sii ntmng.

Related Art: Name and Address of Applicant: Fisher Paykel Limited 39 Mt. Wellington Highway Mt. Wellington Auckland NEW ZEALAND Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia Address for Service: Complete Specification for the invention entitled: Improvements in or Relating to Electronic Circuitry and Methods of Operating the Same The following statement is a full description of this invention, including the best method of performing it known to me/us 5845/3 4 This invention relates to electronic circuitry and methods of operating the same.

It is an object of the present invention to provide electronic circuitry and methods of operating the same which will at least provide the public with a useful choice.

Accordingly, in one aspect, the invention consists in a method of cancelling out spurious variables in electronic circuitry during the measurement of the impedance of one or more sensors in said circuitry using in each sensor circuit a switch having an on and off state to control the circuitry in response to switch control signals from a micro-computer and a .o .calibration impedance the parameters of which are known within defined tolerances together with an associated switch, said :%*'.method comprising the steps of passing a signal from said calibration impedance to said micro-computer, comparing the signal received from saia calibration impedance with a known o *:value for such signal, passing a signal from each of said *oo *sensors to said micro-processor at an appropriate time, each :0 0 signal being indicative of a variable characteristic of said sensor, and adjusting the switch control signals by an amount Swhich is chosen by reference to the variation of said received ,calibration signals compared with said known value of such calibrated signal, and using the adjust switch control signal as a switch control signal substantially free of variations in parameters of the circuit other than variations in the sensor 2 which are indicative of variations of said variable characteristics.

In a further aspect the invention consists in electronic circuitry including one or more sensors the impedance of which is required to be known within defined limits of tolerance, a calibration impedance the impedance of which is known within defined limits of tolerance, a micro-computer and switching means arranged to switch each of said sensors and said calibration impedance into a circuit so that variations in signals received by said micro-computer when said calibration impedance is in circuit are used to modify switch control o signals when each of said sensors is switched in to circuit to S' correct variations in circuit parameters other than each sensor parameter.

1 To those z illed in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest -3 themselves without departing from the scope of the invehtion as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.

One preferred form of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of apparatus for measuring resistance of sensors according to the invention, Figure 2 is a graph of voltage across capacitance and discharge circuit against time showing operation of the circuit of Figure 1, Figure 3 is a further graph of voltage against time with symbols used in formulae demonstrating the mathematics of the said operation, Figure 4 is a modified circuit diagram incorporating the invention, Figures 5 to 8 are voltage-time graphs of operation of the invention, Figure 9 is a flow diagram showing the operation of the invention, Figure 10 is a further voltage-time graph showing all phases required for calibration and measurement, Figure 11 is an alternative circuit diagram using sensors with non-linear characteristics such as thermistors i as temperature sensing devices.

-4- L. The 'requirement of the present invention is to provide 'a system to convert one or more, say three independant analogue channels into a form which can be processed by a micro-computer. That is an analogue to digital converter is provided. In a preferred form the analogue quantity is impedance.

In prior art systems multiplexed sensors (resistance being the active variable) form an RC charging network.

Each sensor is sequentially used to charge a capacitor using switches e.g. transistor switches. The time taken for voltage across capacitor to reach a predetermined level is measured by a micro-computer. This time is thus o. proportional to the quantity being sensed. The capacitator is discharged after each charging cycle, by a switch.

o'o.i The equation describing the transient behaviour of a S simple RC network under charge condition is (See Hughes o Electrical Technology 4th Ed. P.165) t K*C*R*

(A)

where K Ln [1 (1 (vthres V))1 With RC type systems the micro-computer must act as a timer, the sensitivity being dependant on the instruction S'speed of the computer.

For example, the micro-computer interrogates the RC network, and increases or decreases an internal register if the predetermined voltage threshold (vthres) is not reached, the time to perform this task being the gate time. Thus time sensitivities less than the gate time are i I case of the silicon sensor such as the KTY range from Philips and Siemens, the circuit is straightforward as shown in Figure 4. For the thermmistor sensor, the calibration resistor 22 also acts as a linearization resistor (Figure 11).

The advantages of the invention at least in the not realisable.

The sensitivity can be determined from by Dt/DR K*C (B) where D is the partial differential operator.

Hence system sensitivity is mainly dependant on the timing capacitance. Thus a reduction in absolute sensor sensitivity, e.g. TEMPERATURE sensitivity, requires an increase in circuit capacitance. Thus, if z represents a time period increment for the lowest increment in temperature required to be detected, the capacitance must be sized such that z is greater than the micro-computer gate time. 2t/2 Vthres (C) A common method to overcome the above constraint on the ,0 0 0o *0 capacitance is to use a comparator in which the sensor resistance varies the threshold voltage Vthres in the equation of the differential comparator. Hence the 4 I timing capacitance is not solely dependant on the absolute sensor resistance as above, but a higher value of fixed charge resistance R can be traded for a lower value of timing capacitance the capacitance size being a cost factor.

The present invention is a novel development of the above system which has the advantage of trading off a lower timing capacitance for a higher fixed charge resistance.

However the invention in the preferred form requires no differential comparator or discharge switch thus reducing cost.

j 6- I; r

.I,

a Referring to Figure 1 of the drawings, a capacitance C is connected to a series of sensors referenced 1, 2, 3 each sensor in turn having an associated switch, e.g. a transistor switch referenced 4, 5, 6 respectively and a voltage supply is connected to a rail V and a rail 0. The rails V and 0 may be reversed with appropriate changes to the circuit.

A micro-computer 7 in addition to other functions to be described, controls multiplexing of the sensors 1, 2, 3, variations in the resistance of which are to be measured.

Such sensors may be, for example, variable capacitances but preferably comprise temperature sensitive resistances such as thermistors or more preferably silicon temperature sensors the resistance of which may be represented by the So* symbol rs S° The capacitance C has a discharge circuit associated with it, shown in the circuit as resistance Rd. Rd has a much greater resistance than rs but if desired a discharge switch may be provided operated by the micro-computer 7 to 90 be off when a sensor switch 4, 5 or 6 is on and to be on when all the sensor switches are off.

Under control of the micro-computer capacitor C is charged sequentially through each sensor i, 2, 3 in turn by the multiplexing action set up by the micro-computer 7.

The micro-computer also counts out a predetermined time of charging of capacitance C and when that time is ended the micro-computer turns the appropriate switch 4, 5, or 6 from -7the on state to the off state. Capacitance C then discharges automatically through the discharge resistance Rd and the time taken for capacitance C to discharge to a predetermined threshold voltage is measured by the micro-computer. Because the voltage to which the capacitance C is charged is controlled by the capacitance C and the resistance rs of the sensor, then the voltage to which C is charged is a measure of the resistance of r s Further the time to discharge capacitance C is also dependant on the voltage to which capacitance C is charged and thus the time to discharge also gives a measure of the resistance r s Thus, C is charged sequentially via the multiplexed Si.'i sensor network (Figure 1) for a fixed time. Capacitor C is

S

i then allowed to discharge via Rd (Figure 1) simply by turning off the corresponding sensor switch. The discharge time is thus proportional to the corresponding sensor quantity, and is measured by the micro-computer. It should be noted that the charging circuit contains both resistances r s and Rd, but that r s is much smaller than Rd and that the discharge circuit contains only resistance Rd. Preferably Rd is of the order of 10 times rs. As stated above Rd may be controlled by a discharge switch \(not shown) to be out of circuit when a sensor is in circuit.

Figure 2 shows a graph of voltage across on capacitance C and the input to the micro-computer against time. The I charging time tc and capacitance C are fixed, but the voltage at time tc is dependant on resistance rs.

Accordingly the discharge time will vary and from the two curves 8 and 9 shown in the figure, it can be seen that the time variations in discharge y are greater than the time variations in charge x.

The trading of lower capacitance for higher resistance can be shown diagrainatically as in Figure 2. During the charge phase, the time constant is low, by virtue of selecting a lower value of C with respect to rs. Hence any time period variations e.g. x (Figure 2) are in the order of this small time constant. The charge time tc (Figure 2) however is fixed, therefore any time period variation is translated into voltage variation (Figure This voltage i i. variation is then transferred to a higher time constant circuit during the discharge phase (Figure Thus in the discharge phase, higher values of Rd are traded for lower values of C. Hence the charge phase time period variation e.g. x (Figure 2) translates into a magnified discharge time period variation e.g. y (Figure 2).

The lower the value of tc (Figure the lower the S' value of C possible; the limiting value in this case being the micro-computer gate time. Although in the charge phase, time period variations could be less than the gate time, these variations are translated into time period variations greater than the gate time during the discharge phase, or effectively a magnification in the value of C.

i With a charge phase time of 2 milliseconds values of C may be in the order of 0.68 mfd for silicon temperature sensors.

The sensor switches 4, 5 and 6 and/or the discharge switch may be provided integrally in the micro-computer.

Referring to Figure 3, the mathematical analysis of the system is as follows. The governing charge-discharge equations are (as referred to in the above reference to Eughes) charge voltage vu v*(l-exp(-t/C*Ru)) discharge voltage vd (td) vu(tc)*exp(-td/C*Rd) 0 0 o 0.

where v Vsuply*Rd/(Rd ",substituting into gives an equation for the discharge time tc CRd In (1-e -tc/CRu)] .i i where vth vd(td) The sensitivity of the discharge time td with respect to variable rs can be determined from Dtd/Drs, where D is the partial differential operator.

5, From above C*R*vth -v*tc*exp(-tc/C*Ru) Dtd/Dru i

I

V*(l-exp(-tc/C*Ru)) vth*c*RuA2 R~d*tc -exp(-tc/C*Ru) Ru^2 1-exp (-tc/C*Ru) -exp(-tc/2*C*Ru) Ru^2 2*sinh(tc/2*C*Ru) (Refer to Reference Data for Radio Engineers 5th Ed. by

ITT)

2 Let G tc/2*C*Ru Then -Rd*C G*exo Dtd/Dru 4Ru sinh(G) From above 'Dru/Drs -1 w:ere Vu(t) is charging voltage per unit time (t) V is applied voltage C is capacitance Ru is resistance in charge circuit Vd is discharge voltage in discharge time (td) Since Dtd/DrS Dtd/Drs(Dtd/DRu) (DRu/Drs) Rd*C*G*exp Dtd/Drs (<(l/rs)+C1/Rd)>^2*(rs^a21 Ru*sinh(G) -Ru 2

(S

2 Ru*sinh(G) if we assume that Rd>>rs then above reduces to above. The value of the tran~scendental section of is always less than 1.0 as shown.

Table I G*exp sinh(G) 0.1. 0.903 *0.2 0.813 40.3 0.730 0.4 0.4 V.;O 0.50.582 0.6 0.517 0.7 0.458 0.8 0.405 a0.9 0.357 1.00.1 0.075 10.0 0.001 Table I shows that the value of tc must be in the order of the time constant Ru*C, for greatest capacitance magnification, that is lowest value of G.

Comparing with shows the passive multiplication of the timing capacitance, is related directly with the ratio Rd/Ru, which is greater than The irv7 ion is of value in sensing temperature in a clothes c for example as set forth in New Zealand Patent No. 202132 (Australian Patent No. 560520 and Great Britain Patent No. 2128308).

The present invention at least in its preferred form as S above described has advantages over other systems in that: It requires a lower value of timing capacitor, due to Sthe system having passive capacitance magnification and S service. The capacitance value is a cost factor in such systems and this gives a cost factor gain.

A comparator or buffer and discharge switch are not required which could give a cost effective gain.

Additional analogue input channels are possible.

The invention can be implemented using most of the modern transducers which give a variation in capacitance or preferably resistance on a change in physical state occurring.

The above circuitry is subject to some erros due to e.g. voltage variation, transistor saturation and other circuit parameters.

The present invention therefore in a preferred form also provides for circuitry and methods of operation which substantially cancel out variability of parameters in the above circuitry as will now be described.

The circuit of Figure 1 is modified by adding a calibration impedance such as resistor 20 (Figure 4) which has an associated transistor switch 21. The impedance resistance) is one in which the value of the resistance is known within desired tolerances e.g. The micro-computer 7 is programmed to read the values of voltages across the capacitor C when the sensors are in circuit in turn as above described and in addition is programmed to read the voltage across the capacitor C when the calibration resistor'20 is in circuit. Since the resistance (r cal) of resistor 20 is known within desired tolerances, variation in readings must be due to changes in external parameters.

The tolerances of the parameters which most affect the analogue to digital conversion process, can be divided into two sections as follows.

Supply voltage e.g. 5 V 5% 4.75-5.25 V Micro-compute: low voltage threshold e.g. IV +100% -20% 0.8-2.0 V Initial voltage e.g. leakage current dependent Multiplex switch depends on switch type e.g.

voltage drop bipolar lOmv Timing capacitance e.g. Discharge resistor e.g. 1% Micro-computer clock e.g. 10 MHz 1% Voltage tolerance cancellation.

With all voltage parameters included, the governing charge and discharge equations are (refer to Figure vu(t) vs (7) vd(td) (vu(tc)-vs)*(exp(-td/C*rd))+ vs (8) Substituting into gives an equation for the discharge time td. Thus equation 4 now becomes td C*Rd*ln[<v-vs>*<l-exn(-tc/C*Ru)>/<vth-vs>] (9) Consider the case when tc >>C*Ru 0 0 let tref td tc>>C*R) Then the discharge time from equation 9 becomes, d 0,O Stref C*Rd*ln[<v-vs>/<vth-vs>] Subtracting from (10) gives a time period expression which is independent of voltage parameters, including any voltage offset from the multiplex switches Time period P C*Rd*ln[l-exp(-tc/C*Ru)] (11) Thus if two time periods are measured as in Figure 6, taking the difference gives a value which is still S. proportional to the analogue variable Ru (Equation 11), but independent of any voltage parameters. It should also be noted the conversion sensitivity DP/DRu, and hence capacitance magnification is unchanged (Equation ii -p I, DP/DRu -Rd*C G*exo(-G) Ru sinh (G) Hardware component tolerance cancellation Consider the case of the calibration resistor 20 having a resistance real 7 being in circuit by the closing of its switch 21. The graph of voltage against time is given in Figure 7. This would yield a difference time period, using Equation 11. The equation for time Q is Q C*Rd*ln[l-exp(-tc/C*Rucal)] (12) where Rucal Rd*rcal/(rd+rcal) (13) Division of expressions (11) and (12) yields an expression S which is tolerance dependent on ratio tc/C and Rd equation (14).

Iln[l-exp(-tc/C*Ru)] Sp/Q (14) ln[l-exp(-tc/C*Rucal)] Expression (14) has no multiplicative micro-computer clock timing errors, since they are cancelled by the b division process. This is not the case with tc. To remove this hardware parameter tc/C and Rd dependence, two calibration difference time periods are taken using the calibration resistor (Figure That is Qs C*Rd*ln[l-exp(-tcs/C*Rucal)] °ge: Qn C*Rd*ln[l-exp(-tc/C*Rucal)] (16) where tcs is approximately Division of (15) and (16) again yields an expression II~ _I which is dependent only on the ratio tc/C and Rucal, in the exponential term equation (17): In[l-exp(-tc/C*Rucal)] Qn/Qs (17) In[l-exp(-tcs/C*Rucal)] The ratio tc/C can be determined from since Rucal is known. Note that the tolerance in Rd, which is related to Rucal by (Equation 13), is only correctly known for real. However since Rd is much greater than rs (preferably of the order of 10 times) its tolerance effect is reduced by the ratio Rd/rs. Thus only the ratio tc/C need be considered in practical terms.

The ratio tc/C can be related to (14) in two ways and S, thus adjustment of the sensor signals effected in two ways.

A

5 1. The determined ratio value tc/C can be used to directly compensate the value of P/Q (14) for tolerances in C and tc. This requires a further calculation using a nonlinear expression relating P/Q, Ru and tc/C.

2. Using the initial ratio tc/C determined from tc itself can be adjusted for subsequent measurements such that the ratio tc/C is maintained at a theoretical constant. Hence expression (14) will only be dependent on Ru and thus rs (Figure 1 and Figure and not any hardware tolerances. This requires a simple lookup table directly relating the Qn/Qs result to the corrected value of tc. Although this method is not as elegant as 1) above, it requires one less -17- I IIIIII--Y I micro-computer mathematical calculation.

Figure 9 shows a flow chart of the tolerance cancellation alogorithm using the second method. Figure shows the corresponding timing diagram. Note these two figures must be read together since they both use the second method above.

Referring to Figure R is the initial voltage calculation N is the standard time to get time Q S is standard time to get time QSS. Qn/Qs (17) gives value of corrected tc.

Nc gives corrected value of Q from R-Nc.

A summary of the mathematical computations preformed by S the micro-computer are as follows: 2 subtractions S1 division yields value of tc to i 1 lookup table compensate for circuit tolerances.

1 subtraction yields corrected value of Q 1 subtraction 1 division for every sensor 1 lookup table Two circuit configurations are possible depending on whether silicon or thermistor sensors are featured. In the

SI

-1 case of the silicon sensor such as the KTY range from "Philips and Siemens, the circuit is straightforward as shown in Figure 4. For the thermmistor sensor, the calibration resistor 22 also acts as a linearization resistor (Figure 11).

The advantages of the invention at least in the preferred form are 1. By .use of the tolerance compensation alogrithm no hardware adjustments are required, e.g. no potentiometers need be provided and no future calibration is necessary.

2. Cost effective as applicable to the home applicance market.

S 3. Additional analogue input channels possible.

o 4. Can be implemented using most of the modern 0 transducers.

a i -1ii -iq

Claims (6)

1. A method of cancelling out spurious variables in electronic circuitry during the measurement of the impedance of one or more sensors in said circuitry using in each sensor circuit a switch having an on and off state to control the circuitry in response to switch control signals from a micro-computer and a calibration impedance the parameters of which are known within defined tolerances together with an associated switch, said method comprising the steps of passing a signal from said calibration impedance to said micro-computer, comparing the signal received from said calibration impedance with a known value 0 for such signal, passing a signal from each of said sensors to said micro-computer at an appropriate time, each signal being indicative of a variable characteristic of said sensor, and adjusting the switch control signals by an amount which is chosen by reference to the variation of said received calibration signals compared with said known 0ltl G, value of such calibrated signal, and using the adjust switch control signal as a switch control signal substantially free of variations in parameters of the circuit other than variations in the sensor which are 7" indicative of variations of said variable characteristics. 4

2. A method as claimed in Claim 1 which includes the steps i of using a temperature sensitive resistive element for at least one of said sensors. fA I

3. A method as claimed in Claim 1 or Claim 2 which includes the step of using a transistor switch controlled by said micro-computer for each said switch.

4. A method of cancelling out spurious variables in electronic circuitry as claimed in Claim 1 when effected substantially as herein described with reference to and as illustrated in Figures 1 to 3 and Figures 4 to 11 of the accompanying drawings.

Electronic circuitry including one or more sensors the impedance of which is required to be known within defined limits of tolerance, a calibration impedance the impedance of which is known within defined limits of tolerance, a 0: micro-computer and switching means arranged to switch each of said sensors and said calibration impedance into a circuit so that variations in signals received by said micro-computer when said calibration impedance is in circuit are used to modify switch control signals when each Sof said sensors is switched in to circuit to correct variations in circuit parameters other than each sensur parameter.

6. Electronic circuitry as claimed in Claim 5 when constructed arranged and operable substantially as herein I described with reference to Figures 1 to 3 and Figures 4 to 11 of the accompanying drawings. DATED this TWENTY-NINTH day of JUNE 1990 Fisher Paykel Limited Patent Attorneys for the Applicant SPRUSON FERGUSON 21