FR2496360A1 - Ratiometric digital position encoder of potentiometer cursor - uses MOS integrated circuit current sources and counter synchronised to supply waveform - Google Patents

Abstract

The encoder has a fixed resistance, from which a reference current is obtained, and a potentiometer in series with a parallel array of current generators. Eahc generator can be switched into circuit with the potentiometer to provide a current which has a predetermined ratio w.r.t that of the other current generators and the reference current. The outputs of the current generators are controlled by the reference current to follow any fluctuations in the latter. An encoder circuit is connected to a switch to successively connect each crrent generator. A differential amplifier determines the sign of the magnitudes of the voltage drops at the terminals of the fixed resistance to give either a stop or start signal to a counting circuit. The output of the encoder is taken from the counting circuit. The system is used in the central of machine tools.

Description

NUERIOUE CODER RTIOldETRIOUE OF POSITION
OF A POTENTIOMETER CURSOR
The present invention is concerned with an electrical encoder capable of converting the cursor position of a potentiometer into a digital value representing this position, delivered in the form of binary-coded electrical signals.

 The term potentiometer is taken here in a very general sense; it can be a potentiometer with circular track (angular position coding) or linear (linear position coding); it may be a continuous-track or pad-type potentiometer, or even more generally a padded switch in which each pad is connected to a resistor so that the change of position of the switch corresponds directly to a change in resistance, the slider of the potentiometer being constituted by the movable element which can come into contact with a pad or another depending on the position of the switch.

 We are interested here in a conversion of the position of the potentiometer slider into a numerical value, and the relation which links the position to the numerical value is a function of the relation between the position of the slider and the value of the resistance of the potentiometer, this resistance value constituting an intermediate conversion parameter.

 A particular application of the present invention consists in converting into a numerical value the position of an electromechanical machine programmer, this programmer being for example constituted by a switching button that can be rotated and placed in a certain number of positions with a discrete or continuous variation of these positions. Each of the switch positions corresponds to a variable resistor value placed in series in a circuit, and each resistance value corresponds to a function to be performed by the machine or to a particular setting of an operating parameter of the machine. We seek to translate this variable resistance into a digital information that will be used precisely for the numerical control of the operation of the machine.

 One of the aims of the invention is to achieve this conversion by avoiding having to use standard resistances in large numbers to compare the value of the variable resistance to the standard resistances.

 Another particularly important object of the invention is to provide a position encoder in which the digital information obtained at the output does not depend on the supply voltage of the potentiometer whose position is coded.

 Still another object is to provide a digital encoder in which there may be any monotonic relationship between the value of the potentiometer resistor and the digital output value, and not necessarily a linear relationship.

 Finally, it has been sought to produce a digital encoder in the form of an integrated circuit supplied with low continuous voltage (a few volts), which can be coupled to the potentiometer, which itself is supplied with a high voltage which is, for example, that of the alternating sector at 220. volts. This is particularly important for example if the digital encoder according to the invention is coupled to an electromechanical programmer of household appliances (washing machine etc ...) and that it is desired to maintain, for reasons of compatibility, a power supply of 220 volts on the electromechanical programmer.

 To achieve these objects, the present invention provides a digital encoder which comprises a fixed resistor Rg in series with a first current defining element for defining a reference current in this resistor Rg, and, in parallel on the whole, a a series set of a variable resistor R1 comprising a potentiometer P1 and a plurality of parallel current regulating elements, each of which can be operated by a respective switch and establishing a current in a known relationship with the others and with 10 and each being driven by Io 0 to copy 10 to a known proportionality factor, the two sets in series being supplied in parallel by any common voltage source, the encoder further comprising a differential amplifier having its inputs connected to the fixed resistor and the variable resistor to detect the difference between the voltage drops in these two res and to provide a counting enable or stop signal to a counting circuit that increments the number of in-service regulating elements to equalize said voltage drops. The first element used to define the current I01, which will also be called in the usual way "reference current generator", comprises a combined drain and gate MOS transistor (metal-oxide-semiconductor). The second regulation elements, also referred to as "current generators" are also MOS transistors and, to copy the current 10 to a factor, they have their gate and their source connected to those of the transistor of the first element defining 10. The counting circuit is coupled to the switches to successively control the closing of each of them. Stopping the counting occurs when the differential amplifier indicates equal voltage drops in the fixed and variable resistors. The digital output of the encoder is then taken from the counting circuit.

 Any voltage source which supplies the fixed resistor and the variable resistor may be in particular an ac voltage of the mains; it does not need to be a regulated DC voltage at a fixed value.

 However, it is desirable that the counting of the desired numerical value is always done during alternations of a given sign of the voltage of the sector, and preferably at the top of these alternations; it is therefore expected that the counting circuit comprises another authorization input controlled by a synchronization circuit establishing a coder operating authorization signal during a positive alternation fraction of the mains voltage, around the peak of this alternation. .

 The counting circuit comprises in principle a binary counter on the outputs from which the desired output digital information is taken, and a register-decoder registering the outputs of the counter and having p outputs each controlling a switch of the second current regulation element, the decoder register providing, for a number m applied to its inputs by the counter, a signal on the first m outputs of the register-decoder and a complementary signal on the other p - m.

The register-decoder itself normally comprises a decoder 1 among p and a set of p MOS transistors in series, powered by a low DC voltage, the outputs of the decoder 1 among p being each coupled, in the order of their numbering, at a gate of a transistor
Corresponding MOS in the same order; the drains of the transistors serve as an output, possibly after inversion, to the decoder register for controlling the switches of the second current regulating element.

 Other features and advantages of the invention will appear on reading the detailed description which follows and which is made with reference to the accompanying drawing in which the single figure shows a diagram showing the various elements of the digital position encoder performed.

In the single figure; a potentiometer is shown
P1 having a slider whose position one seeks to locate in numerical form. We will not return to what we said in the preamble: the potentiometer can be a switch putting in series a.resistance or another depending on its position.

 Potentiometer P1 is optionally in series with a fixed resistor to form a total variable resistor R1, the value of which will be an analog parameter information on the position of the potentiometer slider.

 This resistive magnitude information is translated into a current value I1 passing through the variable resistor R1 when a voltage is applied across it.

 According to the invention, it is sought to measure the current I1, but, in order to overcome the variations of the voltage giving rise to this current, and in particular to be able to use an unregulated or even effectively variable supply voltage such that At an alternating voltage, it is expected to measure I1 by comparing it with a current flowing through a known fixed resistor Rg and by slaving the current I1 to the variations of R1 so that the voltage drop across Rg is always equal to the voltage drop across R1.

 For this, it is provided on the one hand that the resistor Rg is put in series with an element for defining a reference current lo, on the other hand that the resistor R1 is put in series with a current control element coT ande supplying a variable current I1 in increments, with a further differential amplifier AD detecting the difference in voltage drops across Rg and R1 and controlling, depending on the direction of this deviation, the application of incremental current increments. I1 of the variable current regulating element.

 The set in series of the resistor Rg and the element for defining the reference current is supplied by a voltage Vg which can be constant or variable or alternating, and the series of the variable resistor R1 and its variable current regulation element is also powered by the voltage V0.

 If V0 - V1 is the voltage drop at the Ro terminals and if V0 - V2 is the voltage drop across R1, then when the differential amplifier has equalized these voltage drops, Rg 10 = R1 Il; in this way, the resistance R1 is proportional to Io / Ii, Ro being constant.

 According to the invention, it is arranged that the incrementally variable current I1 under the control of the differential amplifier AD, is kept permanently proportional to the current and that the coefficient of proportionality is easily determinable as a function of the current increments. applied by the variable current regulating element. It will be seen, moreover, that these increments do not need to be regular, that is, all identical.

 To achieve this result, it is expected that the element used to define the current 10 is constituted by a transistor ìMOS T10 whose drain and source are joined in a conventional arrangement.

 Another transistor T20, which will be seen later, is placed in series with the transistor T10 to complete the assembly ending the reference current in the resistor Rn.

As for the variable current regulating element I1, it comprises a plurality of MOS transistors
T11, T12, T13 ... Tlp, each in series with a controlled switch constituted by a corresponding MOS transistor, respectively T21 in series with T11, T22 in series with
T12 .., T2p in series with Tlp. All these elements in series are connected in parallel to form the variable current regulating element I1. The parallel assembly thus formed is, it is recalled, in series with the variable resistor R1, the whole being powered by the voltage V.

 Each of the transistors T11, T12, Tlp receives on its gate the voltage applied to the gate of the transistor T10 so that the gate-source voltages of the transistors T11, T12, Tp are equal to the gate voltage. source of transistor T10. This is a so-called "current mirror" assembly in which each of the transistors T11 to TLP reproduces the current in T10, to a coefficient that corresponds to the ratio between the geometry of a transistor considered and the geometry of the transistor. transistor T10.

 To better understand the pods, we can give an example: if the transistor T11 is three times smaller than the transistor T10, the current flowing T11 will be 10/3; if the transistor T12 is 10 times smaller than the transistor T10, the current flowing T12 will be IO / 10, etc.

 Depending on whether the transistors T21, T22, T2p which serve as switches for the current flowing through the transistors T11, T12, Tlp, are in an on or off state, there will be a current I1, in R1, plus or less important.

 If k1, k2 .... kp are the ratios of the geometries of the transistors T11, T12, etc., to the geometry of the transistor T10, we can say that the current I1 is equal to the current 10 multiplied by a coefficient which is the sum of the factors ki of the transistors which are actually traversed by a current, that is to say of those whose respective switches are on.

 The slaving according to the invention of the voltage drop R1 I1 at the voltage drop Rg 10 during the variations of R1 will consist in controlling the closing of switches selected from T21 to T2p until a combination of closures and switches is obtained. Switching openings such as the sum of the currents in the transistors whose switches are open produce a voltage drop R1 which exactly balances the voltage drop Rg 10.

 According to a particular feature of the invention, it is ensured that the search for balance is made by incrementing the current II in a monotonic manner, that is to say that adjacent switches will be successively closed, without reopening the previously closed switches. , until you get the balance. there will then be a direct correspondence between the numbers of the closed switches and the value of the resistor R1. It is also possible to establish a binary correspondence between the numbers of the closed switches and a numerical value representative of the resistor R1 and therefore of the position of the cursor. potentiometer P1.

 It is advantageous in this case to provide, during the construction of the encoder according to the invention, that the geometries of the transistors Tell, T12 .. 6 Tp are determined as a function of the interesting positions of the cursor of the potentiometer P1 (these positions corresponding to well-determined resistance values R1 and the transition from a position of interest to another position immediately interesting consequent causing a change of current corresponding precisely to the addition of an additional transistor Tli in the circuit).

 It can be seen that there is every latitude to establish any arbitrary but monotonic coding function between resistance values R1 and a digital output value of the encoder.

 It may be mentioned here that it is particularly desirable to provide that the geometries of the transistors rios serving as switches (T21, T22 ... T2n) are the same or in the same ratios as the geometries of the respective transistors they control ( Tll, ..., lp) so that the drop in voltage that they introduce when they pass is the same for all.

In the same way, it is necessary to specify now the role of the transistor T20 in series with the transistor T10 constituting the generator of the current 10: since the switches T21 to T2p are provided in the regulation element of the current II and that they produce a voltage drop even when they are in the on state, a corresponding voltage drop is established in the current branch 1o thanks to transistor T20, permanently conducting (V + gate). The geometry of this transistor T20 must also be chosen in the appropriate ratio. These arrangements improve the current mirror phenomenon between the transistor T10 and the transistors T11 to Tip
In addition, it may be mentioned that the current regulation element constituted by the transistors T11 to Tlp may comprise a transistor, such as T11 operated continuously to let a minimum bias current Il.In this case, it is conceivable that one does not need a transistor operating as a switch in series with T11. However, a transistor T21 is permanently biased in its conducting state by a voltage V + applied to its gate, this transistor creating, like the transistor T20, a voltage drop equivalent to that of the switches T22 to T2p. The geometry of T21 is chosen accordingly.

 We will now indicate how the differential amplifier successively controls the commissioning of transistors T22, T23, etc. until the equilibrium is reached.

 This function is performed by a counting circuit directly connected to the output of the differential amplifier AD which supplies a counting authorization or stop circuit according to the sign of the difference between the voltage drops Rg 10 and R1 II. . The counting circuit comprises, to effect the successive closure of the switches T22 to T2p, a bit counter C on the outputs of which is taken the digital output information of the encoder according to the invention (outputs Q1 to QN). This counter has a count enable or stop input coupled to the output of the differential amplifier. it also comprises a clock input intended to establish the counting, therefore the successive closing of the switches, at a fixed frequency.

 The counting circuit further comprises a decoder and register element, which will be called a decoder-register, which receives the outputs Q1 to QN 'and which has p outputs each controlling a switch of the regulating element of the current II; the register-decoder provides, for a number m applied to its inputs by the counter, a signal on the first m outputs of the register-decoder and a complementary signal on the other p - m.

 In other words, if the content of the counter C is m, the switches T21 to T2m will be closed and the switches Tm + 1 to T2p will be open. During the incre- mentation of a unit of the counter C, an additional switch will close without the others open, thereby creating a certain amount (depending on the geometry factor of the corresponding transistor) the current It circulates in R1.

 The register-decoder preferably comprises a decoder D (4parmi p) constituted by a series of p gates each receiving the outputs of the binary counter C and cabled so as to each deliver a single output for a specific count of the counter C. The register The decoder also includes a part that can be called a register E which comprises a set of p MOS transistors in series, T31 to T3p, powered by a low DC voltage V +.

 Each output of the decoder D controls a gate of a respective transistor T3i and can block it or make it conductive according to the state of the corresponding output of the decoder D.

More specifically, the outputs of the decoder 1 among p are coupled, in the order of their numbering, to a gate of a corresponding MOS transistor in the same order of numbering; thus, the output C1 of the encoder is coupled to the gate of the transistor T31; the exit
C2 the gate of transistor T32; etc; the output Cp to the transistor T3p.

 The decoder V provides, if the counter C has a content m, an output Cm to zero and all others to one.

Thus, only the transistor T3m will be blocked, all the others being conductive.

 The outputs of the register E are taken from the drains of transistors T31 to T3p, after inversion in respective inverters Il I2 .... ip'i. Each of the outputs of the register, that is to say the outputs of these invertors controls a respective switch T22 to T2p (transistor T21 is not controlled to keep a bias current I1 permanently).

The register E has a very simple operation if the counter C has a content m, the decoder D designates the output Cm, the transistors T31 to T3m ground the inputs of the inverters I1 to I1; the transistors
T3m + l to T3p put the inverters I 'to Ip ~ l to V +.

m + 1 pI
Inverters I'1 to I'p-1 reverse this data.

As a result, the first m transistors T12,
T13 .... etc, are put into operation to establish the current and and the other p - m are put out of service. When the counter C increments its contents by one unit to go to m + 1, one more transistor is turned on.

 It is therefore the register E which establishes, under the control of the counter C and the decoder D, the monotonous aspect of the search for I1, it being understood that the monotonic function obtained is not necessarily linear and depends in particular on the geometries relative transistors T11 to Tlp.

 For each position of the slider of the potentiometer P1, the differential amplifier authorizes the counting of the counter C and therefore the cumulative successive commissioning of a certain number of transistors of the current regulation element I1, until the current is such that the voltage drop R1 I1 equilibrium Rg lo Z then, the differential amplifier stops counting and can read on the output of the encoder, that is to say on the outputs of the counter C, a binary digital value corresponding to the desired coding of the position of the cursor of the potentiometer P1.

 It has already been mentioned that one of the advantages of the invention was to allow a supply Vg not necessarily constant and even possibly alternative for the resistors Rg e. R1, the influence of this voltage being eliminated due to the ratiometric measurement of currents carried out.

However, it is desirable, if the tension
V0 is alternative, that the measurement is made when VQ is at its maximum, that is to say for example during a fraction of positive alternation of the voltage around the peak of this alternation. For this purpose, provision is made for a synchronization circuit S coupled to the counting clock of the counter C, to define a positive halfwave around the voltage peak at each period of the sector, to allow the operation of the set of the decoder, and in particular the counter C, cue in the window defined by the circuit.

Claims (10)

 1. Digital encoder position of a potentiometer slider, characterized in that it comprises a fixed resistor (Ro) in series with an element for defining a reference current Io; a variable resistance R1 comprising the potentiometer (P1), in series with a plurality of current generators in parallel (T11, T12 ..) each of which can be put into operation by a respective switch (T21, T22 ..) each generator of the current establishing a current in a known ratio with the others and with 10 and being controlled by 10 to proportionally follow the variations of IO, the two mentioned serial devices being supplied by a common voltage, the encoder further comprising a circuit of counting (C, D, E) coupled to the switches to successively control the closure of each of them, and a differential amplifier (AD) whose inputs are connected to the resistors to determine the sign of the difference between the voltage drops at terminals of the fixed and variable resistors, and to consequently provide the counting circuit with an authorization or counting stop signal, the output of the encoder being taken from the control circuit. counting.  2. Digital encoder according to claim 1, characterized in that the element serving to define the reference current comprises a transistor MCS (T10) drain and gate together, and that each current generator comprises a transistor MOS (Tll, T12 ..) having its gate connected to the gate of the MOS transistor (T10).  3. Encoder according to claim 2, characterized in that the element defining the reference current comprises another MOS transistor, permanently conducting in series with the first  4. Digital encoder according to one of claims 2 and 3, characterized in that the MOS transistors of the current generators have, compared to the MOS transistor de; terminating the reference current, coefficients chosen individually to establish a particular relationship selected between the potentiometer resistance values and the coded output numbers of the encoder.  5. Digital encoder according to claim 4, characterized in that the common voltage applied to the fixed and variable resistors is an AC voltage of the sector.  6. Digital encoder according to one of claims 1 to 5, characterized in that the counting circuit comprises another authorization input controlled by a synchronization circuit (S) establishes a coder operating authorization signal. during a fraction of positive alternation of the voltage of the sector, around the peak of this alternation.  7. Digital encoder according to one of claims 1 to 6, characterized in that the counting circuit comprises a bit counter (C) on the outputs of which is taken a digital output signal of the encoder, and a register-decoder ( DE) receiving the outputs of the meter, and having p outputs each controlling a switch of the second current regulating element, the registredécodeur providing, for a number m applied to its inputs by the counter, a signal on the first m outputs of the register- decoder and a complementary signal on the other p - m.  8. Digital encoder according to claim 7, characterized in that the register-decoder comprises a decoder 1 among p (D) and a set of transistors. MOS series, fed by a low DC voltage, the outputs of the decoder 1 among p being each coupled, in the order of their numbering, to a gate of a corresponding MOS transistor in the same order, and the drains of the transistors serving of outputs, possibly after inversion, to the register-decoder.  9. Digital encoder according to one of claims 1 to 8, characterized in that the switches are constituted by MOS transistors controlled by their gate.  10. A digital encoder according to claim 9, characterized in that the ratios between the geometries of the MOS transistors serving as switches are the same as the ratios between the geometries of the MOS transistors serving as current generators.