GB1599115A - Electronic characterizer - Google Patents

Description

(54) ELECTRONIC CHARACTERIZER (71) We, FISCHER & PORTER COM PANY, a corporation organised and existing under the laws of the State of Pennsylvania, of Warminster, Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: Background of invention This invention relates generally to electronic characterizers, and more particularly to a simple, low-cost characterizer providing negative or positive compensation above a predetermined breakpoint.

Characterizers or function generators are electronic devices whose output signal is caused to vary as a prescribed function of an input signal. These devices are usable for correcting a non-linear element so that the output signal of the characterizer becomes a linear function of an input variable applied to the non-linear element.

Existing types of function generators or characterizers in many instances include elaborate networks of diodes and resistors such that a given arbitrary function can be approximated by a number of straight line segments. Bias and resistor values determine the junction points and slopes of each segment of a plot of the output voltage. A more detailed discussion of function generators may be found in U.S. Pat. 3,560,727 of Schussler.

Function generators of the type commercially available are relatively complex and costly instruments. Typical of such instruments is the characterizer manufactured by Fischer & Porter Co. and described in their Instruction Bulletin (1974)fun the Series 55 FG 3000 Function Generator. This device consists essentially of five input amplifier stages, each adapted to adjust a segment of a process input curve, a summing amplifier and an output voltage-to-current converter.

The need exists for a much simpler and less expensive characterizer capable of providing negative or positive compensation of non-linear element whereby the characterizer yields an output signal which is a linear function of the input variable applied to the element.

Thus in U.S. Patent No. 4,083,237 entitled "Differential Reluctance Transducer System," there is disclosed a displacement detector whose electrical output is a function of input motion. Ideally, this electrical output should be linearly proportional to the extent of input motion. But, in practice, the upper end of the displacement range tends to deviate in the negative or positive direction from linearity, and the system is therefore somewhat inaccurate.

While existing function generators or characterizers are capable of characterizing the signal from the displacement detector to correct for its deviation from linearity, such instruments, because of their complexity, add substantially to the overall cost of the measuring system.

Summary of invention This invention provides an electronic characterizer which is adjustable to effect negative or positive compensation above a predetermined breakpoint. The characterizer operates in conjunction with a non-linear device such as a displacement detector, is responsive to an input variable to yield an input current, and produces in response to the input current an output signal which is a linear function of the input variable.

According to the present invention, there is provided an electronic characterizer operating in combination with a non-linear device responsive to an input variable to yield an input current, the characterizer effecting negative or positive compensation to produce an output signal which is a linear function of the input variable, said characterizer comprising; (A) a first branch connected to said non-linear device and having a first resistor therein across which is developed a first voltage as a function of the input current flowing therethrough which reflects the nonlinear characteristics of said device;; (B) a second branch connected to said device whereby said first voltage is applied across said second branch, said second branch consisting of a second resistor in series with a diode having a given threshold such that no input current flows through said second branch until said first voltage exceeds said threshold, after which a second voltage is developed across said second resistor, said second resistor being a potentiometer whose adjustable tap acts to select a portion of the second voltage, and (C) means to combine said first voltage with said selected portion of said second voltage to produce said output signal.

The characterizer may thus have an exceptionally simple, low-cost design.

Outline of drawings For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following exemplary detailed description to be read in conjunction with the accompanying drawings, wherein: Figure 1 is a schematic diagram of an electronic characterizer in accordance with the invention; Figure 2 is a graph illustrating the relationship between the input current from the non-linear element and the output current due to current in the non-adjustable path only.

Figure 3 is a graph illustrating the effect of the characterizer on the current relationships with compensation; and Figure 4 is a schematic diagram of the characterizer in an actual application therefor.

Description of invention Referring now to Figure 1, there is shown the circuit diagram of a characterizer according to the invention adapted to correct a non-linear element 10 which may be a displacement detector of the type described in the above-identified U.S. Patent No.

4,083,237 or any other non-linear element in need of positive or negative compensation above a prescribed breakpoint.

The input current yielded by non-linear element 10 is applied through a fixed resistor Rx to an ammeter 11 or other suitable current-responsive device. Shunted across non-linear element 10 are two parallel branches, the first containing a resistor Ra, and the second a solid-state diode 12 in series with a potentiometer Rb. In practice, the diode may be constituted by a transistor whose base is connected to its collector. The adjustable tap of potentiometer Rb is connected to the junction of ammeter 11 and resistor Rx through a fixed resistor Ry.

The current flow through resistor Rx is represented by symbol Ix, and the current flow through resistor Ry by symbol Iyw Hence the current 1o through ammeter ilis the combined currents Ix and Iyo As is characteristic of solid-state diodes, no current flows in the forward direction until a predetermined voltage, called the "threshold voltage," is exceeded. From then on, the forward drop is roughly linear.

Hence the characteristic diode current has a knee which represents the point of inflection at which the current proceeds to flow above the threshold voltage. We shall assume, therefore, that diode 12 has a sharp knee at voltage Vf. Thus in the arrangement shown, current always flows through resistor Ra in the first branch shunted across element 10 to produce a first voltage Ea, whereas current flows through resistor Rb only when voltage Ea exceeds the threshold level of diode 12.

We shall further assume that the current 1o passing through ammeter 11 is very small compared to the current Ij passing through non-linear element 10 (e.g. in a 1 to 10 ratio). This relationship is achieved by proper selection of the values of resistors Ra, Rx and R The relationship between ammeter current 1o and non-linear element current Ij, is represented by the graph in Figure 2. It will be seen that the relationship between currents I and Ii is linear until a break point BP determined by the threshold voltage of the diode 12 is reached, at which point the current deviates from linearity in the negative direction.

When diode 12 is non-conductive because the first voltage applied thereto is below the breakpoint level, current Ij only flows in the first branch across resistor Ra. Voltage Ea developed across resistor Ra is then directly proportional to Ij. Hence Ea = Ii Ra.

Current Ij below the breakpoint is less than Vf, the diode breakpoint voltage, divided by the resistance of resistor R When current Ij exceeds the breakpoint value, diode 12 is rendered conductive and voltage Ea increases at a rate proportional to the parallel combination formed by resistors Ra and Rb in the first and second branches.

Since the combined resistance of the parallel branches is less than the ohmic value of resistor Ra alone, the rate of change of voltage Ea with respect to current Ij is reduced above the breakpoint.

The current Ix through fixed resistor Rx is equal to voltage Ea divided by resistance Rx; hence current Ix will have the same reduced slope above breakpoint BP as was derived for voltage Ea. Beyond breakpoint BP, current flow through diode 12 results in a second voltage drop Eb across potentiometer Rb. A portion represented by symbol 6 of this voltage is applied to fixed resistor Rb to develop the current ly which is combined with the current Ix across resistor Rx, thereby producing output current lo across ammeter 11.

Therefore, when 0=0, current ivy = 0 and ammeter current 1o = Ix. The circuit transfer function for 0 = 0 is shown in Figure 2.

Beyond breakpoint, when current ly is produced, the additional current increases the slope of current 1o with respect to current Ij, the amount of increase being a function of the value of the voltage portion 6. Figure 3 shows output current 1o as a function of non-linear element current 1o and portion 6. It will be seen that below breakpoint BP, the transfer function is independent of 0.

Thus by selecting the value of resistor Ra in the first branch across element 10, one can select the location of breakpoint BP as a function of Ij; for the higher the value of resistor Ra, the lower the level of current necessary to attain the threshold of diode 12.

By selection of the value of resistance Rb, the amount of reduction in slope for 0 = 0 can be controlled. And by selecting the ratio RX/Ry, the amount of increase in the slope can be controlled.

The following values make possible a non-linearity of + 1.5% at 6 = 1 for an input Ij in the range of 4 to 20 mAdc.

Ra = 50 ohms Rb = 750 ohms Rx = 250 K ohms Ry = 1.5 megohm Diode = 2N2484 transistor base junction.

In the characterizer shown in Figure 1, an ammeter is provided as the output device to illustrate the functional characteristics of the circuit. In actual practice, however, the characterizer may be used in conjunction with any current-responsive output device wherever it is necessary to effect negative or positive compensation above a predetermined breakpoint.

Figure 4 shows one actual application for the characterizer in accordance with the invention. The non-linear device in this instance comprises a summing amplifier 13 and a transducer, the characterizer being included in the feedback loop of the summing amplifier 13 to whose input is applied the input voltage from the transducer, which may for example be in the form of a displacement detector, through a range resistor 14, the output of the amplifier 1out being the input current Ij to the characterizer.

While there has been shown and described a preferred embodiment of an electronic characterizer in accordance with the invention, it will be appreciated that many changes and modifications may be made therein without, however, departing from the scope of the claims.

WHAT WE CLAIM IS: 1. An electronic characterizer operating in combination with a non-linear device responsive to an input variable to yield an input current, the characterizer effecting negative or positive compensation to produce an output signal which is a linear function of the input variable, said characterizer comprising:: (A) a first branch connected to said non-linear device and having a first resistor therein across which is developed a first voltage as a function of the input current flowing therethrough which reflects the nonlinear characteristics of said device; (B) a second branch connected to said device whereby said first voltage is applied across said second branch, said second branch consisting of a second resistor in series with a diode having a given threshold such that no input current flows through said second branch until said first voltage exceeds said threshold, after which a second voltage is developed across said second resistor, said second resistor being a potentiometer whose adjustable tap acts to select a portion of the second voltage, and (C) means to combine said first voltage with said selected portion of said second voltage to produce said output signal.

2. A combination as set forth in claim 1, further including an ammeter to indicate said output signal.

3. A combination as set forth in claim 2, further including a first fixed resistor connecting said device to said ammeter, and a second fixed resistor connecting said tap to said ammeter whereby the current applied thereto is the sum of the currents flowing through said first and second fixed resistors.

4. A combination as set forth in claim 1, wherein the non-linear device comprises a non-linear transducer and a summing amplifier to whose input is applied the output voltage of the non-linear transducer, and wherein the characterizer forms the feedback loop of the amplifier, the output current of said amplifier flowing through said first branch to produce said first voltage which is applied across said second branch to produce said second voltage, the output signal derived from the combination of said first voltage and a portion of said second

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. equal to voltage Ea divided by resistance Rx; hence current Ix will have the same reduced slope above breakpoint BP as was derived for voltage Ea. Beyond breakpoint BP, current flow through diode 12 results in a second voltage drop Eb across potentiometer Rb. A portion represented by symbol 6 of this voltage is applied to fixed resistor Rb to develop the current ly which is combined with the current Ix across resistor Rx, thereby producing output current lo across ammeter 11. Therefore, when 0=0, current ivy = 0 and ammeter current 1o = Ix. The circuit transfer function for 0 = 0 is shown in Figure 2. Beyond breakpoint, when current ly is produced, the additional current increases the slope of current 1o with respect to current Ij, the amount of increase being a function of the value of the voltage portion 6. Figure 3 shows output current 1o as a function of non-linear element current 1o and portion 6. It will be seen that below breakpoint BP, the transfer function is independent of 0. Thus by selecting the value of resistor Ra in the first branch across element 10, one can select the location of breakpoint BP as a function of Ij; for the higher the value of resistor Ra, the lower the level of current necessary to attain the threshold of diode 12. By selection of the value of resistance Rb, the amount of reduction in slope for 0 = 0 can be controlled. And by selecting the ratio RX/Ry, the amount of increase in the slope can be controlled. The following values make possible a non-linearity of + 1.5% at 6 = 1 for an input Ij in the range of 4 to 20 mAdc. Ra = 50 ohms Rb = 750 ohms Rx = 250 K ohms Ry = 1.5 megohm Diode = 2N2484 transistor base junction. In the characterizer shown in Figure 1, an ammeter is provided as the output device to illustrate the functional characteristics of the circuit. In actual practice, however, the characterizer may be used in conjunction with any current-responsive output device wherever it is necessary to effect negative or positive compensation above a predetermined breakpoint. Figure 4 shows one actual application for the characterizer in accordance with the invention. The non-linear device in this instance comprises a summing amplifier 13 and a transducer, the characterizer being included in the feedback loop of the summing amplifier 13 to whose input is applied the input voltage from the transducer, which may for example be in the form of a displacement detector, through a range resistor 14, the output of the amplifier 1out being the input current Ij to the characterizer. While there has been shown and described a preferred embodiment of an electronic characterizer in accordance with the invention, it will be appreciated that many changes and modifications may be made therein without, however, departing from the scope of the claims. WHAT WE CLAIM IS:

1. An electronic characterizer operating in combination with a non-linear device responsive to an input variable to yield an input current, the characterizer effecting negative or positive compensation to produce an output signal which is a linear function of the input variable, said characterizer comprising:: (A) a first branch connected to said non-linear device and having a first resistor therein across which is developed a first voltage as a function of the input current flowing therethrough which reflects the nonlinear characteristics of said device; (B) a second branch connected to said device whereby said first voltage is applied across said second branch, said second branch consisting of a second resistor in series with a diode having a given threshold such that no input current flows through said second branch until said first voltage exceeds said threshold, after which a second voltage is developed across said second resistor, said second resistor being a potentiometer whose adjustable tap acts to select a portion of the second voltage, and (C) means to combine said first voltage with said selected portion of said second voltage to produce said output signal.

2. A combination as set forth in claim 1, further including an ammeter to indicate said output signal.

3. A combination as set forth in claim 2, further including a first fixed resistor connecting said device to said ammeter, and a second fixed resistor connecting said tap to said ammeter whereby the current applied thereto is the sum of the currents flowing through said first and second fixed resistors.

4. A combination as set forth in claim 1, wherein the non-linear device comprises a non-linear transducer and a summing amplifier to whose input is applied the output voltage of the non-linear transducer, and wherein the characterizer forms the feedback loop of the amplifier, the output current of said amplifier flowing through said first branch to produce said first voltage which is applied across said second branch to produce said second voltage, the output signal derived from the combination of said first voltage and a portion of said second

voltage being applied to the input of said amplifier.

5. A combination as set forth in claim 4, wherein said transducer is a displacement detector.

6. An electronic characterizer substantially as described with reference to the accompanying drawings and substantially as illustrated therein.