METHOD AND APPARATUS FOR CONTROLLING A CURRENT GENERATOR

Abstract: Method of controlling a current generator (1) having two voltage inputs (9, 10) and an output (13) connected to a load (6). In case the impedance of the load is lower than a first value, the generator is controlled in a non-feedback way to generate a signal, the current value of which is determined by the input voltage. On the other hand when the impedance of the load is higher than a second value larger than the first value, the output signal is negatively fed back to generate a signal on the output, the voltage value of which signal is determined by the input voltage. Thus, the output signal is automatically adapted to the desired mode of operation of the receiving apparatus (5). The invention also relates to an apparatus for carrying out the method.

Description: Method and apparatus for controlling a current generator TECHNICAL FIELD The invention relates to a method of controlling a cur- rent generator, which generator has two voltage inputs and a current output connected to a load, and generates a current on the output that is proportional to the diffe¬ rence voltage of the voltages on the inputs. The inven¬ tion also relates to an apparatus for use with the me- thod. BACKGROUND OF THE INVENTION In many devices in the field of control engineering, e.g. regulators, actuators and sensors for proportional con- trol, output values, i.e. desired values and actual va¬ lues for e.g. valve positions and temperatures, are ge¬ nerally represented by the voltage of an analogue signal that can range between e.g. 0 and 10 volts. In many other devices receiving these analogue signals, these values are expected to be in the form of a current signal, e.g. 0 to 20 mA. This constitutes a problem, as not all devices will in¬ teract, while the output of a device does not correspond to the expected input of another device. In certain de¬ vices the mode of operation, i.e. current control or voltage control, can be switched by means of a jumper, e.g., but this switch means one more installation step and it can also lead to an incorrect installation. Also, two separate solutions, one for current control and one for voltage control, lead to a higher cost for electronic circuitry. OBJECT OF THE INVENTION Accordingly, the main object of the present invention is to achieve such a method of controlling the output of a current generator that this output is automatically adap¬ ted to the preferred mode of operation of a receiving device. The invention also relates to an apparatus for implementation of this method. SUMMARY The present invention is based on the realization that the abovementioned object can be achieved using the fact that a current controlled input has a much lower input impedance than a voltage controlled input. According to the invention, this object is achieved by a method of controlling a current generator having two voltage inputs and an output connected to a load, which generator generates on the output a current proportional to the difference voltage of the voltages on the inputs, so that, if the impedance of the load is lower than a predetermined first lower limiting value, the current generator is controlled in a non-feedback way to generate a signal on the output of the generator, the current value of which signal is determined by said input vol- tage, and that, if the impedance of the load is higher than a predetermined second limiting value larger than said first limiting value, the output signal of the cur¬ rent generator is negatively fed back to generate an output signal on the output of the generator, the voltage value of which signal is determined by said input vol¬ tage. The invention also relates to an apparatus using the method according to the invention. Other features of the invention are stated in the depen¬ dent claims. The invention provides a technique that eliminates the abovementioned problems connected with adaption of the modes of operation of interconnected devices. BRIEF DESCRIPTION OF THE DRAWING The invention will be further described by means of an example with reference to the accompanying drawing, in which: Fig. 1 is a general diagram of an apparatus according to the present invention. Fig. lb shows the characteristics of an amplifier incor- porated in Fig. 1, and Fig. 2 is a circuit diagram of an embodiment of an appa¬ ratus according to the present invention. EXAMPLE OF AN EMBODIMENT The device according to Fig. 1 comprises, besides a cur¬ rent generator 1, a high gain amplifier 3 with essential¬ ly infinite input impedance, which amplifier has ground and a voltage +v as power supplies. The output signal U^ of the current generator is supplied to the positive input 11 of the amplifier 3 and an input U^ is supplied to the negative input 12 of the amplifier through an input voltage input 2 connected to the negative input of the amplifier, which input signal can represent e.g. a de- sired or an actual value. The output of the amplifier is connected to the negative input 10 of the current genera¬ tor. The characteristics of the amplifier 3 are given by Fig. lb showing the output voltage U of the amplifier as a function of the difference voltage across the inputs 11, 12 of the amplifier, in this case U^ - U^. The output 13 of the current generator is also connected to a unit 5 to be controlled, via a connection 7. This unit that can be either current or voltage controlled has an input impedance value corresponding to a load resistor 6, " the value of which usually is low for current control, for example less than 300 ohms, and high for voltage control, preferably more than 10 kohms. The method according to the invention will be described in detail below. The input signal U k is supplied to the input 2, which produces a current on the output 13 of the current generator 1, which current in turn generates a voltage U,,,, over the load resistor 6. As the input impedance of the amplifier 3 is essentially infinite, the output current of the current generator essentially cor¬ responds to the current flowing through the load resistor 6. The device is so adapted that if the load is current controlled, the output voltage U^ will not be higher than the input voltage Uj„. Thus, the amplifier 3 receives a negative voltage U^ - U^ across the inputs 11, 12, which leads to it providing an output signal with a voltage essentially equal to ground to the negative input 10 of the current generator, due to that the amplifier has ground and voltage +V as power supplies. The entire de- vice then operates as a current generator generating an output current I^ determined by the input voltage U^. However, if the load is voltage controlled, i.e. the value of the load resistor 6 is high, the output voltage U ou , will be higher than the input voltage U^ and the amplifier then has a positive difference voltage across the inputs 11, 12. This leads to the amplifier supplying a signal to the negative input 10 of the current genera¬ tor, which signal is proportional to the difference vol- tage between the output voltage U^ and the input voltage U ta . It can easily be shown that the output voltage U ∞t in this case, with the gain of the amplifier 3 and thus the loop gain being high, due to the negative feedback of the output voltage adjusts so that U^asU^. The entire device then functions as a generator generating on its output a voltage essentially corresponding to the input voltage, that is, the device operates as a voltage follower. The embodiment shown in Fig. 2 corresponds in essential parts to the design of the construction described above, so that the same references are used for corresponding parts. Close to each resistor is a resistor value used for this specific embodiment. The current generator 1, which is of a common type de¬ scribed in e.g. IC Op-Amp Cookbook, Third Edition, Howard W. Sams Co., ISBN-O-672-22453-4 comprises two input re¬ sistors 14, 15 connected to the negative and the positive input, respectively, of a first operational amplifier 16. The operational amplifier can be of a conventional type, e.g. LM324. Across the inputs of the operational ampli¬ fier 16, there are three resistors 17, 18 and 19 connec¬ ted in series. Because the two inputs of the operational amplifier 16 are essentially at the same potential, the so called virtual ground, the three resistors 17, 18 and 19 form a closed voltage loop producing a current on the output 13 of the current generator 1 located between the resistors 18 and 19, which current is proportional to the difference voltage across the two inputs 10, 11 of the current generator, i.e. across the two input resistors 14, 15. In the embodiment shown, a second operational amplifier 20 and a resistor 21 are provided in parallel with the resistor 18 to improve the driving capability of the current generator. The output of third operational amplifier 22 is connected to the negative input 10 of the current generator, i.e. to the input resistor 14. The positive input of this amplifier is connected to the output 13 of the current generator, while the negative input is connected to the input 2 of the device via a resistor 23. Resistors 24 and 25 are connected between the outputs of the first and third operational amplifiers 16, 22, re- spectively, and ground, to improve the driving capability of the amplifier to ground. A diode 4 is provided between the output and the negative input of the third amplifier 22, for reasons of stability and in order to improve the capability of the operational amplifier 16 to drive its output to the negative supply. The load resistor 6 is connected to the output 7 of the device. As previously mentioned, its value depends on whether the load is current or voltage controlled and it can in the first case be approximately 200 ohms and in the latter approximately 10 kohms. A preferred embodiment of a device according to the invention has been described above. This can be varied in several aspects within the scope of the claims. For example, the second operational amplifier 20 and the resistor 21 can be omitted if less driving capability of the current generator 1 is required, and the supply volt¬ ages of the operational amplifiers could have a positive and a negative voltage, in which case the output signal could be a positive as well as a negative signal. A method and an apparatus as above can be of use in many applications, as followers in controllers, actual value outputs in actuators or other outputs for physical quan- tities, where the output signal represents velocity, flow etc.

Claims: CLAIMS 1. Method of controlling a current generator by means of an input voltage, wherein the generator has two vol- tage inputs and an output connected to a load, which generator generates on the output a current proportional to the difference voltage between the voltages on the first positive input and the second negative input, c h a r a c t e r i z e d i n that, in case the impedance of the load is lower than a predetermined first lower limiting value, the current generator is controlled in a non-feedback way to generate a signal on the output of the generator, the current value of which signal is determined by said input vol¬ tage, and that, in case the impedance of the load is higher than a predetermined second limiting value larger than said first limiting value, the output signal of the current generator is negatively fed back to generate a signal on the output of the generator, the voltage value of which signal is determined by said input voltage. 2. A method according to claim 1, c h a r a c t e ¬ r i z e i n that the negative feedback is provided by means of feeding the input voltage to the positive input of the generator, and that said input voltage, U^, is compared with the voltage value of the output signal of the generator, U^, wherein if U out ≥U f c, the negative input of the generator is supplied with a signal essentially equal to ground, and if U ou ^U jn , the negative input of the generator is supplied with a signal proportional to the diffe¬ rence between the voltage value U^ and the input voltage U^. 3. A method according to claim 2, c h a r a c t e ¬ r i z e i n that the comparison is effected by means of an operational amplifier. 4. A method according to any of the preceding claims, c h a r a c t e r i z e d i n that the generator, when negatively fed back, generates a signal on its output, the voltage value of which essentially corresponds to the input voltage. 5. A method according to any of the preceding claims, c h a r a c t e r i z e d i n that the first lower limiting value is set to 300 ohms. 6. A method according to any of the preceding claims, c h a r a c t e r i z e d i n that the second higher limiting value is set to 10 kohms. 7. Apparatus for controlling a current generator by means of an input voltage, which generator has two vol¬ tage inputs and an output connected to a load, and which generator generates on the output a current proportional to the difference voltage between the voltages on the first positive input and on the second negative input, c h a r a c t e r i z e d i n that it comprises an input voltage input (2) connected to the positive input (9) of the current generator, and a negative feedback loop (8) provided between the output (13) and the negative input (10) of the current genera¬ tor, which loop is so formed that if the impedance (6) of the load is lower than a predetermined first lower limiting value, the loop (8) is opened, whereby a signal essentially equal to ground is supplied to the negative input (10) of the current generator, and if the impedance (6) of the load is higher than a predetermined second limiting value larger than said first limiting value, the loop (8) is closed, so that a signal proportional to the difference between the output voltage U^, and the input voltage U^ is supplied to the negative input (10) of the current generator. 8. An apparatus according to claim 7, c h a r a c - t e r i z e d i n that the loop (8) comprises an opera¬ tional amplifier (3) having a positive input (11) con¬ nected to the output (13) of the current generator (1) , a negative input (12) connected to the input voltage input (2) , and an output connected to the negative input (10) of the current generator. 9. An apparatus according to claim 8, c h a r a c ¬ t e r i z e d i n that the supply voltages of the opera¬ tional amplifier (3) comprise ground and a voltage that is higher than the highest possible voltage potential for the voltage of the input voltage input (2) .

Inventors: HELLQVIST OLE, HELLQVIST, OLE
Applicants: HELLQVIST OLE, LANDIS & GYR BUILDING CONTROL, LANDIS & GYR BUILDING CONTROL AB
Application year: 1993
Main classification: G05B19
IPC codes: G05F1, H03F1, H03H11