DE2721371A1 - PROCESS FOR REDUCING LOSSES AND IMPROVING THE TRANSMISSION BEHAVIOR ON MEASURED VALUE TRANSMITTERS AND DEVICE FOR PERFORMING THE PROCESS - Google Patents

Description

Process for reducing losses and improving transmission behavior on measured value transmitters and device for carrying out the method

The present invention relates to a method for reducing losses and improving the transmission behavior Measured value transmitters for isolated measured value transmission of static electrical signals, with transformer transmission means and with chopping means which are connected in such a way that a field flux occurs in the switching rhythm of the chopping means alternating polarity in the transmission means and a device for carrying out the method, wherein the transmission means comprise at least one transformer, which is provided with an intermediate tap primary

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has winding, and wherein the chopper means for closing, respectively. Opening of the circuits from the intermediate tap are formed over each part of the primary winding.

As is known, transformer-based ones are generally available for potential separation in measured value transmitters or opto-electrical transmission means. The usage of opto-electrical transmission means that preamplifier and offset circuit means must be provided in order, on the one hand, to achieve sufficient resolution, especially in the case of small ones to be transmitted To ensure measured values and, on the other hand, the most linear possible characteristic curve range of the optoelectrical To be able to use transmission means. To ensure sufficient stability of the zero point even in the long term and to ensure linearity, must be added when designing and selecting the above-mentioned additional Switching means are operated at a great expense.

To circumvent the above mentioned difficulties with optoelectrical Transmission means are conventionally used transforming means.

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For this purpose, a current proportional to the measured value, be it a voltage or a current, is used Center tap of a symmetrical transformer primary winding and the circuit through chopper means alternately closed over one or the other half of said transformer winding. In the transformer secondary winding This induces a DC voltage-free AC voltage and thus, depending on the load, a current, which is then subjected to rectification and smoothing.

Usually at least one pair of switches is used as a chopper provided, which is complementary to the 5current returns closes and opens.

As will be explained below, the abrupt current switching causes relatively large losses, which leads to a Reduction of the efficiency of such transformers and at the same time, due to transient interactions in the transformer, also leads to major transmission errors.

The present invention aims to remedy the above-mentioned disadvantages. To this end, it is characterized by it from the fact that the chopper means the induction

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interrupts the flow causing current through the transmission means for a predetermined period of time, in each case before the polarity changeover of the induction flux.

The device for carrying out the method is characterized in that the chopping means are control means are assigned, which emit at least one control signal for controlling the chopping means in such a way that that the latter alternately close and open the corresponding circuits, so that in each case only one is closed, and that before closing one of the circuits, both circuits during predetermined Time are interrupted.

As is known, there are two ways of feeding the chopping and control means. Will be a External feed is provided, the circuit complexity for such measured value transmitters is thereby increased completely

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essential in that additional feed units must be provided. Become the chopper and control means operated with self-supply, this is the case, especially when the measured value transmitter is used for power transmission and are not used for voltage transmission, d. H. practically measured a power source should be, difficulties in that the current consumption of the chopper unit falsifies the measurement current.

Since part of the measuring current is branched off for the self-supply, the transformer means on the one hand and the chopper and control means on the other hand act as a current divider, the ratio of which is of course dependent on the load on the measured value transmitter. It can be seen from this that in the case of measured value transmitters for currents, the errors caused by the internal consumption of the current are load-dependent. For this reason, measured value transmitters are usually used for voltage transmission and not for current transmission, although there is a notable need for the latter uses .

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The present invention now also aims at a device for carrying out the method on measured value transmitters for direct current, the chopper and control means from the signal to be transmitted are self-fed, which only has the above-mentioned disadvantages to an extremely reduced extent. to For this purpose, it is characterized by the fact that current sources are provided for the supply.

This ensures that the internal power consumption is no longer dependent on the load and the remaining measurement error can be taken into account without further details, for example by calibration.

The invention will then be explained using figures, for example.

Show it:

1 shows a function block diagram of a measured value transmitter of known design,

Fig. 2 schematically shows the timing of the current supplied to the transformer and its

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Division into both primary winding parts and the resulting transformer field profile, in a measured value transmitter according to FIG. 1,

Fig. 3 schematically shows the desired time courses of the currents in both primary winding parts and the resulting field course as well as the course of the current supplied to the transformer,

4 shows a functional block diagram of a chopper unit for obtaining current / field profiles according to Fig. 3,

5 schematically shows the time profile of the chopper control signals and the resulting transformer current profile in a chopper unit according to Fig. 4,

6 shows a possibility of realizing a chopper unit using a conventional one Multivibrators,

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7 shows a preferred embodiment of the chopper unit according to FIG. 4 for self-fed measured value transmitters for direct currents,

FIG. 8 schematically a further switch arrangement for obtaining current curves according to FIG. 3,

9 shows a digital embodiment of a control unit for the chopper switches,

10 shows a possible, temporal pulse sequence as a control unit for the chopper switch usable oscillator.

1 shows the block diagram of a conventional measured value transmitter for direct current or direct voltage. The measured value transmitter is connected to either a voltage or a current source 5 via two input lines 1 and 3, idealized. The first input line 1 is fed to a center tap 7 of a symmetrical transformer primary winding 9, with winding halves 9a and 9b, the latter in turn via chopper switch S, respectively. S connected to the return line 3 a D

are. The chopper switches S and S. are part of one

away

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Chopping unit 11, which also has a control unit 13 for controlling the chopper switch includes. The chopper unit 11 can be externally fed via a feed line 15, or, as indicated by the dashed line 17, be self-powered. The input lines 1 and 3, a current I proportional to the measured value to be transmitted is fed. The two chopper switches S and S,

a ο

are controlled complementarily by the control unit 13, so that uninterrupted the current I alternately through one of the Mmärwicklungshälf th 9a or 9b to the return line 3 flows. As a result, an alternating voltage is induced in a transformer secondary winding 19, which a rectifier unit 21 and then smoothed in a low pass 23, one of the load corresponding output direct current I.

Without first going into the usability of this measured value transmitter for current or voltage measured values, The basic course of the current and field variables is to be illuminated with reference to FIG. 2.

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As a complementary switch, astable multivibrators are conventionally available Switching transistors switch with more or less steep switching edges, with a switch either is closed or open, the other complementary to it.

Let the two chopper switches S and S, from Fig.

away

For example, switching transistors, with complementary on / off control, alternating currents I and I _? flow. In this case, the greatly increased switching times in FIG. 2 are intended to illustrate the transistor switching times which, with the inductive load from the transformer windings, are extended beyond the intrinsic switching times of the transistors.

As can be seen from FIG. 2, the chopper switch S is already more or less, for example

switched on before the chopper switch S is switched off and vice versa. By the One switch becomes conductive and blocked

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of the other switch in the same switching period T, a rapidly changing field H is created in the transformer. As is known, this causes a relatively large induced voltage, depending on the steepness. This is depends of course on the load and leads to a relatively large magnetizing current in the Transformer, which in turn causes measurement errors. Under this aspect it is to be striven for to reduce induced voltage as such as possible because it, especially when using the transmitter for voltage transmission, can only be reduced slightly by the load. In the The induced voltage can largely be used for the transmission of currents by low-resistance load can be reduced, although the aim should nevertheless be to keep them as small as possible from the outset.

In Fig. 3 it is shown how the induced voltage can be reduced, at the same time an additional Loss reduction is effected.

If one switch is only used for a period of time Z

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later switched on than the other switch was switched off, so there are current curves through the two halves 9a and 9b of the primary winding 9 corresponding to the currents I ₁ , and I, of Fig. 3 Switch is separated in time, there is a significantly reduced steepness of the field change in the transformer. This reduces the induced voltage, theoretically by a factor of two, compared to that resulting from the curves in FIG. 2, whereby the transmission errors of the measured value transmitter caused by the magnetizing current are greatly reduced.

Since neither the one nor the other of the switches S and S are switched on during the dead time periods C

from ^

is, the current I proportional to the measurement signal is also interrupted during this time C. During idealized in Fig. 2, a constant DC current I is supplied to the center tap 7, this current I ¹ falls in Fig. 3, during the dead times * »to zero. It can be seen from this that both the power supplied to the transformer and the power loss through

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the aforementioned measures will decrease. Overall considered, there is a significantly improved efficiency of the transformer, in addition to a significant one Reduction of transmission errors.

For example, FIG. 4 is a functional block diagram
a circuit which is suitable for realizing switching functions for obtaining the current curves shown in FIG. 3. If complementary output signals, corresponding to square-wave pulses, of an oscillator 25 z. B. a multivibrator each in integration units
27 and the so integrated signals each to a threshold-sensitive unit 9 as input signals and respectively. u_, which then emits an output signal when its input signal reaches one, preferably two threshold value units 29, the same predetermined threshold value S, and its output signal a resp. b used the

Chopper switches S and S, so the current

a D

runs through the corresponding transformer primary winding halves accept the desired course.

In Fig. 5, the two are integrated complementary
Output signals as pulse trains with ramp-shaped im-

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pulses u. and u shown. These two signals are fed to the threshold-sensitive units 29. Thus, as control signals for the corresponding chopper switch, the switching signals a and, respectively, shown idealized in FIG. b. A dead time U results between the transients of the chopper switches, as a result of which, as shown in FIG. 5, the transformer ^{input current I 1 is also} chopped up.

With the described current chopping with dead time, must it can be avoided that the measuring current chopping affects the source 5 to be measured (FIG. 1). This will, as shown in dashed lines in FIG. 1, achieved by a low-pass element 31. Is this low-pass element 31 already preferably to be provided with complementary operated chopper switches without dead time, so it is now when between the switching transients dead times ι- are provided, absolutely necessary.

FIG. 6 shows a first possible implementation of the block diagram of FIG. which correspond to those of Fig. 1, with the

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provided with the same reference numerals.

As a control unit 13, respectively. Oscillator 25 for the chopper switches S and S, is an astable multi

away

vibrator 30 provided, shown in the usual basic circuit. No further details should be given on how it works should be discussed, since this is a well-known circuit. The multivibrator 30 is connected between the return line 3 and the feed line, wherein, as is analogous to FIG. 1, too is indicated by dashed lines, a self-feed via the line 17 can also be provided, which, however can affect the accuracy of the transmission without further measures.

As is known, the collector currents I of the multivibrator transistors TR 1 and TR 2 are relatively good rectangular, complementary courses. In the case of conventional measured value transmitters, it was therefore possible to say Switching transistors of the multivibrator 30 can be used directly as a chopper switch. To obtain the In Fig. 3 shown current curves through the transformer primary winding 9, the fact can be exploited be that the collector-emitter voltage

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of the transistors TR 1 and TR 2, when these elements are turned off, are not changed abruptly, but that their rise time due to the charge reversal process of the capacitor C via the corresponding collector resistance R is given. If such a multivibrator is used in conventional circuits, one will strive to choose the capacitor resistor R as low as possible in order to reduce the voltage gradient to improve.

According to FIG. 4, however, is aimed to produce as with little additional switching elements, a pulse having ramp-like rising edges, according to u, or u For this purpose, the collector resistance R substantially higher impedance is chosen so that the collector-emitter voltage of each blocking switching transistor slowly increases. The resulting collector-emitter voltages of the multi vibratortransistors TR 1 and TR 2 are used to ver, the chopper switch S and TR 4 realized for example with transistors TR 3 and TR 4, respectively.

S. to control. For this purpose the collector b

Both transistors TR 1 and TR 2 are connected to the base of one of the chopper transistors TR 3 and TR 4.

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Said collectors are further with the corresponding end taps of the transformer primary winding 9, the Each emitter is connected to the return line 3 via an emitter resistor R. Due to the current-voltage negative feedback the chopper transistors TR 3, respectively. TR 4 across the emitter resistors R_ will cause their

Base-emitter control voltage, via the forward voltage u_

must rise before they are switched on. This negative feedback thus practically becomes the threshold-sensitive Unit 29 realized, the threshold value largely determined by the degree of negative feedback is.

If difficulties arise with the circuit shown in FIG. 6 with regard to the oscillation of the multivibrator, can be provided in its place a circuit in which both transistors operating points in the have active area and not in the saturation resp. be driven into the cut-off area. this can be implemented, for example, by an emitter-coupled, astable multivibrator.

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At this point it must be noted that the idea is the control voltage for the chopper switch S, S, on an integrating component a D

a conventional multivibrator to tap on a variety of known multivibrator circuits can be realized.

As mentioned above, the aim is to achieve that Chopper to operate self-fed.

As can now be seen from FIG. 6, this is possible without any particular problems when using conventional multivibrators if the measured value transmitter is used to transmit voltages . If the measured value transmitter is used to transmit direct currents, the load on the measured value transmitter changes the dividing ratio of the transformer and converter unit currents, so that load-dependent errors arise as a result.

In FIG. 7 a circuit is shown which represents a self-spun chopper unit on a measured value transmitter for direct current. Basically,

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The circuit preferably proposed in FIG. 7 differs from that in FIG. 6 in that the chopper unit is fed by power sources, which generate a load-independent current and thus ensure that the current to be measured is also load-independent will. In this figure, too, the position symbols used in the figure are for corresponding parts accepted.

Analogous to FIG. 1, a first feed line 1 for the direct current I to be transmitted is the center tap the symmetrical transformer primary winding 9 is supplied. This current I is passed through the two chopper switches S and S. on the return line 3

el D

switches. The low-pass element 31 at the input of the measured value transmitter is designed as a 77 * element with a series resistance R ₁ and two shunt capacitors C- and C_. The secondary winding 19 of the transformer is connected to the rectifier unit 21 with two diodes G. and G_, which is followed by a low-pass filter 23, again designed as an I) element, with a series resistor R and two shunt capacitors C ₃ and C ₄ .

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The chopper control unit 13 consists of an astable multivibrator with two switching transistors T. and T. Their emitters are connected to the return line 3. Since the two switching transistors T- and T_ are connected symmetrically, only the connection of T is described below. Its base-emitter path is through a capacitor C ₁ . bridged. The base is further through a capacitor

C to the collector of the second switching transistor ο

T connected. In analogy to the collector power supply via the collector resistor R of FIG. 6, the transistors T ₁ and T_, the collector currents via current sources 41, respectively. 42, which take their highly constant current from the supply line 1. The function of the charge reversal resistors R of FIG. 6 approximately corresponding to the bases of the two transistors T. T_ charging currents from two 'further power sources 43, respectively. 44 supplied.

Without going into the design of these power sources for the time being, the following is a brief description of the mode of operation this astable multivibrator will be explained. It is assumed that the device is switched on that means that all charge storage systems should be used for the time being

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be discharged.

Since the base-emitter voltages of the two switching transistors T. and T are approximately zero, both block.

The highly constant currents I., ^and I. ", the collector

41 42

Current sources 41 and 42 charge the capacitor pairs of C _c and C _{r connected} in series. Because of the specimen scatter of the components used for the current sources, the two currents I .. and I. «are not exact

41 42

the same, as are the two capacitor series circuits. Therefore, the base capacitor becomes C ₁ . one of the two switching transistors will first reach the required forward voltage u. The corresponding transistor, for example T ₁ , is switched on. The collector-emitter potential of the transistor switched through is practically zero and the capacitor C- connected to said collector on one side with the base capacitor

C_, which is connected to the base of the non-conductive transistor T- is connected in parallel.

Since the capacitors C, are dimensioned smaller

than the capacitors C-, during their series-

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circuit built up a greater voltage across the latter than across the former. If one of the two pairs C ₁ -FC is now connected in parallel, for example, a charge reversal process takes place, as a result of which the voltage at the base-emitter capacitor C ₁ . decreased. _{This ensures that the transistor T 2} , which is not switched on, remains blocking. The current I ₄₂ ' ^{of the} current source connected to the collector of the non-conductive transistor T now charges the capacitor C, connected on one side to the base of the conductive transistor T. on. This increases the potential at the collector of the non-

conductive transistor T ramp-shaped. The base-emitter path of the transistor switched on acts as a voltage source. The current from sources 42 and 43 flows across the base-emitter resistance of this transistor. Depending on the size of the currents fed to the base this transistor may be driven into saturation.

In contrast to the example in FIG. 6, the primary capacitor in the corresponding base capacitor C ₁ . stored charge is used to build the collector base capacitor

C- reloading and not an externally supplied current, b

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When the reloading process is complete, the parallel connection is established the two capacitors are further charged by the charging current of the current source 44, for as long as until the base-emitter voltage of the still blocking transistor T reaches the forward voltage u. Begins to conduct the second transistor T_, an analog charge reversal is immediately applied to the second capacitor pair

C _c , C _c initiated, whereby the previously conductive transistor jb

C ^ is quickly blocked. By making the transistor conductive T, the collector-emitter potential that had previously risen in the form of a ramp falls returns to zero and remains at zero until T again switched to the blocking state in a next cycle will. The collector-emitter voltages thus have the desired complementary sawtooth curve, which is now used to control the chopper switch

S resp. S is used.
a D

The chopper switches S and S are through in FIG

a ο

two Darlington circuits D. and D_ realized. This means that the collector-emitter voltage of the multivibrator transistors T., T at a minimum reach twice the value of the forward voltage u until the corresponding Chopper switch becomes conductive. If this value is sufficient as

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Switching threshold value and the resulting dead time L do not, the input transistor of the Darlington circuit is fed back with a correspondingly dimensioned emitter resistor R.

In order not only to provide a load-independent feed current for the chopper unit, but also to keep this current itself as small as possible, FET current sources are used as current sources. For this purpose, two FETs 45 and 46, shown in the exemplary embodiment as self-conducting n-channel _{FETs, each provided with a negative feedback resistor R r} , are connected to the supply line 1.

As is known, the source current is equal to the quotient of the source-gate voltage and the negative feedback resistance R "". The one caused by the negative feedback resistance flowing current is called collector current I .. resp. I._

41 42

fed to the multivibrator. To form the power sources 4 3, respectively. 4 4 becomes the Drane Source Current of the corresponding FETs by means of a current divider from the negative feedback resistor R "_ and a resistor R_, divided.

SG O

The current of the sources 43 and 44 is preferably selected to be smaller than the collector currents I .. and 42.

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By using FETs as constant current sources, it is achieved that extremely small currents are fed to the multivibrator, which falsify the measurement signal only very little. This also ensures that the transistors T ₁ and T _{7 are} not driven into saturation, which improves their oscillation behavior and the cutoff frequency.

In Fig. 8, a further switch arrangement of the chopper unit is shown schematically. There are the end taps the primary winding halves 9a and 9b each have a switch connection A, C of a three-stage switch S, fed. The switch is moved from one of the switch connections A or C to the other via a center connection unswitched. In this switch position the circuits are interrupted and the time in which the switch remains in this position corresponds to the dead time Γ. How a three-pole works Changeover switch can easily be implemented with electronic switches or electromechanical switches.

In Fig. 9, a circuit possibility of a digital control unit is shown. As a chopper switch you can here z. B. TTL compatible analog switches can be used.

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An oscillator 50 emits a pulse train e which is the clock input of a flip-flop 52, for example a JK flip-flop. The flip-flop 52 is connected as a frequency divider and emits a pulse train of half the clock frequency at its Q output. If the clock signal and the flip-flop output signal are on the one hand in a first AND gate 54 to form a Control signal for the first chopper switch S linked, on the other hand, in a second AND gate 56, the Clock signal e with the inverted flip-flop output signal to form the control signal for the second chopper switch, the switches are each switched on during a clock pulse width and blocking for three pulse widths, both switches being actuated out of phase by two pulse widths will. This also creates the required dead times.

The sequence of control signals shown in FIG. 10 basically corresponds to that with the circuit of FIG. generated.

Obviously, the pulse trains must be designed in such a way that that the chopper switches are conductive for less than half the pulse repetition period. The pulse trains

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must then, for equally long dead times, mutually by half the sum of the pulse width and period duration be out of phase.

It can be seen from these embodiment variants listed that the person skilled in the art has a multitude of other possibilities offer to control the chopper switch with dead time intervals. Depending on the measured values, both electronic and electromechanical ^ switches as chopper switches.

By separating the switch-off and switch-on transients of the chopper switches in measured value transmitters with transformer means for potential separation, it is achieved that significantly smaller losses occur. It is advantageous for the transmission of direct current signals to feed the chopper unit via current sources, unless one chooses the much more complex way of external feed. If FET current sources are used as current sources, then the residual current, which is load-independent anyway, is very small and can be corrected without any problems by a slight zero point shift at the transformer output.

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Claims (1)

1256 / ot / wi
May 6, 1977 272137Ί Patent claims : 1. Procedure for reducing losses and improving the Transmission behavior on measured value transmitters for electrically isolated measured value transmission of static electrical Signals, with transformer transmission means and with chopping means which are connected in such a way that a field flux changes in the switching rhythm of the chopping means Polarity arises in the transmission means, characterized in that the chopper means the induction flux interrupts causing current through the transmission means during a predetermined period of time, respectively before switching the polarity of the induction flux. 2. Apparatus for performing the method according to claim 1, wherein the transmission means has at least one transformer include, which is a primary winding provided with an intermediate tap having, and wherein the chopper means for closing, respectively. Opening of the circuits from the intermediate tap are formed over each part of the primary winding, characterized in that the chopping means Control means are assigned which emit at least one control signal for controlling the chopping means and in such a way that the latter alternately close and open the corresponding circuits, so that in each case only one is closed, and that in each case before the closing of one of the circuits, both circuits during predetermined Time are interrupted. 3. Apparatus according to claim 2, characterized in that the chopper means comprise at least two electronic and / or electromechanical switches (S, S.), and that the control means (13) to at least one pulse generator (oO) 809836/0497 ORIGINAL INSPECTED 1256 / ot / wi 6th May 272137] grasp as well as selection means (54, 5b), to each second impulse one of the switches and the adjacent ones. to the other switch. 4. Apparatus according to claim 2, characterized in that the chopping means at least two electronic and / or Electromechanical switches (S, S,) comprise, and the control means, oscillator means (25, 30) for outputting one each Comprise control signals for the switches, the control signals being two pulse trains. 5. Apparatus according to claim l, characterized in that the Zer
grasp. the chopper means a three-stage switch (S.) ao 6. Apparatus according to claim 4, characterized in that the pulse trains (u, u,) pulse widths smaller than the sa so have half a pulse repetition period, and the two pulse trains at least nearly half a pulse repetition period are also shifted by half a pulse width. 7. Apparatus according to claim 4, characterized in that the pulse trains (U ₁ , u) have pulses with at least approximately continuous switching edges, and that the oscillator means (25, 30) threshold-sensitive means (2'J, Ti * 3, TR 4, D1, D2) are connected downstream. 8. Apparatus according to claim 7, characterized in that the oscillator means at least one astable multivibrator include, and that further integration means (R, C; C ,; 27) are provided for complementary output signals ο of the multivibrator. 809836/0497 1256 / ot / wi _ 6th May 19 77 3 - yg - 9. The device according to claim d, characterized in that;} the integration means integrating Scnaltraittel (R, C; C _f ) of the multivibrator circuit itself. 10. Apparatus according to claim 7, characterized in that the switches are semiconductor switches, and that these are fed back in such a way that they themselves are threshold-sensitive Means with a predetermined threshold are used. 11. The device according to claim 7, characterized in that the switches Darlington circuits of semiconductor switching elements include. 12. The device according to dependent claim 9, characterized in that the astable multivibrator _{comprises two switching transistors (T 1} , Ί.Λ, whose base-emitter path are each bridged with first capacitive means (C-), wherein the base of a transistor with the Collector of the other with second capacitive means (C ₆ ) is connected, and vice versa. 13. Apparatus for carrying out the method according to claim on measured value transmitters for direct current, wherein the chopper and Control means are self-powered by the signal to be transmitted, characterized in that current sources for the supply (41, 42, 43, 44) are provided. 14. Apparatus according to claim 12 or 13, characterized in that the current sources are FET current sources .