Device for transmission of a measured value between sensors and a monitor unit

0001The invention relates to a device for transmitting measured values ​​between sensors and a monitoring unit with a first operational amplifier, the output current of which is conducted to ground through the diodes of a first and a second optocoupler and the inverting input of which is connected to the emitter of the transistor of the second optocoupler, the collector of which is connected to potential , on a first grounded resistor and on a first capacitor leading to its output, is arranged.

0002A similar device is known from DE-A-4 024 002. The sensors, which are located at a great distance from a monitoring system, can be connected to a bus system. The sensors can be connected to this bus system. Each sensor generates a current that contains a DC component in the transmission current. Because of the large distance of the sensors, the direct current component is no longer sufficient to supply the electronics, so that the function of the devices and units to be monitored by the monitoring unit is endangered and therefore require their own power supply.

0003From the above-mentioned DE-A-4 024 002 an LF transformer with galvanic isolation is known. A frequency signal with a frequency of 300 - 3000 kHz is amplified by an operational amplifier and drives two photodiodes in series, which emit a signal corresponding to the LF signal, which is received by two phototransistors and converted into an electrical signal. The output signal is present at the first photo transistor. The electrical signal of the second transistor is fed back to the inverting input of the operational amplifier and has an amplifying effect on the characteristic curve of the LF transmitter.

0004It is disadvantageous that the transmission of the measured values ​​recorded by distant sensors to a transmission device is not possible with the AF transmitter. The signal is decoupled by the photodiodes and transistors. However, they are used to implement a controllable current sink. Dar Amplkar only has the task of linearizing the characteristic curve of the LF transmitter through its negative feedback effect.

0005Furthermore, a circuit arrangement for a potentially decoupled speed controller is known from DE-B-3 720 996. A comparison value of a setpoint speed value and an actual speed value is given to the inverted input of an operational amplifier. Two luminous diodes connected in series are arranged in the output circuit of the operational amplifier. One optocoupler is used for signal transmission, the other acts with its phototransistor, which has a load resistor in the collector string, via a return path into which an RC element is inserted, on the inverted input of the amplifier.

0006This solution is also not suitable for the transmission of measured values ​​from sensors located far away to a monitoring device, even if a potential separation is carried out. The negative feedback effect of the amplifier only serves to linearize the signal. The returned control signal is used for processing that determines the control characteristic.

0007DE-B-4 016 400 specifies a device for monitoring sensor points with a wireless transmission of data, in which the monitoring between transmitters and receivers is carried out by infrared light. The device is used as a safety device. It is suitable for transferring data between moving parts that are a few meters apart.

0008DE-A-3-445 521 discloses an interface module for the symmetrical transmission of data stations in a data network with separate power supplies for the individual bus stations. Here the signals are transmitted from an interface module via a two-wire bus system. An OR circuit, which is acted upon by a signal derived from the differential receiver of the interface module and delayed by an RC element and by a signal generated by an optocoupler connected in parallel with the input of the differential amplifier, is used as a means for bridging the bus station. This interface is only suitable for the transmission of digital signals.

0009It is also known to transmit a measured voltage in a potential-free, electrically insulated manner to a receiver part (Tietze, U., Schenk, Ch .: semiconductor circuit technology, seventh, revised edition, Springer-Verl., 1985, pp. 781-783). A DC voltage is transmitted directly with optocouplers. In order to compensate for the linearity error of the optocouplers, the current is controlled by light emitting diodes with the aid of operational amplifiers in such a way that a photocurrent is equal to a desired value. A negative feedback loop is then closed via a reference coupler. Since the photocurrent cannot change its sign, a constant portion is superimposed in order to be able to process bipolar input signals.

0010The advantages of an optical coupling compared to an inductive one, for example by means of a transformer, are only highlighted here in a very general way, so that the potential-free coupling described in this way cannot simply be adopted for a transmission of measured values ​​between sensors and a monitoring unit with a bus system, it is not intended to continue a risk to the function of the devices and units to be monitored by the monitoring unit.

0011The object is to develop a device for potential-free transmission of the type mentioned at the outset in such a way that simple and precise transmission of the measured values ​​from sensors located far away to a monitoring unit is possible.

0012According to the invention, the object is achieved by<ul id="ul0001" list-style="none" compact="compact"><li>that the non-inverting input of the first amplifier is led to a control line via a second resistor and a low-pass filter and to a control line via a third resistor,</li><li>that the emitter of the transistor of the first optocoupler is connected to the series-connected diodes of a second and a third optocoupler, the emitters of which are connected to ground and the collectors of which are connected to a SYNC line,</li><li>that a bus system is connected to the collector of the transistor of the first optocoupler and to the output of the diode of the fourth optocoupler via a power supply unit and a synchronization voltage is applied to it,</li><li>that a microbridge air flow sensor, the SYNC line, the signal line and the control line are arranged on a microcomputer unit and with the microcomputer unit<ul id="ul0002" list-style="none" compact="compact"><li>a) the microbridge air flow sensor is queried and a measured value with a pulse-width-modulated voltage is applied to the signal line in the time window in accordance with its assigned address, and</li><li>b) the presence of the synchronization voltage is detected on the basis of a counting current generated with the aid of the second and the third resistor via the SYNC line, so that a transmission current can be taken from the bus system and can be transmitted to the monitoring device as the AC component of the measured value representing the measured variable.</li></ul></li></ul>

0013The bus system is a 2-wire bus system via which the microbridge airflow sensors transmit their measured values ​​to the monitoring unit in time-division multiplexing.

0014In a further embodiment of the invention, the four optocouplers are located in an integrated circuit.

0015It is advantageous if a fourth resistor is arranged between the output of the first operational amplifier and the first input of the first diode of the first optocoupler.

0016It is advantageous if a second capacitor connected to ground and a fifth resistor connected to potential are arranged on the SYNC line.

0017It is advantageous if the low-pass filter consists of a second operational amplifier, a sixth and a seventh resistor and a third and fourth capacitor, wherein<ul id="ul0003" list-style="dash" compact="compact"><li>the sixth and seventh resistors in the PWM line are at the non-inverting input,</li><li>the third capacitor between these two resistors and the inverting input and</li><li>the fourth capacitor is at the non-inverting input of the second operational amplifier,</li></ul> wherein the inverting input leads to the output of the second operational amplifier, which is connected to the fourth resistor lying in front of the inverting input of the first operational amplifier.

0018The advantages resulting from the invention are described in connection with an exemplary embodiment which is shown schematically in the drawing.

0019A 2-wire bus system BUS is advantageously used for microbridge airflow sensors, in the following sensors 12, which are located at a great distance from a monitoring unit ÜE.

0020As an interface arrangement shown in the drawing shows, the 2-wire bus system is connected both via a PTC resistor R44 and also directly to the points of a rectifier bridge circuit V76. A Zener diode V77 and the series connection of a resistor R43 and a Zener diode V75, to which a capacitor C64 is connected in parallel, are arranged parallel to the other two bridge branches. Before that, three transistors V94, V95 and V96 are connected, the collectors of which are connected to the resistor R43. A rectifier V78 is arranged between the emitter and the base of the transistor V96. In addition, the emitter of transistor V96 is connected to the base of transistor V95 and the emitter of transistor V95 is connected to the base of transistor V94. The emitter of transistor V94 is at input 16 'of optocoupler U117. This optocoupler is connected at its connection 1 'via a resistor R41 to an output 7' 'of an operational amplifier N116A. A further operational amplifier N116B is arranged at the non-inverting input 5 ″ via a resistor R39 via an output 1 ″. A resistor R36 and a resistor R37 are connected to the non-inverting input 3 ″ in a PWM line PWM. A capacitor C59 is connected to the inverting input 2 ″ of the operational amplifier N116B between the two resistors R36 and R37. This input 2 ″ is also connected to the output 1 ″ of the operational amplifier N116B. In addition, a resistor R38 is connected in a control current line R-ON between the resistor R39 and the non-inverting input 5 ″ of the operational amplifier N116A. The inverting input 6 ″ of the operational amplifier N116A leads to a resistor R40 which is at zero potential, via a capacitor C61 to the output 7 ″ and to a connection 13 ′ of the optocoupler U117A, the connection 14 ′ of which is connected to a potential + VCC . Its connection 3 'is connected to the connection 2' of the optocoupler U117, while the connection 4 'is connected to the ground forming a zero potential. A connection to connection 5 of optocoupler U117 leads to connection 15 of optocoupler U117, whose connection 6 'leads to connection 7' ​​of an optocoupler U117C. The connection 8 'of the optocoupler U117 leads to the Zener diode V75, the capacitor V64, the Zener diode V77 and the rectifier bridge circuit V76. While the outputs 11 and 9 of the optocouplers U117B and U177C are connected to the ground carrying zero potential, the connections 10 'and 12' are connected to ground via a capacitor C62 as well as via a resistor R42 and a SYNC line SYNC guided. The control line R-ON, the PWM line PWM and the SYNC line SYNC are connected to a microprocessor device MP. The sensor 12 is connected to this microprocessor device MP, which is also designated 41. Each of the optocouplers U117, U117A, U117B and U117C consist of a transistor and a diode, which are known to be arranged and interact as a phototransistor and a photodiode.

0021Up to 127 sensors 12 can be connected to the two-wire bus system BUS, which transmit their measurement results in a time-division multiplex process. The measured value is transmitted using an alternating current, the frequency of which represents the measured variable. The alternating current is taken from the two-wire bus system. The frequencies are transmitted using time division multiplexing. For this purpose, each of the sensors 12 is assigned a different address, which defines the time of transmission (time window). The length of a time window is determined with a certain length of time, for example two seconds. After that, the beginning of the time window address x time period. The synchronization of all sensors is triggered by applying a voltage UATX.

0022Each of the sensors 12 causes a current in the two-wire bus system BUS. A distinction is made here:<ul id="ul0004" list-style="none" compact="compact"><li>a) counting current, that is the current caused by each sensor 12 when it is not transmitting. It adds up according to the number of sensors and causes a voltage drop on the two-wire bus system BUS and limits the range, among other things. It should be as small as possible, for example between 50 and 200 µA per interface.</li><li>b) transmission current, it only occurs during the time window described above and is composed of a direct current component and an alternating current component. The DC component can only be used to power the sensor in certain cases. The AC component, on the other hand, represents the measured variable. The transmission current is taken from the two-wire bus system and always occurs only once in the time window.</li></ul>

0023In the case of sensors 12 in which the direct current component of the transmission current is not sufficient to supply the electronics, an additional voltage supply must be provided. The voltage supplies for the flow measuring devices with a microbridge airflow sensor can all be at different potentials and may therefore only be connected to the 2-wire bus via a potential separation. For this purpose, the operational amplifier N116A forms a voltage-controlled current source, the output current of which is passed through the diodes of the optocouplers U117 and U117A. The voltage at connection 6 of the optocoupler U117B follows the voltage at connection 5. The current through the resistor R40 is thus<maths id="math0001" num=""><img file="EP0973016A2_D0001.tif" /></maths>

0024The current through resistor R41 is not linear because the optocoupler U117A is a non-linear current transformer. The non-linear current through R41 is transmitted through the optocoupler U117 and appears linear at the output if it has the same non-linearity as U117A. This is sufficient because all four optocouplers U117, U117A, U117B and U117C are in an integrated circuit and the voltage at the collector of U117 through V75, V76, V77, V78, V94, V95, R43, R44, C64 is so great is selected as the voltage at the collector of the U117A optocoupler. With the help of this circuit part of the interface arrangement described in this way, currents can thus be transmitted. This part thus represents a voltage-controlled, potential-free current sink.

0025The low-pass filter, which is formed by the operational amplifier N116B, the resistors R36 and R37 and the capacitors C59 and C 60, converts a pulse width modulated voltage into a sinusoidal voltage with a frequency between 1000 and 2000 Hz, which is passed through the resistor R39 to the current source formed by the operational amplifier N116A is passed. With the help of the resistors R38 and R 39, a DC voltage is generated at the connection 5 ″ of the operational amplifier N116A, the size of which defines the counting current. The PWM line PWM then has the potential + VCC and the quiescent current line R-ON the potential GND.

0026The U117B and U117C optocouplers detect whether the counting current is flowing, i.e. UATX is switched on. Their diodes are connected in series, while their collectors are connected in parallel. This results in greater sensitivity. Since all optocouplers U117, U117A, U117B and U117C belong to an integrated circuit, good error compensation also results with regard to temperature errors.

0027The bridge rectifier V76, the Zener diode V77, the capacitor C64, the Zener diode V75 and the diode V78, the resistor R43 and the transistors V94, V95 and V96 form a power supply unit. The PTC resistor R44 serves as short-circuit protection, the bridge rectifier V76 as reverse polarity protection, the Zener diode V77 in cooperation with R44 and the bridge rectifier V76 as overvoltage protection. The voltage reference is formed from the Zener diode V75, the resistor R43 and the capacitor C64. The transistors V94 to V96 are connected as emitter followers.

0028The microprocessor unit MP (41) can therefore transmit measurement results potential-free from the sensors to the two-wire bus system. This is only possible because the interface carries out electrical isolation with the following properties:<ul id="ul0005" list-style="none" compact="compact"><li>1. Detection of the connection of the synchronization voltage UATX with the smallest possible counter current, and</li><li>2nd Transmission of the AC component of the transmission current with constant amplitude and low harmonic distortion.</li><li>3rd In contrast to a transformer coupling, it is not only possible to transmit the alternating currents from the sensor to the monitoring unit, but also to transmit the synchronization DC voltage in the opposite direction.</li></ul>

0029The optocouplers also have the following advantages:<ul id="ul0006" list-style="dash" compact="compact"><li>The most accurate transmission possibility of the smallest currents (approx. 5.0 mA), since strong non-linearities of the optocouplers, eg due to temperature errors of the optocoupler semiconductors in the overall system, are compensated for by the optocoupler U117A and</li><li>bidirectional galvanic isolation from the two-wire bus system.</li></ul>

0030With this potential isolating sensor interface it is possible to transmit measured values ​​to a monitoring unit up to 100 km away. With the help of the synchronization voltage applied by the monitoring unit (UATX = 20 ... 120V), it is able to flow a sinusoidal modulation current (typically: iss = 3.64mA, f = 1000 ... 2000 Hz) representing the measured value to let. It is also able to set a quiescent current (also called a counting current) (typically: I = 0.2 mA, DC), with the aid of which the synchronization voltage is switched on and the start of time multiplexing is recognized.

0031Only by combining several measures is it possible to precisely set or detect such a small current with optocouplers.

0032The measures include:<ul id="ul0007" list-style="none" compact="compact"><li>a) Linearization of the optocoupler characteristics and minimization of the temperature errors of the optocouplers U117 to U117C. (This measure is important not only because of the sinusoidal modulation current, but also for the correct setting and detection of the quiescent current).</li><li>b) Increase in sensitivity by connecting the optocoupler diodes U117B and U117C in series and connecting the corresponding phototransistors in parallel.</li><li>c) Stabilization of the synchronization voltage with a Metz part V 94, ... R44 from 20 ... 120V to the same voltage level as Vcc (approx. 10V), so that the same voltage ratios between collector and emitter result for the phototransistors on both sides.</li></ul>