EP3268700A1

EP3268700A1 - Low cost transceiver circuit for flow meter

Info

Publication number: EP3268700A1
Application number: EP15709175.2A
Authority: EP
Inventors: Lars Jespersen
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 2015-03-11
Filing date: 2015-03-11
Publication date: 2018-01-17
Also published as: CN107407584A; WO2016141982A1; EA033712B1; EA201791854A1; CN107407584B

Abstract

The present invention relates to a transceiver circuit for a flow meter comprising a common signal path for signals to be transmitted and/or received via one or more associated transducers, the transceiver circuit comprising a generator circuit, a signal processing circuit and an active circuit, wherein the active circuit comprises a first and a second transistor being operatively connected via their respective emitter terminals thereby forming a combined input/output terminal, said combined input/output terminal being operatively connectable to one or more associated transducers, the active circuit being adapted to act as a buffer for signals to be transmitted, and adapted to act as an amplifier for received signals.

Description

LOW COST TRANSCEIVER CIRCUIT FOR FLOW METER FIELD OF THE INVENTION

The present invention relates to transceiver circuits for flow meters. In particular, the present invention relates to low cost and power saving transceiver circuits for ultrasonic flow meters. BACKGROUND OF THE INVENTION

Various types of transceiver circuits suitable for in particular ultrasonic flow meters have been suggested over the years.

In general the performance of ultrasonic flow meters, especially at low flow rates, is directly dependent on delay differences in the signal paths (upstream vs. downstream) of the front end circuit connected to the ultrasonic transducers. Very small delay differences may cause measurement errors in the order of some percent's at low flow rates. Furthermore, delay differences may be induced by differences in the ultrasonic transducers. These errors can be removed during calibration, but their variation over time and temperature will still pose a problem. Moreover, calibration at low flow rates is time consuming and hence costly. Other circuit topologies that solve the above-mentioned delay issues exist, but these circuit topologies often require expensive amplifiers.

One very simple approach has been suggested in EP 1 426 739 where a single transistor acts as a buffer and an amplifier when ultrasonic signals are to be transmitted and received, respectively. It is a drawback of the circuit suggested in EP 1 426 739 that the transistor needs to be operated in class A in order to preserve linearity for both transmitted and received signals. It is well-known that operating a transistor in a class A mode of operation requires current levels that are orders of magnitudes higher compared to what is required by other types of bipolar topologies.

It may be seen as an object of embodiments of the present invention to provide a power saving transceiver circuit for flow meters, such as ultrasonic flow meters.

It may be seen as a further object of embodiments of the present invention to provide a simple and low cost transceiver circuit for flow meters, such as ultrasonic flow meters. DESCRIPTION OF THE INVENTION

The above-mentioned objects are complied with by providing, in a first aspect, a transceiver circuit for a flow meter comprising a common signal path for signals to be transmitted and/or received via one or more associated transducers, the transceiver circuit comprising a generator circuit, a signal processing circuit and an active circuit, wherein

- the generator circuit is operatively connected to an input terminal of the active circuit,

- the signal processing circuit is operatively connected to an output terminal of the active circuit,

- the active circuit comprises a first and a second transistor being operatively connected via their respective emitter terminals thereby forming a combined input/output terminal, said combined input/output terminal being operatively connectable to one or more associated transducers, the active circuit being adapted to act as a buffer for signals to be transmitted, and adapted to act as an amplifier for received signals, and

- the input terminal of the active circuit being operatively connected to base terminals of the first and second transistors, and the output terminal of the active circuit being operatively connected to collector terminals of the first and second transistors.

The transceiver circuit may be operated as a front end transceiver circuit. The term front end is here to be understood as a circuit being operatively connected to, either directly or indirectly, the associated transducers. The present invention finds its use in relation to flow meters adapted to measure flow speeds of liquids and/or gasses.

Preferably, the emitters of the first and second transistors are directly connected. The term emitters should be understood as emitters in relation to bipolar transistors as well as sources in relation to Field Effect Transistors (FETs). Similarly, the terms bases and collectors in relation to bipolar transistors correspond to gates and drains of FETs, respectively. The active circuit is adapted to act as a buffer for signals to be transmitted, and act as an amplifier for received signals. The aim of the buffer is to facilitate that more current may be delivered to the transducer. The buffer may be a unity gain buffer having a voltage gain of essentially one. It should be noted however, that the voltage gain of the buffer may differ from one. The active circuit forming the buffer and the amplifier may ensure that each of the associated transducers experiences an essentially constant impedance while transmitting and receiving signals.

In the present content the term transducer is to be understood as either a transmitting or receiving transducer. As an example piezo-electric transducers being capable of both generating and detecting ultrasonic signals may be applicable in relation to the present invention.

Similarly, the term transceiver circuit is to be understood as a circuit that is capable of both transmitting and receiving signals via a number of associated transducers. The signals being transmitted and/or received by the associated transducers may be ultrasonic signals. Such ultrasonic signals are here to be understood as signals having frequencies from 100 kHz to 10 MHz, such as preferably around 1 MHz. It should however be noted that other frequency ranges may be applicable as well.

The proposed transceiver circuit is advantageous in that it is simple, cheap, and it has a common signal path for transmitted and received signals. Hence, delay differences in the upstream and downstream signal paths at zero flow are essentially avoided.

Moreover, the first and second transistors may be configured in a low-power consumption topology where the first and second transistors are operated in a class AB mode by a circuit adapted to set a bias point for the first and second transistors. The transceiver circuit may further either comprise, or being connected to, a controllable switch/multiplexer for providing signals to and/or from associated receiving and/or transmitting transducers. Thus, the controllable switch/multiplexer may provide signals to and from a pair of transducers. Each of said pair of transducers may be operated as a transmitting transducer as well as a receiving transducer. The transceiver circuit may further comprise, or being connected to, additional controllable switches/multiplexers for providing signals to and/or from associated receiving and/or transmitting transducers. The additional switches/multiplexers may be controlled individually so that signals may be directed to and/or from a plurality of transducers in an independent manner. The switches/multiplexers may be implemented as a number of different switch/multiplexer types - either as single pole, single throw (SPST) switches or as a multiplexer. Each element of the switch or multiplexer may be a simple integrated or discrete MOSFET switch or a more elaborated T-type switch to enhance cross talk performance between transducers. It may also be combined with short circuit switches across the transducers to further enhance cross talk between transducers.

Preferably, the generator circuit has an output impedance, Zout, which is essentially constant. The generator circuit is adapted to generate periodic signals, such as sinusoidal signals, square wave signals etc. The signals from the generator circuit may be provided in bursts having durations of an appropriate number of periods. Thus, the burst signals may comprise an appropriate number of periods of sinusoidal signals, square wave signals or even a single-step function.

A negative signal feedback from an output terminal of the active circuit to the combined input/output terminal of the active circuit may be provided as well. The amount of negative signal feedback may be variable, such as variable on-the-fly. The term on-the-fly is here to be understood as variable at any time. As an example a first amount of negative feedback may be used during transmitting, whereas a second, and different, amount of negative feedback may be used during receiving.

In a second aspect the present invention relates to a flow meter comprising a transceiver circuit according to the first aspect of the present invention. In terms of implementation the transceiver circuit may be implemented and configured as disclosed in relation to the first aspect.

The flow meter may further comprise a plurality of transducers being connectable to the combined input/output terminal of the active circuit. At least a number of the plurality of transducers may be adapted to both transmit and receive signals. The plurality of

transducers facilitate that signals, such as ultrasonic signals, may be sent both upstream and downstream relative to the direction of a given flow.

An amplifier may be provided to amplify signals from the output terminal of the active circuit. The gain of this amplifier may be variable. This may be advantageous in relation to the following signal processing. In fact, the gain of the amplifier may be variable on-the-fly, i.e. it may be changed at any time, such as between transmitting and receiving. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further details with reference to the accompanying figures, wherein

Fig. 1 shows the principle of the present invention, Fig. 2 shows the principle of the invention including an embodiment of an output circuit, and

Fig. 3 shows the principle of the invention including embodiments of a bias circuit and an output circuit.

While the invention is susceptible to various modifications and alternative forms specific embodiments have been shown by way of examples in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION In its most general aspect the present invention relates to a low cost and low power consumption transceiver circuit topology that is capable of providing a stable zero flow offset of zero in flow meters, such as in ultrasonic flow meters. Ideally the transceiver circuit topology of the present invention is independent on influences from temperature and varying transducer impedances. To avoid delay differences in the transceiver circuit, the present invention suggests a circuit topology with a single common signal path for both upstream and downstream signals, i.e. for signals to either transmitted or received. Moreover, the transceiver circuit of the present invention offers that associated transducers can be operated in a reciprocal manner in order to avoid influences from different transceiver impedances. Referring now to Fig. 1 the components of the transceiver circuit 100 of the present invention are depicted. The main components of the transceiver circuit 100 are the two transistors 101 and 102 which are connected via their respective emitters at point 114. The emitters may be directly connected as shown in Fig. 1 or they may be connected via other components, such as via resistors. During transmitting the two transistors 101, 102 are driven by the signal generator circuit 104 through the bias circuit 103. The main advantages of the transceiver circuit topology shown in Fig. 1 are as follows:

1) Very simple and very low cost topology 2) Low current consumption due to the bipolar class AB topology

The transceiver circuit is operated as follows:

A low impedance transmit signal is generated at the emitters of transistors 101, 102 that during transmitting is operated as a bipolar class AB emitter follower. The bipolar class AB emitter follower drives one of the transducers 112, 113 through the transducer termination impedance 110 and the switch/multiplexer 111.

It should be noted that the number of transducers may differ from the two shown in Fig. 1. Thus, the number of transducers may be three, four, five or even more. Also, the number of switches/multiplexers may be more than one. In that case each of the switches/multiplexers may be controlled individually. During receiving, the signal generator circuit 104 is not transmitting, and the signal from one of the two transducers 112, 113 is provided to the emitters of transistors 101, 102 through the switch/multiplexer 111 and transducer termination impedance 110.

The transistors 101, 102 now work as a bipolar class AB common base amplifier through the output circuit 105. The output signal from the output circuit 105 may be further amplified in the signal processing circuit 107 before the final output signal 109 is provided. This further amplification may be a variable or a fixed amplification, and it may depend on whether signals are transmitted or received. The power supply signal is denoted 108.

It should be noted that the transceiver circuit shown in Fig. 1 is illustrated with bipolar transistors 101, 102, but also Field Effect Transistors (FETs) are applicable. In that case the sources of the FET's are operatively connected, either directly or via other components, such as via resistors.

As indicated by the dashed line in Fig. 1 the transceiver circuit could optionally include a negative feedback 106 from the output of the output circuit 105 to the emitters of transistors 101, 102. The negative feedback 106 would increase the overall linearity of the active circuit. The negative feedback may be implemented in various ways, such as by using an operational amplifier, transistors, transformers etc.

The transducers 112 and 113 are only shown schematically. In practice, the transducers 112, 113 may include various components, such as series and parallel impedances. The transducers 112, 113 may be capable of transmitting and/or receiving signals, such as ultrasonic signals. The switch/multiplexer 111 may be implemented as a number of different switch/multiplexer types - either as SPST switches or as a multiplexer.

Fig. 2 shows a possible implementation of the output circuit 105 in Fig. 1. As seen in Fig. 2 the transceiver circuit 200 still comprises a signal generator circuit 204, a bias circuit 203 and two transistors 201, 202 being operatively connected (directly or indirectly) via their emitters in point 214. The transducer termination impedance 210, the switch/multiplexer 211 and the transducer 212, 213 are similar to the components shown in Fig. 1. The transceiver circuit is powered by the power supply 208.

The shown implementation of the output circuit 105 of Fig. 1 includes two capacitors 205 and 206 is depicted. The output signal is provided from a node between these two capacitors. Resistors 215 and 216 are inserted in the power supply line and the connection to ground, respectively. The output signal from the node between the two capacitors 205, 206 may be further amplifier in amplifier 207 before the final output signal 209 is provided. Again this further amplification may be variable or fixed, and it may depend on whether signals are transmitted or received.

Fig. 3 shows a possible implementation of the bias circuit 203 in Fig. 2. The output circuit in Fig. 3 is similar to that of Fig. 2 in that it includes two capacitors 305 and 306, and the output signal is provided from a node between these two capacitors. Resistors 315 and 316 are inserted in the power supply line and the connection to ground, respectively. The output signal from the node between the two capacitors 305, 306 may be further amplifier in amplifier 307 before the final output signal 309 is provided. As previously stated this further amplification may be variable or fixed, and it may depend on whether signals are transmitted or received.

As seen in Fig. 3 the transceiver circuit 300 further includes a signal generator circuit 304, a bias circuit including two transistors 317, 318 operatively connected via their respective connectors. The signal generator circuit 304 provides signals to the bias circuit via the common point 303. Similar to Figs. 1 and 2 the two transistors 301, 302 are operatively connected (directly or indirectly) via their emitters in point 314. The transducer termination impedance 310, the switch/multiplexer 311 and the transducer 312, 313 are similar to the components shown in Figs. 1 and 2. The transceiver circuit is powered by the power supply 308.

In the bias circuit shown in Fig. 3 pairs of matched transistors 317, 302 and 318, 301 are included. The pairs of matched transistors eliminate the use of emitter resistances without introducing the risk of having thermal run-away in transistors 301, 302. In the present content thermal run-away is to be understood as an uncontrolled increase of current flow and power dissipation leading to a destructive result.

It should be noted that the bias circuit 103 and the output circuit 105 may be implemented in alternative ways which may deviate from the implementations depicted in Figs. 2 and 3.

Claims

1. A transceiver circuit for a flow meter comprising a common signal path for signals to be transmitted and/or received via one or more associated transducers, the transceiver circuit comprising a generator circuit, a signal processing circuit and an active circuit, wherein - the generator circuit is operatively connected to an input terminal of the active circuit,

2. A transceiver circuit according to claim 1, wherein the emitters of the first and second transistors are directly connected.

3. A transceiver circuit according to claim 1 or 2, wherein the first and second transistors are operated in a class AB mode by a circuit adapted to set a bias point for the first and second transistors.

4. A transceiver circuit according to any of claims 1-3, further comprising one or more controllable switches/multiplexers for providing signals to and/or from one or more associated receiving and/or transmitting transducers.

5. A transceiver circuit according to any of claims 1-4, wherein the generator circuit has an output impedance, Zout, which is essentially constant.

6. A transceiver circuit according to any of claims 1-5, wherein the generator circuit is adapted to generate periodic signals.

7. A transceiver circuit according to any of claims 1-6, further comprising a negative signal feedback from an output terminal of the active circuit to the combined input/output terminal of the active circuit.

8. A transceiver circuit according to claim 7, wherein the amount of negative signal feedback is variable.

9. A transceiver circuit according to claim 8, wherein the amount of negative signal feedback is variable on-the-fly.

10. A flow meter comprising a transceiver circuit according to any of claims 1-9.

11. A flow meter according to claim 10, further comprising a plurality of transducers being connectable to the combined input/output terminal of the active circuit.

12. A flow meter according to claim 11, wherein at least a number of the plurality of transducers are adapted to both transmit and receive signals.

13. A flow meter according to any of claims 10-12, further comprising an amplifier for amplifying signals from an output terminal of the active circuit.

14. A flow meter according to claim 13, wherein the gain of the amplifier is variable.

15. A flow meter according to claim 14, wherein the gain is variable on-the-fly.