GB2315651A

GB2315651A - Electrical signalling

Info

Publication number: GB2315651A
Application number: GB9704246A
Authority: GB
Inventors: Mihai Bucsa
Original assignee: Ampy Automotion Digilog Ltd
Current assignee: Landis and Gyr Ltd
Priority date: 1996-05-08
Filing date: 1997-02-28
Publication date: 1998-02-04
Anticipated expiration: 2017-02-28
Also published as: GB2315651B; GB9704246D0; NZ314741A; NO972088L; NO972088D0

Abstract

Apparatus for sending and receiving electrical signals representative of (for example) the amount of a metered commodity consumed comprises receiving means (30) connected to sending means (32) via a transformer (T). The receiving means (30) is operable to supply a time varying current to the primary windings of the transformer (T) and has detection means (20) responsive to changes in the effective impedance of the primary windings resulting from alterations in the impedance of the circuit in which the secondary windings of the transformer (T) are connected. The sending means has transmission means, such as a transistor switch, for altering the impedance of that circuit in response to the signal to be transmitted, and that variation causes a corresponding variation in the effective impedance of the primary windings, which variation is detected by the detection means. The transformer, in effect, isolates the receiving means and sending means, whilst enabling the receiving means to provide the voltage necessary to operate the sending means. A transistor switch connected in series with the primary winding enables signals to be sent from receiver to sender.

Description

Title: Electrical signalling Field of invention The invention concerns electrical signalling apparatus and methods.

Background to the invention In a commodity measuring meter, e.g. Water, Heat or Electrical Energy, a signal is generated, representative of a specific quantity of the commodity consumed, for use by a monitoring device. Such a signal may be a pulse. In this case, the number of pulses generated/received relates to the amount of commodity consumed e.g. 1000 pulses could mean 1 kWh of electrical energy consumed. Alternatively 1 pulse might signify the flow of 1 litre of water. Alternatively the signal may be in the form of a serial data stream.

The signal can take the form of a digital signal or may be what is known as "Voltage free". A device for sending voltage free signals to a receiving device may comprise switching mechanism, such as a set of relay contacts or an open collector semi-conductor.

In either case the switching mechanism has to be provided with a potential, supplied by the receiving device, that can be switched by the sending device, in order for the receiving device to detect the switching action of the sending device.

An advantage of a voltage free method arises where the sending device is battery powered: The receiving device can provide the signalling current which therefore does not drain the sending device's power source.

However, it can be seen that, conventionallv, there has to be a direct connection between the sending device and the receiving device to supply the switching potential to the sending device. Where the two devices are operated at different reference potentials, they need to be electrically isolated from each other. Furthermore, isolation must also be maintained where connection of the devices would cause one of them to become unsafe, e.g. when a receiving device's reference potential is at an unsafe level and that of the sending device must remain at 'safe' levels in the hands of a consumer.

A second example requiring isolation is where direct electrical inter-connection of the devices can induce electrical noise in the sender causing the sender/receiver to malfunction.

A normal method of removing the need for a direct connection is to use an opto-Isolator between the devices. This method still requires a voltage source to provide a potential to be switched by the sending device. Moreover this potential has to be isolated from the receiving device's reference potential, which leads to extra expense and/or complexity in the receiving device.

Summarv of the invention According to the present invention there is provided a means of obtaining the electrical isolation whilst obviating the need for power supply for generating signalling potential, in which an isolating transformer is employed wherein a change in the secondary load impedance thereof is reflected in its primary impedance. Thus when a square wave signal is applied through a resistor to the primary winding of a transformer, the signal measured across the primary winding of the transformer is a function of the impedance presented by the secondary winding of the transformer. For example if the secondary winding of the transformer is short circuited, the signal measured at the primary winding is practically zero, and when it is open circuit, the signal is much greater.

In one embodiment there is provided a low power, high frequency transformer, a square wave oscillator. and a voltage sensing circuit in the primary circuit of the transformer.

The insulation between the windings provides the isolation between the sender to the receiver. In the sending unit, a switchable device is provided which operates to change it's impedance from high to low in response to data to be transmitted. This device, which may be a transistor. is connected to the secondary winding.

Since a transformer is "bi-directional", it can be used to transmit data from primary to secondary, or vice versa, and it can therefore be used as an electrically isolated, two wire, bi-directional communication system.

According to a second aspect of the invention, there is provided apparatus for sending and receiving electrical signals, the apparatus comprising receiving means and sending means connected together via a transformer, the receiving means being operable to supply time varying (for example alternating) current to the primary windings of the transformer and having detection means responsive to changes in the effective impedance of the primary windings, the sending means having transmission means for altering the impedance of the circuit in which secondary windings are connected, in response to the signal to be transmitted, wherein said variation in the impedance of the circuit causes a corresponding variation in the impedance of the primary winding which is detected by the detection means.

Preferably, the transmission means comprises a switch connected in the circuit which includes the secondary windings. The switch may, for example, comprise a semi conductor device such as a transistor.

Preferably, when the switch is closed, the secondary windings are short circuited, whilst substantially no current flows in the secondary windings when the switch is open.

The invention also lies in apparatus comprising sending means and receiving means respectively for sending and receiving signals, wherein said signals are conveyed from the sending means to the receiving means via a transformer which isolates the sending means from the receiving means.

Brief description of the drawings Figures 1 and 2 illustrate prior art systems, Figure 3 is a practical example of a receiver embodying the invention.

Voltage wave forms across various parts of the circuit of Figure 3 are shown in Figure 4.

Figure 5 shows another embodiment of the invention constructed as a receiver where DC current is used in the cable, Figure 6 corresponds to Figure 4, and illustrates the associated wave forms for the circuit shown in Figure 5, Figure 7 is an example of a transmitter-receiver for two way communication, Figure 8 corresponds to Figures 4 and 6, and shows the associated wave forms for the circuit of Figure 7; and Figure 9 is a block diagram representation of a transmitter-receiver for two way communication, an example of which is the transmitter-receiver shown in Figure 7.

Detailed Description The apparatus shown in Figure 1 employs the voltage free method to transmit signals from a sending device 1 to a receiving device 2. In this apparatus, the sending and receiving devices are directly connected to each other so that the power supply from the sending device provides a voltage which can be switched by a transistor switch 4 in the receiving device 1.

The apparatus shown in Figure 2 also has a receiving device 6 directly connected to a sending device 8 to provide a voltage for switching by a switch 10, but also employs an opto isolator 1'.

The apparatus shown in Figure 3 has a receiving device 14 which includes a square wave generator 18. which can be any astable circuit capable of producing a frequency in the range of 10kHz to 2 MHz, depending on the required baud rate, transformer parameters and circuit constants. The amplitude of the square wave voltage could be any value that allows a detectable signal. The generator 18 is connected to the primary windings of a 1 : 1 transformer (although a step down or step up transformer could be used, depending on the voltage requirements of the sending device) and through a series resistor R1 that allows the coupling of the high frequency square wave (generated by generator 18) to the transformer T without unduly loading the transformer and detector circuitry 20 connected across the primary winding of the transformer T. This essentially forms a potential divider between the resistor and the transformer primary inductance. It is chosen to provide the maximum sensitivity with respect to the transformer parameters.

The secondary windings of the transformer T are connected in series to a bipolar or MOS transistor switch TR in a sending device 16. When TR is in its off State, the secondary windings are in an open circuit condition, in which no current flows through them. On the other hand, when TR is in its on state, the secondary windings are short circuited so that large currents can flow in the secondary windings. The base 22 of the transistor TR constitutes an input for a binary signal to be transmitted.

When the transmitted data at the base of the switching device TR is in the low state TR is in the off state (high impedance) and the transformer secondary is unloaded. Therefore the reflected impedance in the primary is zero and the impedance is that of the primary inductance of the transformer. The potential V1 across the primary is at a high value.

The circuitry 20 has a diode D which rectifies this potential to a positive DC potential, and a reservoir capacitor C for smoothing that potential. The circuitry 20 also includes a resistor R2 which forms a discharge path for C.

When the transmitted data at the base of TR is in the high state (positively biased) the switching device TR conducts and presents a practically short circuit to the secondary of the transformer. As a result of this the primary impedance of the transformer will be the leakage inductance of the transformer. Because of the divider action of R1, most of the voltage produced by the generator 18 will appear across R1 and the voltage (V1) across the transformer primary will be a very small amplitude. Therefore the DC potential across the reservoir capacitor C will be relatively small. Thus, the potential across C and R2 represents the state of the transmitted data and constitutes a received signal at output 23. In order to achieve high baud rate the potential across C will have to discharge at a greater rate than the required maximum baud rate. The value of R2 in conjunction with the value of C controls the discharge rate.

Figure 5 shows apparatus with many circuit components which are similar to those used in the apparatus shown in Figure 3 and which are therefore denoted by the same reference letters and numbers. The apparatus shown in Figure 5 applies a DC potential to the wires connecting the receiving device 24 to the sending device 26, and is therefore particularly suitable where the cable joining the receiver and sender is long or is of high capacitance.

The difference between the circuits of Figures 3 and 5 is that, in the latter, the current from the secondary of the transformer T is rectified and smoothed by D2, C2, and R3 which causes a DC current to flow in the cable 28 instead of an a.c. signal.

With reference to Figure 5, the action of circuit elements R1, T, D1, R2 and TR are the same as described in Figure 3 (C1 replaces C and D1 replaces D in the description).

In this case the DC is used along the cable instead of the high frequency signal. D2 rectifies the AC at the secondary of T and the AC is smoothed by C2 reservoir capacitor.

R3 has the same function as R2 in Figure 3 the operation is the same as described in Figure 3. Thus, the switch action of TR is reflected in the primary impedance of the transformer T, and the resulting potential across C1 represents the transmitted data.

Because the windings of the transformer are connected such that the signals V1 and V2 are in phase the two diodes D1 and D2 are both conducting at the same time (positive half cycle), and there is thus no residual voltage across R1 and R2 when the transmitted data is high other than that caused by circuit inefficiencies e.g. diode speed etc.

With further expansion of the circuit, bi-directional simplex transmission is possible, and this is the case with the apparatus shown in Figure 7, in which circuit components corresponding to those of Figure 5 are denoted by the same reference letters and numbers as are used in Figure 5.

In addition to the components of the apparatus shown in Figure 5, the circuit shown in Figure 7 includes a modulation transistor TR1 connected to the transformer primary in the receiving device 30, in addition to the switching transistor TR2 in the sender 32. The modulation transistor modulates the square wave input to the primary of the transformer 1 so that the receiving device 30 can also act as a master for sending data to the slave/sending devices 32.

When the master transmitted data (ie from receiver 30 to sender 32) is in the low state at the base of TR1 the transistor is OFF, there is then a high frequency potential V1 developed at the primary of T. This signal is present at the secondary of T, with an amplitude dependent on the transformer ratio, and is rectified by D2 and smoothed by C2.

In the slave this DC potential can be detected at V2 for use as received data with amplification to a suitable level, or by suitable choice of the square wave amplitude and transformer ratio no further processing may be required. When the master transmitter data is high, the transistor TR1 is turned on. This effectively short circuits the primary of the transformer T, resulting in practically zero primary signal. Therefore V2 will be zero. R3/C2 are chosen to achieve the required baud rate. The result is that the potential at V2 represents the state of the transmitted data as from the master. To allow the slave (ie the sender 32) to transmit its data TR1 must be biased OFF. The circuit action is then the same as that of the apparatus of Figure 5.

Figure 9 is a more general block diagram of a system which provides bi-directional communication between master and slave devices. In Figure 9, a square wave generator 41, is connected to a modulator 42, which is, in turn, connected to the primary of a transformer T. The voltage across the primary is monitored by a voltage sensing circuitry 43, and the transformer secondary is connected to a rectifier circuit 44 (for use where DC is to be used). Reference number 46 denotes a slave voltage sensing circuit for sensing and decoding the signal contained in the rectified voltage at the output of the rectifier 44, and 45 a slave modulating unit for sending signals back to the master device. The square wave signal can be of any frequency. By suitable selection of the circuit components high baud rates can be achieved.

Thus, signals from the master device 48 to the slave device 50 are generated by the modulator 42, which modulates the current supplied to the transformer T, and transmitted to the slave through the inductive link provided by T and the rectifier 44, to be processed by the sensing circuit 46. Signals from the slave 50 to the master 48 are provided by the slave transmitter 45 (by, for example, modulating the impedance of the circuit containing the transformer secondary) and are picked up (and optionally amplified) by the circuitry 43.

The invention permits two wires to transmit and receive data with full isolation between transmitter and receiver. It has the advantage that the communication between master and slave is performed without any significant power consumption from the slave. Therefore it is very good when the slave is battery powered and power consumption is a major issue.

Because of its simplicity a circuit embodying the invention gives good results, with low cost and good reliability.

A number of the wave forms shown in Figures 4, 6 and 8 correspond with each other, and are therefore indicated by the same reference numerals. Each of the Figures show a number of graphs of voltage wave forms (against time) across different parts of the circuitry shown in Figures 3 to 7, and all of the Figures include a graph 60 showing the voltage output of the square wave generator for each sending device and a graph 62 which represents a binary signal to be sent from the sending device to the receiving device in each case. In addition, Figure 8 includes a graph 63 representative of the binary signal to be transmitted from the receiver/master 30 to the sender/slave 32. The signals, in all cases, modulate the voltage across the transformer primary, and this voltage (V1) is represented by the graphs 64.

The currents flowing in the primary windings of the transformers also induce voltages across the secondary windings which give rise to voltages (V2) across the collector and emitter of the transistors TR/TR2 in the sending devices. The voltages V2 are represented by the graphs 66. In Figure 4, the voltage V2 is alternating because there is no rectification of the voltage induced in the transformer secondary. By contrast, the voltages V2 shown in Figures 6 and 8 are rectified, and consequently are in the form of DC pulses.

Finally, Figures 4, 6 and 8 include graphs 68 which represent the voltages of the signals received by the receiving devices, after having been processed by the rectification circuitry connected to the primary windings of the transformers.

Any of the sending devices of the embodiments may be incorporated into a commodity meter and may be arranged to generate a signal representative of the amount of commodity consumed. That signal is received by the receiving device, which is remotely located and which thus avoids the need for someone to visit the meter in order for the latter to be read.

Claims

1. Apparatus for sending and receiving electrical signals, the apparatus comprising receiving means and sending means connected together via a transformer, the receiving means being operable to supply time varying current to the primary windings of the transformer and having detection means responsive to changes in the effective impedance of the primary windings, the sending means having transmission means for altering the impedance of the circuit in which secondary windings are connected, in response to the signal to be transmitted, wherein said variation in the impedance of the circuit causes a corresponding variation in the effective impedance of the primary windings which is detected by the detection means.

2. Apparatus according to claim 1, in which the transmission means comprises a switch connected in the circuit which includes the secondary windings.

3. Apparatus according to claim 2, in which the switch comprises a semi-conductor device.

4. Apparatus according to claim 2 or claim 3, in which, when the switch is closed, the secondary windings are short circuited, and in which substantially no current flows in the secondary windings when the switch is open.

5. Apparatus according to any of the preceding claims, when adapted for use as a bidirectional communications system.

6. Apparatus according to claim 5, in which the transformer primary windings are connected to further transmission means for modulating the current supplied to the primary windings, and the secondary windings are connected to further detection means responsive to the signal from the further transmission means.

7. Apparatus according to any of the preceding claims in which the receiving means includes an oscillator operable to supply said time varying current to the primary windings.

8. Apparatus according to any of the preceding claims, in which the sending means is incorporated into a commodity meter and generates a signal representative of the quantity of the commodity consumed.

9. Receiving means for apparatus according to any of the preceding claims, the receiving means comprising a transformer having primary windings connected to means for supplying a time varying current thereto and detection means responsive to changes in the effective impedance of the primary windings, the transformer also having secondary windings for connection to sending means for altering said effective impedance.

10. Apparatus comprising sending means and receiving means respectively for sending and receiving signals, wherein said signals are conveyed from the sending means to the receiving means via a transformer which isolates the sending means from the receiving means.

11. Apparatus substantially as described herein with reference to, and as illustrated in, Figures 3 and 4 of the accompanying drawings.

12. Apparatus substantially as described herein with reference to, and as illustrated in, Figures 5 and 6 of the accompanying drawings.

13. Apparatus substantially as described herein with reference to, and as illustrated in, Figures 7 and 8 of the accompanying drawings.

14. Apparatus substantially as described herein with reference to, and as illustrated in, Figure 9 of the accompanying drawings.