GB2136583A

GB2136583A - Data transmission systems

Info

Publication number: GB2136583A
Application number: GB08406448A
Authority: GB
Inventors: Anthony Bernard Thorpe
Original assignee: British Hovercraft Corp Ltd
Current assignee: British Hovercraft Corp Ltd
Priority date: 1983-03-17
Filing date: 1984-03-12
Publication date: 1984-09-19
Also published as: GB8406448D0

Abstract

The value of a variable, such as temperature at thermistor 15, changing by small amounts and sensed at one location 11 is reproduced at a remote location 12 by transmitting thereto a signal representative of the difference between the sensed value and a reference value, and using the difference signal to control the reference value. A digital reference value held in an up/down counter 23 is converted to analogue form at 19 and subtracted at 18 from the sensed value, and the difference signal transmitter as a current over line 2 DEG c to A/D converter 24. If the difference signal as received by microprocessor 25 is greater than that corresponding to one increment of counter 23, a voltage pulse is passed back along line 2 DEG c, and via count-down switch 21 and pulse length discriminator 22, to increment or decrement the counter 23. The reference signal value which is stoned in microprocessor 25 for combination with the difference signal is similarly altered. True temperature is obtained using look up table 26. The difference and updating reference signals may alternatively be VHF and microwave signals. <IMAGE>

Description

SPECIFICATION Data transmission systems This invention relates to data transmission systems and is more particularly concerned with a method and apparatus for transmitting data relating to the value of avariable quantity being sensed at one location to a remote receiving location.

A particular problem exists in transmitting between remote locations data relating to the value of a variable quantity such as, for example, temperature or pressure, being sensed at one of the locations when the variable quantity is changing by small amounts relative to its totai value.

For example, if small changes in temperature have to be detected with a required accuracy of, say 0.1 C and a resolution between adjacent temperature sensors of, say, 0.002"C and, having detected a small change in temperature, a signal corresponding to the new temperature value has to be transmitted to a remote location over, say, a 400m cable length, then signal degradation may completely mask the temperature change when the signal is received at the remote location for display or storage.

It is an object of the present invention to provide a method and apparatus for transmitting between remote locations data relating to the value of a variable quantity being sensed at one of the locations so that a true value of the variable quantity is accurately recorded at the other location.

Accordingly, one aspect of the invention provides a method of transmitting data relating to a variable quantity being sensed at one location to a remote data receiving location, comprising transmitting to the receiving location a difference signal representative of a change in the sensed value of the variable quantity relative to a reference value, and using the said difference signal to update the reference value to the sensed value.

Another aspect of the invention provides apparatus for transmitting data relating to a variable quantity being sensed at one location to a remote data receiving location, comprising meansfortrans- mitting from the sensing location to the receiving location a difference signal representative of a change in the sensed value of the variable quantity relative to a reference value, and means for using the said difference signal to update the reference value to the sensed value.

Transmitting means providing a signal transmission link between the sensing locaton and the data receiving location may be single channel or multichannel and may comprise a multi-core cable line connecting the two said locations. Alternatively, the transmitting means may comprise a radio frequency (RF) system such as, for example, a system using very high frequency (VHF) signals to carry the signals representative of small changes in the sensed variable quantity from the sensing location to the receiving location, and microwave signals for transmitting from the receiving location to the sensing location information to update the reference signal at the sensing location.

In an embodiment of the invention a signal representative of the sensed value of the variable quantity is amplified by a low noise amplifier and fed to one input of a summing amplifier. The other input of the summing amplifier is fed from the output of a digital to analogue converter, the polarity of the output voltages of the digital to analogue converter being opposite to that of the low noise amplifier so that for equal output voltages from the digital to analogue converter and the low noise amplifier the output from the summing amplifier is zero.

The output voltage of the digital to analogue converter is representative of a reference value of the variable quantity. This reference value is preferably obtained by processing, at the receiving location, signals output by the summing amplifier which are transmitted to the receiving location in analogue form.

Signals representative of the reference value are transmitted back to the sensing location, as voltage pulses in the case of cable line transmission, and the pulse length of these signals is detected by a pulse length discriminator and used to cause an up/down counter to drive the digital to analogue converter so that it is counted up or down and the reference value signal supplied to the summing amplifier is updated to the sensed value of the variable quantity.

A signal output by the summing amplifier is representative of a change in the sensed value of the variable quantity relative to the reference value. This signal is preferably transmitted to the receiving location in analogue form.

In the embodiment having the data transmission link means comprised by a cable line the signal is preferably transmitted as electrical current so as to keep signal degradation to a minimum.

When the signal is transmitted in analogue form it is fed first at the receiving location to an analogue to digital converter whose output is then fed to a microprocessor. The microprocessor is programmed to read the signal and if the change is within a predetermined range it is added to or subtracted from, as appropriate, the reference value to obtain the measured or sensed value of the variable quantity. After linear correction, if required, the true value of the variable quantity is displayed and/or stored in a memory for future processing.

The invention will now be further described by way of example and with reference to the accompanying drawing which shows a schematic block diagram of a single channel data acquisition and display system.

Referring to the drawing a single channel data acquisition and display system 10 incorporates a data transmission method and data transmission apparatus in accordance with one embodiment of the present invention.

The system 10 comprises a data sensing location 11 linked with a remote data receiving location 12 by signal transmitting means 13. Forthe purposes of this description it will be assumed that temperature is the variable quantity being sensed at the data sensing location 11.

Atemperature sensing element 14 at the sensing location 11 comprises a thermistor 15 mounted in one arm of a Wheatstone bridge configuration. The bridge is energised from a thermistor bridge voltage supply circuit 16 which keeps the excitation voltage low, say, for example, 120mV, in order to minimise any self heating due to current flow through the thermistor. The expected output signal from the bridge over a temperature range 0 C to 30"C is approximately OmV to 34mV so that a 0.001"C change in temperature causes an output signal change of approximately 1.1 ;V.

To avoid losing these small output signal changes a first stage amplification is provided by an extremely low noise and low drift amplifier 17.

The amplified signal from amplifier 17 is fed to one input of a summing amplifier 18 which has its other input fed from the output of a digital to analogue (D/A) converter 19. The polarity of the output voltages of the D/A converter 19 and the amplifier 17 are chosen such that they oppose one another. Thus for equal output voltages from the D/A converter 19 and the amplifier 17 the output from the summing amplifier 18 is zero. The output of the D/A converter 19 represents a reference temperature which is first established at the outset of a temperature measurement sequence and subsequently held constant between updating operations.The output of the summing amplifier 18 is, therefore, a signal representative of changes in the sensed temperature value over the reference value and this signal is transmitted to the data receiving location 12 by the signal liaison link means 13 as an analogue current signal in order to keep signal degradation to a minimum during transmission.

In this embodiment the signal liaison link means 13 comprises a three core screened cable 20 having two cores 20a and 20b carrying power to the thermistor bridge voltage supply circuit 16. The third core 20c is used to transfer data between the data sensing and receiving locations 11 and 12, respectively.

At the same time as analogue current signals representative of temperature difference are passed over core 20c from the sensing location 11 to the receiving location 12, superimposed momentarily on the line are voltage pulses which pass information from the receiving location to the sensing location.

These voltage pulses are fed by way of a down-count switch 21 and a pulse length discriminator 22 to an up/down counter 23. The up/down counter 23 drives the D/A converter 19, the two devices 23 and 19 being scaled such that a single step up or down from the counter 23 causes the output of the D/A converter 19 to change by an amount corresponding to 0.15"C, the width of the voltage pulse determining whether the D/A converter is to be incremented up or down.

The analogue current signals passing over core 20c are fed to an analogue to digital (A/D) converter 24. The binary word output from the A/D converter represents temperatures in the range -0.1 6"C to +0.159"C in 0.0125cm increments. This output is read by a microprocessor 25 and if the temperature difference is within the range 0.15CCto +0.1sic, it is added to/subtracted from the reference temperature value which is stored in the microprocessor 25 to give the measured or sensed value of temperature. To correct for any non-linearity a linearity correction factor stored in a look up table store 26 is also fed to the microprocessor 25 where it is added to/subtracted from the measured temperature to obtain a true temperature value. The true temperature value is then displayed on either a digital output 27 or a binary output 28, or after conversion to analogue form by a digital to analogue converter 29, it is displayed on an analogue output 30.

If temperature difference is outside of the range -0.1 5CC to +0.15"C then the up/down counter 23 is pulsed up or down as required to bring the differential temperature back into range. At the same time 0.15cm is added to/subtracted from the reference temperature value and scanning continues to enable the true temperature value to be displayed accurately over the full 30"C range.

Claims

1. A method of transmitting data relative to a variable quantity being sensed at one location to a remote data receiving location, comprising transmitting to the receiving location a difference signal representative of a change in the sensed value of the variable quantity relative to a reference value, and using the said difference signal to update the reference value to the sensed value.

2. Apparatus for transmitting data relative to a variable quantity being sensed at one location to a remote data receiving location, comprising means for transmitting from the sensing location to the receiving location a difference signal representative of a change in the sensed value of the variable quantity relative to a reference value, and means for using the said difference signal to update the reference value to the sensed value.

3. Apparatus as claimed in Claim 2, wherein a low noise amplifier at the sensing location is arranged to amplify a signal representative of the sensed value of the variable quantity and to feed the amplified signal to one input of a summing amplifier.

4. Apparatus as claimed in Claim 3, wherein the other input of the summing amplifier is connected to receive a signal output by a digital to analogue converter, and representative of the reference value, the polarity of the digital to analogue converter output signal being arranged to be of opposite polarity to the output signal of the low noise amplifier so that for equal output signals the output from the summing amplifier is zero.

5. Apparatus as claimed in Claim 3 or 4, wherein signal output by the summing amplifier are transmitted to the receiving location in analogue form.

6. Apparatus as claimed in Claim 5, wherein analogue signals received at the receiving station are fed to an analogue to digital converter whose output is fed to a microprocessor.

7. Apparatus as claimed in Claim 6, wherein the microprocessor is programmed to read signals received from the analogue to digital converter and to add to or subtract from the reference value a change of said signals which is within a predetermined range so as to obtain the sensed value of the variable quantity.

8. Apparatus as claimed in Claim 7, wherein signals representative of the updated reference value generated at the receiving location are transmitted to the sensing location.

9. Apparatus as claimed in Claim 8, wherein a pulse length discriminator senses the pulse length of the signals representative of the updated reference value received at the sensing location and causes an up/down counter to drive the digital to analogue converter so that it is counted up or down and the reference value signal output to the summing amplifier is updated.

10. Apparatus as claimed in any one of Claims 2 to 9, wherein the transmitting means providing a signal transmission link between the sensing location and the receiving location comprises a multicore cable line connecting the two said locations.

11. Apparatus as claimed in any one of Claims 2 to 9, wherein the transmitting means providinvg a signal transmission link between the sensing location and the receiving location comprises a radio frequency system.

12. Apparatus as claimed in Claim 11, wherein the radio frequency system uses VHF signals to carry the signals representative of small changes in the sensed variable quantity from the sensing location to the receiving location and microwave signals for transmitting from the receiving location to the sensing location information to update the reference signal at the sensing location.

13. Apparatus substantially as hereinbefore described with reference to the accompanying drawing.

14. Any new or improved features, combinations and arrangements described, shown and mentioned or any of them together or separately.