GB2130035A - Isolator system for data loggers - Google Patents

Abstract

An isolation system for low level output sensors 10-13 typically associated with a multi-channel data logger. The respective sensor outputs are applied across capacitors 50-53 and the capacitor voltage is connected periodically to respective transformers 56-59 via time-multiplexed switches 40-46. Rectangular amplitude modulated pulses resulting therefrom at the secondary transformer winding pass via the multiplexed switches 41-47 to the capacitor 61 and thence to buffer 62. The voltage across capacitor 61 represents the sensor voltage from the most recently switched channel now referenced to ground potential and can be used by the data logger after conversion by ADC 21. <IMAGE>

Description

SPECIFICATION Isolator system for data loggers The invention relates to an isolation system suitable for handling low level signals, especially for use in data loggers.

In known data logging systems, as exemplified in Figure 1, a number of data channels receive information from associated sensors 10-13.

Typically the sensors would be thermocouples and the voltage from each would be passed via channel switches 1 spa, 1 sub etc., to an isolator 20 within the data logger 14 prior to the conversion of the voltages into digital signals in converter 21.

Each channel would be connected to the inputs 20a and 20b of the isolator 20 on a time multiplex basis by operating the associated switches 1 5-1 8.

Typically these switches would take the form of reed relays.

In such systems, the incoming signals from the sensors may be referred other than to ground potential, and the purpose of the isolator is to refer the sensor signals to ground potential to achieve system compatibility. For example, a sensor may produce a series mode voltage, which is the desired signal, of the order of 1 OmV, whereas this may be superimposed on a common mode (reference) voltage of 1 00V. The isolator would allow the 1 OmV signal to pass through, but block the common mode voltage.

A prior art isolator capable of handling these low levels of signal is shown in Figure 2. It includes an input amplifier 30, a pulse-width modulator 31 with associated transformer 32, a detector 33 and an output buffer 34.

The pulse width modulator shown requires an input signal of a few volts if it is to operate satisfactorily, and the input amplifier 30 is provided to amplify the signal, which is only in the mV range, to several volts so as to allow the modulation to be achieved. The modulated frequency is typically within the range 500KHz to 1 MHz. The input amplifier requires a power source but, because of the presence of the common mode voltage at the amplifier input, it cannot be directly driven by the data logger supply as this will be at a ground related voltage.

To overcome this problem a dc to dc converter formed by an oscillator 39, a transformer 38 and a rectifier 37, is used to provide an isolated power source with non-ground related voltage rails.

The provision of an isolated power supply is however inconvenient both on account of its cost and size.

The present invention is concerned with providing an improved isolator system which does not require an isolated power source.

According to the invention there is provided an isolator system for a data logger comprising, input means for receiving an input signal superimposed on a common mode voltage and derived from a first signal source, switching means for periodically switching the input signal to generate pulses having an amplitude representing the input signal, detector means for detecting said pulses, and transformer means for feeding said pulses to said detector means whilst isolating the common mode voltage therefrom.

The system may include a plurality of channels for input signals derived from a plurality of signal sources, each of said channels including associated switching means, transformer means and detector means operable on a time multiplex basis.

In order that the present invention may be more clearly understood and readily carried into effect, it will now be more fully described, by way of example, with reference to Figures 3 to 5 of the accompanying drawings; in which Figure 1 and Figure 2 have already been referred to, Figure 3 shows an embodiment of an isolator system according to the present invention, Figure 4 shows typical control signals for operating the associated switches in the said embodiment, and Figure 5 shows an arrangement capable of providing control signals such as illustrated in Figure 4.

The arrangement of Figure 3 does not require an input amplifier and hence an isolated power source because the system no longer uses the pulse width modulation technique previously employed. It can be seen that each channel is not provided with its own independent isolator. The first channel (channel 0), receives an output from a sensor 10, which is assumed to be a thermocouple in this example, and this is applied across a capacitor 50. The capacitor 50 acts as a reservoir which is charged via the thermocouple to ensure sufficient current flow on actuation of the switch 40. It also reduces rapid changes in the signal due to noise, for example, from being passed through the system. The voltage across the capacitor 50 is applied via a switch 40 to the primary winding of a transformer 56.The switch 40 is periodically closed for short periods to apply the signal from the sensor 10 across the primary winding intermittently. The switching action (which may occur typically at 50KHz) will produce substantially rectangular pulses across the secondary winding of the transformer 56 having an amplitude representative of the voltage across capacitor 50 at the time of closing the switch, which voltage itself is dependent on the thermocouple output at the given time.

It will be understood that the transformer 56 will inherently achieve the desired isolation of the sensor 10 from the data logger. The voltage derived via the transformer is applied via a second switch 41 across a capacitor 61, and thence to a buffer 62. The output of the buffer 62 is applied to the analogue to digital converter (ADC) 21 in the system. Switch 41 is closed co-incidentally with the switch 40 (though for slightly shorter periods) and when it is closed the capacitor 61 is charged to the peak value of the voltage transmitted via the transformer 56; this peak value is the input to the buffer 62.

Thus capacitor 61 acts as a memory after switch 41 is opened.

The remaining channels with associated capacitor 51-53, input and output switches 42-47 and transformers 57-59 operate in similar manner on a time multiplex basis.

The voltage across capacitor 61 will represent the sensor voltage from the most recently switched channel referenced however to ground potential. The fixed rate switching by the switches effectively produces amplitude modulated pulses and also achieves isolation as required.

In order to achieve the desired result the switches have to be operated in time sequence as illustrated by the control pulses shown in Figure 4. As shown, switch 41 is arranged to be closed within the period of closure of switch 40 to give the required synchronism. The other switches are similarly operated but in time multiplex sequence.

An arrangement suitable for controlling the switching is shown in Figure 5. To initiate scanning of the sensors, a strobe signal is applied to an oscillator 70 which starts operation at the desired frequency. The oscillator output periodically triggers a first monostable 71 which produces a sequence of relatively long pulses for switches 40, 42 etc. The oscillator also triggers a second monostable 72 which produces the shorter pulses for switches 41, 43 etc. The monostable outputs are received respectively by demultiplexers 74 and 73. A counter 75 has an N bit output sufficient for the number of channels in a given system (e.g. 128) and it provides a circulating output which is used to sequence the demultiplexers to produce the control pulses in the manner illustrated in Figure 4.

The switches 40-47 could be realised by reed-relays, but solid state switches are preferred for reliability. However in the case of the switches 40, 42, 44 and 46, this could re-introduce the difficulty of achieving isolation, but it is avoided by opto-isolation techniques. This is illustrated in Figure 5, which shows the switch 40 as comprising a light emitting diode 65 and FET 66 optically coupled, (e.g. General Electric type H 11). The appropriate control pulse to close the switch is applied to diode 65 which becomes iiluminated and activate the FET. Opto-isolators employing bipolar transistors would not be preferred because of their power requirements.

The system which has been described is sensitive enough to handle low level signals without employing an input amplifier and hence removes the requirement for an isolated power supply. All the components of the system can be powered from the data logger power system referenced to ground.

Although in the system as described, switching pulses are derived using dedicated circuits, such as Figure 5, the pulses could alternatively be generated under software control using the computer included in data loggers.

Although the isolator has been described specifically for multi-channel data loggers, the arrangement could be used for single channel operation, to effect input isolation in other areas of technology. The invention is also applicable to process control systems and other systems where low level sensors are employed.

The term 'ground' has been used to be indicative of a reference rail voltage on a device to which the system is to be coupled. It need not be held at ground potential and may be at a potential other than zero volts.

Claims (5)

Claims

1. An isolator system for a data logger comprising, input means for receiving an input signal superimposed on a common mode voltage and derived from a first signal source, switching means for periodically switching the input signal to generate pulses having an amplitude representing the input signal, detector means for detecting said pulses, and transformer means for feeding said pulses to said detector means whilst isolating the common mode voltage therefrom.

2. A system according to claim 1, wherein said switching means comprises a switch on the primary side of the transformer means, and a second switch is provided on the secondary side of the transformer means, which second switch is closed co-incidentally with said first switch but for shorter periods.

3. A system according to claims 1 or 2, adapted to receive input signals from a plurality of sources, and wherein individual switching means and transformer means are provided for each said input signal, and said input signals are multiplexed at said detecting means.

4. An isolating system substantially as herein described with reference to the accompanying drawings.

5. A data handling system including an isolating system according to any preceeding claim.