GB2449275A

GB2449275A - Driving a sensor pad for electrical impedance tomography (EIT)

Info

Publication number: GB2449275A
Application number: GB0709398A
Authority: GB
Inventors: Thomas William Bach
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-05-15
Filing date: 2007-05-15
Publication date: 2008-11-19
Also published as: GB0709398D0

Abstract

The function of a selected pad 8 is changed from receiver to transmitter/impedance measurer by driving the power rails 11 17 of an inverter 10 with feedback to AC follow an oscillator signal 20. The changing of function is ideally done at zero crossings of the AC oscillator for a fixed whole number of cycles per measurement with switches SW1 SW2 SW3 in a T configuration with two series switches SW1 SW3 and a parallel switch SW2 capable of connecting the unused signal line 21 to OV in order to better isolate the functioning as a transmitter or receiver. The inverter virtual-earth input 23 is directly, or indirectly through a series capacitor and /or through switches, coupled to a sensor pad 8 with the rails of such switches as well as any guard plates 9 driven to AC follow the oscillator signal in order to eliminate the effects of stray impedances to the guard plate 9 or rails. A further embodiment (fig 3) has a guard amplifier 24, which may have two level shifted outputs, in the feedback loop in front of an inverter 10 forming an improved inverter with the guard input 23 the improved inverter input. The output 25 of the guard amplifier now AC or DC drives any guard plate 9 or rails of a switch in front of the improved inverter input 23.

Description

No Input Multiplexer Impedance Sensor

Background

EIT

Electrical Impedance Tomography, EIT, is a technique applied to determine the distribution of impedance of a space. It generally uses contact sensors and six or more plates to cylindrically or spherically enclose the space so that the boundary conditions of the space are fixed. The technique can also be used with a linear or planar array but in these cases, generally the boundary conditions are less well defined.

Conventionally one or more plates act as voltage or current transmitter and the remaining plates are scanned and used as a voltage or current receivers.

Alternatively, with a larger set of plates a group of plates can be selected as transmitters or receivers. The data collected can be used to identify and locate inhomogeneous areas in the total space and thus used as the basis of a medical instrument, for example to find breast cancer or dental caries. An inverse mathematical transform can be applied to the total data set collected to determine the 2-D or 3-D profile of the distribution of impedance in the space.

Scanning transmitter! receiver pairs An example is to use eight plates in a cylindrical configuration with one plate, as a transmitter. The transmitter plate is injected with a fixed sine wave current and the remaining seven plates are used as current receivers. The current is measured, typically with a synchronous demodulator locked to the transmitter frequency and phase, and stored. This is repeated for all possible choices of transmitter generating a data set of 8 X 7 independent data points.

Setting up voltage gradients in a linear array Another example is the use of a linear array of sensor pads. The two pads at the ends are used as transmitters, for example driven with equal amplitude voltages 180 degrees out of phase with respect to each other. The voltage gradient set up across the remaining six plates allows voltage or current to be measured generating six independent data points.

Standing capacitances to directing guard plates Typically for high impedance measurements, a directing guard plate will be used behind and/or around the sensor pad to make the sensor pad more sensitive in one direction. This adds parallel standing capacitance to the sensor pad that may be sensitive to temperature and vary from pad to pad.

Selecting pad transmitter! receiver functioning with switches Typically the selection of which plates are used as transmitters or receivers or as a pad whose impedance is to be measured is done with semiconductor switches or a multiplexer, a set of semiconductor switches in single package with a common signal line. However, it can be done with reed or optical switches. These route the transmitter signals from the transmitting oscillators to the selected plates and the received signals to the receiver circuits and/or connect unselected pads to a isolating signal line that my be guard or OV.

Advantages of using switches 1. The main advantage of using switches or multiplexers is that their needs to be fewer transmitter and receiver circuits and more complication can be invested in the ones that exist. This may result in a less expensive Circuit.

2. Much of the transmitting and measuring circuitry being common means errors due to external capacitive or resistive loading, temperature variations in the gain of the guard amplifier, mechanical ageing and internal capacitance changes of the electronic components can be corrected by fractionally comparing real-time measurements of on-board stable reference capacitors with calibration values stored earlier.

Disadvantages of using switches are: 1. The capacitance from the two FET control gates to the output and input pins means when the switches changeS state then current is injected into the signal line and this cause switching glitches to appear in the receiver and transmitter circuits. This is especially true with semiconductor switches when both series and parallel switches are used in order to get good isolation when selecting a pad.

2. The capacitance of approximately O.2pF across the open switch means there is some of feed-through signal across an open switch.

3. There is a total of approximately 7pf to the positive and negative rails from each switch end. This adds parallel standing capacitance to the sensor pad and to the transmitting and receiving circuits that may vary from pad to pad and may result in temperature sensitive effects besides generally reducing the ability to measure very small impedances. For example eight switches used as an 8-way multiplexer has 56pF on the common line even with all switches open. In order to reduce these effects guard amplifiers are needed to bootstrap out the capacitance but these are prone to oscillations especially when they have a gain near very one.

4. They are in the transmitter and receiver signal path and are a source of component noise, imperfect non-zero on resistance and non-linearity.

State of art dual T arrangement of switches Figure 1 shows a typical currently used switching circuit to change the function of a sensor pad 1 from transmitter to receiver. It uses a T arrangement of switches in the top path SW 1,2,3 and a T arrangement of switches in the bottom path SW 4,5,6.

One of the pair of switches in series with a possible signal path SW 1,3 or SW 4,6 will be closed if that function is chosen. Lines 3 and 5 are connected to Dv. If one of the pair of switches is open then one of the switches in parallel SW2 or SW5 will be closed connecting the unused line 6 or 7 to OV. This is done in order to stop the feed through of unwanted signals across the approximate 0.3pF feed-through capacitance of an open series switch by.

Figure 1 illustrates the case where the function of a sensor pad 1 is chosen as transmitter. In the top T arrangement from the transmitter 2 SW1 and SW3 are closed so that the sensor pad is connected to the transmitter 2. SW2 is open so that the sensor pad is not connected to ov at 3 through line 6. In the bottom T arrangement to the receiver 4, the switches SW4 and SW6 are open so that the sensor pad is not connected to the receiver 4. The switch SW5 is closed connecting signal line 7 to a known isolating signal 5, in this case ov so that the transmitter signal does not get to the receiver 4 across the 0.3 pF feed through capacitance across SW4.

The control lines are not shown to the switches. Neither is any level shifting necessary to correctly interface to the switches from an assumed micro controller.

Statement of Invention

This invention is for a device that changes the function of a selected pad from receiver to transmitter/impedance measurer by driving the power rails of an inverter with feedback to AC follow an oscillator signal. The changing of function is ideally done at zero crossings of the AC oscillator for a fixed whole number of cycles per measurement with switches in a T configuration with two open series switches and a closed switch connecting the unused signal line to OV in order to better isolate the transmitter and receiver signals. The inverter virtual-earth input is directly, or indirectly through a series capacitor and /or through switches, coupled to a sensor pad with the rails of such switches as well as any guard plates driven to AC follow the oscillator signal in order to eliminate the effects of stray impedances to the guard plate or rails. An improved device has a guard amplifier, which may have two level shifted outputs, in the feedback loop in front of an inverter forming an improved inverter with the guard input the improved inverter input. The output of the guard amplifier now AC or DC drives any guard plate or rails of a switch in front of the improved inverter input.

The advantages of this invention are: 1. The circuits can measure, using long coaxial cables connecting to a sensor pad, high frequencies and small capacitances without spurious oscillations than similar functioning accomplished by circuitry with multiplexers in front that have their power supply lines driven to make capacitances to the rails effectively small.

2. This way of selecting the functionality, without multiplexers in the input lines, eliminates glitches especially when done at zero crossing points with a fixed number of cycles of the transmitting oscillator. The non-glitch performance increases the speed in which the different plates can be selected as transmitters or receivers.

3. The use of a separate receiver/transmitter for each pad eliminates feed-through effects across switches and increases isolation between the transmitter on and the receiver on states. The increased isolation between transmitter on and receiver on states and the signal on different pads means the data measured is pertaining to the impedance or receiver characteristics only of the selected pad.

This is especially true when the T configuration shown above, implemented with switches SW 1,2 3 in Figures 2 and 3, are used to turn the drive to the rails on and off.

4. One plate can be chosen to act as voltage or current transmitter and its injected current or transmitting voltage respectively measured. The data collected can be used to determine the contact impedance, i.e. the coupling of each plate to the target object.

5. One or more plates can be chosen to act as voltage or current transmitter and the remaining plates can be scanned and used as a voltage or current receivers.

A basic embodiment Figure 2 shows a basic embodiment of this invention using an inverter. The inverter can be constructed from two opposite polarity drain connected enhancement mode MOSFETs or from an operational amplifier with the non-inverting input biased at some point between the rails. The sensor pad 8 has a guard plane 9 underneath connected to the VSS rail of the inverter 10. The power supply rail driver 13 has DC voltage offset outputs 11 and 17 to the VDD and VSS rails of the inverter 10. The input 19 to the power supply rail driver 13 comes from SW3 that is a part of the T arrangement SW1, SW2, SW3. If SW3 and SW1 are closed and SW2 is open then the oscillator 20 is connected to the input 19 of the power supply rail driver 13 so that the DC voltage offset outputs 11 and 17 AC follow the oscillator 20. SW2 is used to increase the isolation when the circuit is to be used as a receiver, i.e. when the rails are not to be driven, by connecting line 21 to OV.

The virtual earth input 14 follows closely in an AC manner the inverter rails. This means the guard plane 9 can be driven by the VSS rail 17, or VDD rail or input 19 to rail driver 13, to reduce the effective capacitance between sensor pad 8 and guard plate 9.

When SW1 and SW3 are off the oscillator is no longer connected to the input 19 of the power supply rail driver. SW2 is closed so that the line 2lconnecting SW1 to SW3 is connected to OV through SW2. This eliminates any feed through of the oscillator signal across the capacitance across SW1.

An improved embodiment Figure 3 shows an improved device with a guard amplifier 24 in front of the inverter but in the feedback loop. For best results the rails 11, 17 of the guard amp 24 should also be driven either by the power rail driver used for the inverter or a separate one following the guard output 25. The guard amp 24vthen does not have to have a gain very near one but will still follow well the virtual earth input 23. The output 25 of guard amplifier 24 is used to drive the guard plate 9. This makes parallel impedances to the input larger. If multiplexers are used in front of the virtual earth, they should have their power rails driven to follow the virtual earth 23 by using the guard amplifier output 25 as the input to a circuit like power rail driver 13.

Figure 4 shows how a switch 27, which may be a multiplexer, in the signal line in front of the inverter input is driven. 26 is the line to the sensor pad and 31 is the line to the improved inverter input. The output of the guard amplifier in front of the inverter now is the input 30 to the switch rail driver 29. Rail driver 29 drives the rails 28 and 32 of the switch 27so that they AC follow the output of the guard amplifier.

Demodulation of signals Each pad requires the equivalent of one of the embodiments shown above.

Typically, multiplexers pass a selected inverter output and rail signals to a demodulator giving a DC output proportional to the size of the inverter signal output.

This will work whether the sensor pad functions as transmitter or receiver, measuring respectively either current injected or current received.

Figure 5 shows such a circuit. The inverter outputs 33 are presented to MUX1 34.

MUX1 34 chooses one of the inverter outputs 33 and puts it on its common line 35.

The guard outputs 39 are presented to MUX2 40. MUX2 40 chooses one of the guard outputs 39 and puts it on its common line 41. Differential amplifier 36 subtracts these two signals 35 and 41. Multiplier 37 synchronously demodulates this difference with the oscillator signal 42, or a 90 degree shifted oscillator signal to find the complex component. Filter 38 smoothes the result giving a DC output 43.

Typically, this would be output to a micro controller for further processing.

The difference amplifier uses the guard as one input. If the simpler circuit without a guard in front of the inverter is used then one of the rail voltages can be used instead.

Application Information Use as a receiver When the rails are not driven the AC difference between the voltage on the rails, or guard output for the improved device, and the inverter output is a measure of the current received.

Use as a transmitter When the rails are driven the AC difference is a measure of the current injected and from this and the drive voltage the complex contact impedance of each pad to the environment can be found. Knowledge of this contact impedance greatly improves the ability to calculate a satisfactory mathematical inverse of the distribution of impedance of in the space.

If there are many such driven pads then the 3-D shape of target objects can be found from this data. Alternatively, we can use a multiplexer in front of one such driven inverter so that many pads are scanned in order to accomplish the same purpose. The target 3-D shape is found most easily when the contact impedance, i.e. a vacuum or a gas, is much higher than the impedance of the target object.

Implementing scanning of transmitter and receiver pairs and setting up voltage gradients This invention can implement both scanning of transmitter and receiver pairs and by driving pads with out of phase voltages create voltage gradients.

Using "guard planes" A transmitter or receiver pad can made directional, so it functions the same as a "guard plane", by connecting a pad behind the receiver pad to make it AC follow the power rails, typically this is done by connecting it to one of the power rails or in the improved device to the output of a guard amplifier in front of the inverter.

Effect of open loop gain and why a guard amplifier in front of the inverter is an improvement If current is injected into a sensor pad connected to a virtual-earth input then the voltage on this input equals the output voltage divided by the open loop gain. It follows if the power lines of a virtual-earth inverting amplifier are being driven and if the virtual-earth input sees any impedance the AC output voltage will not be zero and there will be an AC voltage on the virtual-earth input. Thus for any device such as "guard" plane or other element whose impedance to the input line needs to be made larger than it is best to add a high impedance amplifier with gain near one in front of the inverter following the virtual-earth input and use this to bootstrap the element.

Claims

Claims 1. A device comprising a sensor pad, an inverter, an oscillator,

a power supply rail driver with two DC voltage offset follower amplifier outputs for the power supply rails of the inverter and switches that change the pad function from receiver to transmitter/impedance measurer by turning on and off the oscillator drive to the input of the power supply rail driver of the inverter.
2. A device according to claim 1 that uses a high input impedance guard amplifier in front of the inverter, but in the closed feedback loop, with gain near one as a guard amplifier for making the input impedance of the inverter larger and for bootstrapping impedances that need to be made larger in front of the inverter.
3. A device according to claim 2 where the impedance that needs to be made larger in front of the inverter is the impedance to a guard plate.
4. A device according to claim 2 where the impedance that needs to be made larger in front of the inverter is the capacitance to the rails of switches or multiplexers selecting different sensor pads or connecting the unused pad to set signal.
5. A device according to claim 1 where if the pad is being used as a transmitter or receiver then the injected or received current and phase can be found by using the difference between the AC output voltage from the inverter and the AC rail drive voltage with the feedback resistor value.
6. A device according to claim 2 where if the pad is being used as a transmitter or receiver then the injected or received current and phase can be found by using the difference between the AC output voltage from the inverter and the guard amplifier output with the feedback resistor value.
7. A device according to claim 1 or claim 2 where the zero crossing point of the oscillator signal is determined and used to switch the function of the selected pad for minimal glitch generation.
8. A device according to claim 1 or claim 2 where a certain number of cycles of the oscillator signal is used with the zero crossing point so that the demodulation to a DC signal always uses the same number of cycles and thus the measurement is more repetitively correct.
9. A device according to claim 1 or claim 2 where said driving of the power lines improves on noise performance, as no multiplexers are need in the signal path.
10. A device according to claim 1 or claim 2 where said driving of the power lines improves on feed through an glitch performance with the selection being done by switches in the input path as a separate amplifier is used for each sensor pad.
11. A device according to claim 1 or claim 2 where said driving of the power lines is chosen by using three switches in T configuration for determining whether or not the rails are driven so that there is better isolation between tranmitter and receiver functionality.