WO2005107313A2

WO2005107313A2 - Sensor bus

Info

Publication number: WO2005107313A2
Application number: PCT/GB2005/001716
Authority: WO
Inventors: James Redmon Jordan; Ronald David Lessels Mackie; Brian William Flynn
Original assignee: University of Edinburgh
Current assignee: University of Edinburgh
Priority date: 2004-05-04
Filing date: 2005-05-04
Publication date: 2005-11-10
Anticipated expiration: 2006-11-04
Also published as: WO2005107313A3; GB0409927D0

Abstract

A sensing arrangement comprises a plurality of sensor nodes (1…n) each adapted to measure a physical property which affects an impedance component (Cs) of the node, the nodes being arranged in series along a bus, and a control element (µCc, S1) connected to the bus for interrogating the nodes and receiving a signal determined by each said impedance component (Cs), the control element comprising means (S1) for activating a first sensor node (1), which first node comprises means (S12) for activating a further sensor node (2) and so on. The impedance component (Cs) may be an interdigitated finger capacitor.

Description

I SENSOR BUS

Background to the Invention

The present invention relates to a sensing circuit comprising a plurality of sensor nodes each adapted to measure a physical property which affects an impedance of the node.

Conventional field bus sensor circuits are complicated. It is also relatively expensive to provide each of a plurality of sensors with a battery and the requirement to replace spent batteries is inconvenient.

Summary of the Invention

The invention provides a sensing arrangement according to claim 1. A switching arrangement provides power in sequence to each of a plurality of nodes via a bus on which the nodes are arranged. When powered, each nodes receives and sends data from and to a controller.

To realize their full commercial potential, low cost sensors require low cost connection methods. The serial bus connection method is attractive, especially when it is implemented without the complication of a full implementation of field bus protocols. The connection may be made using a twisted pair of cables to minimize electrical interference, particularly over long distance. In addition, power can be supplied via the bus and the elimination of a battery will significantly increase the reliability and reduce the cost of a sensor system.

At the scale of a circuit board or integrated circuit the serial bus will provide useful reductions in circuit wiring complexity and power consumption. In one embodiment of the invention, each node comprises a capacitor, e.g. an interdigitated finger capacitor, the capacitance of which varies e.g. in response to the presence of moisture or by touching.

Each sensor node may be being adapted to respond to the controller with a pulse of duration dependent on the impedance of the impedance component. Each sensor node may comprise a controlling means such as a microcontroller, or a microcontroller may be arranged to directly activate a plurality of sensor nodes in sequence.

In an alternative embodiment, the control means comprises means for generating a signal of varying frequency for addressing each of the sensor nodes to determine the resonant frequency of the node.

Each of a plurality of control means could control an array of such resonant frequency sensors. If the sensors are positioned remotely and connected using the bus techniques of the invention, it will be possible to simplify sensor circuits by eliminating the need for a microcontroller associated with each sensor. In this case a control node of a main bus' could control a sub-bus connecting together a number of resonating sensors with each sensor arranged to resonate at a different frequency.

The arrangement may comprise inductive means for receiving power from and transmitting sensed data to a portable device.

A further aspect of the invention provides a method of sensing a touch by a human finger, comprising providing sensor means including an interdigitated finger capacitor (IFC) and means for generating a signal depending on the capacitance of the capacitor, which capacitance varies as the sensor means is touched. The sensor means may for example be comprised in an input device such as a cursor controller, in which case the sensor means may include a plurality of IFC's for sensing different commands. The sensor means may comprise means for distinguishing between finger touches of different durations and/ or pressures.

The sensor means may comprise a printed circuit board on which the IFC is formed or a microfabricated IFC formed using integrated circuit technology.

Finally, the invention provides an input device for controlling electronic equipment, comprising at least one interdigitated finger capacitor (IFC) sensor and means for generating a signal depending on the capacitance of the capacitor, which capacitance varies as the sensor is touched.

In a particular embodiment, a plurality (e.g. four) IFC sensors is provided, and the input device may comprise a cursor control device, each sensor being arranged to move a cursor of the equipment in a particular direction.

Brief Description of the Drawings

Specific embodiments of the present invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is a schematic circuit diagram of a sensor bus according to the invention;

Figure 2a shows a sensor including a real inductor;

Figure 2b shows a sensor including a virtual inductor; Figure 3 shows a sub-bus according to the invention;

Figure 4 shows an arrangement according to the invention using resonant sensor nodes;

Figure 5 shows an alternative arrangement using resonant sensor nodes;

Figure 6 shows a further alternative arrangement using resonant sensor nodes; and

Figure 7 shows an inductively coupled sensor according to the invention.

Detailed Description of Particular Embodiments

Serial Connection Method

A schematic diagram of a basic bus and its sensing nodes is shown in Figure 1. The bus is connected to a field bus interface and includes a control means comprising a control microcomputer μC_c, a bus interface and a switch Si. A plurality of sensing nodes (only one shown) is arranged along the bus. Each node comprises a data switch Sni, a node microcomputer μC_n, a bus switch Sn2 in the bus, where n is the number of the node, and a sensed capacitance

Cs.

The capacitance may in particular be provided by an IFC arranged to sense the presence in the area above the IFC of a finger or other object or substance of dielectric constant greater than 1, such as water, or a biological or chemical analyte. The operating sequence is as follows:

(i) Before interrogation of sensing nodes - all switches are open.

(ii) Interrogation starts when switch Si closes. μCi is then powered, it wakes up and monitors sensor 1. It determines a period proportional to capacitance of the sensor. This is reported back to the control microcomputer (μC_c) by appropriately closing and opening switch Sii to change the load on the bus and hence reduce the volts across the bus. After the transfer of information, μCi closes switch S12 and the first sensor unit adopts a low power-operating mode.

(iii) Since S12 is now closed, the second sensor unit is powered and its μC wakes up and repeats the operating procedure of the first sensor unit.

(iv) All sensors on the bus are sequentially addressed in this way. At the end of the sequence μCc removes power from the bus by opening switch Si. At the start of the next measurement cycle switch Si is closed.

(When the switch Sn2 is closed it will be expected that μC_n will be performing low level, low power, background tasks.)

The serial network of sensors could be made to be part of a fieldbus system by using a fieldbus interface.

A period proportional to capacitance can be obtained by using well known capacitance charging techniques.

Each microcomputer can control more than one sensor by using CMOS switches to sequentially connect each capacitance sensor to the timing circuit. An alternative conversion method for capacitance based sensing is obtained by connecting the sensor to be part of a series resonant circuit using real inductors as shown in Figure 2a, where

= 2π)L_RC_s res

or virtual inductors as shown in Figure 2b.

The sensor microcontroller will in this case generate a variable frequency sine waveform and sweep its frequency to discover the resonant condition and hence the resonant frequency. A period proportional to the period of the resonant frequency i.e. 1/Fr-s will be signalled back to the bus controller μC.

Each sensor microcontroller could control an array of resonant frequency sensors. If the sensors are positioned remotely and connected using the bus techniques described here, it will be possible to simplify the sensor circuits by eliminating the need for a controlling sensor μC. In this case a control node controls a sub-bus connecting together a number of resonating sensors with each sensor arranged to resonate at a different frequency. The resonant frequencies are sufficiently separated to allow modulation of each frequency to be monitored. The resonating frequency tells the sub-bus control node μC what sensor it is communicating with. Figure 3 shows the sub-bus as a spur on the basic bus shown in Figure 1. However, it should be noted that the sub-bus could be interfaced directly to a field bus node or other convenient monitoring point.

The basic series resonating circuit is shown in Figure 4. Note that since signal and power are supplied over the bus, the circuit shown uses the amplifier power supply connection to monitor the resonant condition. At resonance the current taken by the series resonant circuit is at its maximum value and the current taken by the amplifier is at its maximum value. Hence the ac signal across the amplifier voltage supply pins (i.e. across the bus) will be at its maximum value because of the voltage drop across the resistor supplying the ac signal to the bus.

In practice the loss resistance associated with the "real" tuning inductor causes the quality factor (Q) of the resonant circuit to be low. A positive feedback circuit can be used to generate a negative resistance that will significantly reduce the overall loss resistance and hence increase the Q. This circuit is shown in Figure 5.

As before the ac voltage drop across the bus indicates the resonant condition and each sensor is identified by its unique resonant frequency.

The "real" resonating inductor can be replaced by a virtual inductor. This is shown in Figure 6. In this case:

Note that this circuit introduces the possibility of monitoring sensors using resistance as a variable, rather than capacitance.

In some circumstances it may be desirable to interrogate remotely located, bus connected sensors by using an inductive coupling method. This would be used:

(i) for safety reasons in a hazardous environment and/ or (ii) when the cost of permanent connection to a larger monitoring system cannot be justified.

A hand-held inductively coupled device, shown in Figure 7, can be designed to provide power and monitor sensor outputs. Operating sequence for this device is: (i) Hold Hand Held Device (HHD) on to monitoring position.

(ii) Sensor μC or sensor bus μC is powered and wakes up. Period (T) proportional to sensor output is determined.

(iii) Csi is switched on to slightly detune the coupling resonant circuit: this causes a drop in amplitude which is monitored by the HHD. Hence bus-controlling μC can signal a period back to the HHD by appropriately opening/ closing the switch in series with Csi .

Low cost sensors

A wide range of measurement options is based on the use of capacitance sensors. The interdigitated finger capacitor (IFC) is a good example of a low cost capacitance sensor that can be fabricated using printed circuit board (pcb) or microfabrication techniques. The depth sensing zone above the finger electrodes is approximately defined by the pitch of the fingers (P): 95% of the current through an IFC flows in a surface layer above the electrodes to a depth equivalent to the pitch of the electrodes. Pcb IFCs with a finger pitch of 1 mm and width of 0.5 mm, and active sensor area of 2cm x 2cm have produced a capacitance of approximately 10 pF in air. This increases to 100 pF with a surface layer of water. A microfabricated IFC on a glass substrate with finger width 2 μm, and overall dimensions 10.7 mm x 9.3 mm gave a capacitance of 730 pF in air. This increased to 1600 pF with a moisture film on top of the electrodes. The IFC responds to the dielectric constant of the surface layer above the electrodes; it is not troubled by hand capacity effect's. The IFC sensor and other low cost sensors will benefit from the availability of a low cost interconnection method.

A finger can be viewed as a sack containing a saline solution so a finger placed on top of an IFC will directly increase its capacitance. If a finger is pushed onto the surface of an IFC it will spread and cover an increased surface area causing a further increase in capacitance. Finger tap can be distinguished from touch by timing the duration of significant capacitance change. The IFC can be viewed as a finger operated machine interface responding to finger touch, push, tap and roll. This will find application in, for example, access control systems using remotely located IFC finger operated sensors and cursor controllers, replacing the widely used joystick and touch pad.

Cursor control is a widely used method for controlling software-based machines. Software interpretation of the capacitance outputs from a quad array of IFCs can be used to form a very effective cursor control system. The quad array effectively performs the functions of a joystick without using any moving parts.

Multiplayer computer games based on this concept use an IFC cursor controller as a replacement for a joystick. For example, if the game is screen based, each player has an IFC cursor controller to control his actions on the game screen. The cursor controllers are linked together by a simple twisted pair serial bus according to the invention, thereby minimising wiring complexity in the player area and eliminating the need for batteries.

The IFC sensor is well suited to monitor moisture in, for example, the fabric of a building. Sensors linked by a serial bus do not require a battery. This IFC sensor is robust, so long-term operation of permanently installed sensors can be reasonably envisaged.

Overcoats can be designed to cover the surface of an IFC with a chemically sensitive material that will attract, for example, pollutants into the material and change the dielectric constant of the sensing zone and hence change the capacitance of the IFC. It is likely that arrays of sensors with different selectivity will be required. Implemented on a pcb or microsystem, the arrays will be connected by a serial bus to reduce wiring complexity. Impedance measurement rather than capacitance measurement may be required to obtain maximum benefit from these chemical sensors.

In the above description, a low cost approach to monitoring low cost sensors has been presented. The serial bus concept can be used with a wide range of sensors. However, it has been shown that the IFC based sensor offers many low cost possibilities that will benefit from being used in association with a low cost bus. Possible applications include cursor control for screen based machine control, access control, multi-player games, large-scale moisture monitoring systems and pollution monitors.

Claims

1. A sensing arrangement comprising a plurality of sensor nodes each adapted to measure a physical property which affects an impedance component of the node, the nodes being arranged in series along a bus, and a control element connected to the bus for interrogating the nodes and receiving a signal determined by each said impedance component, the control element comprising means for activating a first sensor node, which first node comprises means for activating a further sensor node and so on.

2. A sensing arrangement according to claim 1, wherein each impedance component is a capacitance.

3. A sensing arrangement according to claim 1 or 2, wherein the capacitance comprises an interdigitated finger capacitor.

4. A sensing arrangement according to claim 3, wherein the surface of the capacitor is covered with a chemically sensitive material adapted to change the dielectric constant of a sensing zone of the capacitor.

5. A sensing arrangement according to claim 4, wherein different ones of the sensing nodes have different sensitivities and/ or selectivities.

6. A sensing arrangement according to any preceding claim, wherein the control element is arranged to supply power to the sensor nodes via the bus.

7. A sensing arrangement according to any preceding claim, wherein the bus comprises a twisted pair of cables.

8. A sensing arrangement according to any one of claims 1 to 6, comprising a printed circuit board on which the bus is formed.

9. A sensing arrangement according to any one of claims 1 to 6, comprised in an integrated circuit.

10. A sensing arrangement according to any preceding claim, wherein each sensor node is adapted to respond to the control element with a pulse of duration dependent on the impedance of the impedance component.

11. A sensing arrangement according to any preceding claim, wherein each sensor node comprises a microcontroller for controlling its operation.

12. A sensing arrangement according to any preceding claim, wherein each sensor node has a resonant frequency dependent on the impedance component, and comprises means for generating a signal depending on said resonant frequency.

13. A sensing arrangement according to claim 12, wherein each sensor node comprises means for generating a signal of varying frequency to determine said resonant frequency.

14. A sensing arrangement according to claim 12, wherein each sensor node has a different unmodulated resonant frequency and the control element comprises means for applying to the bus a signal of varying frequency, and means for determining which of the sensor nodes is generating a signal depending on the frequency of the applied signal when the signal so generated is received.

15. A sensing arrangement according to claim 12, 13 or 14, wherein each sensor node includes a positive feedback circuit for increasing the quality factor of the node.

16. A sensing arrangement according to any preceding claim, wherein the bus is a sub-bus formed as a spur of a main bus, the control element forming a control node of the main bus.

17. A sensing arrangement according to any preceding claim, comprising inductive means for receiving power from and transmitting sensed data to a portable device.

18. A method of sensing a touch by a human finger, comprising providing sensor means including an interdigitated finger capacitor (IFC) and means for generating a signal depending on the capacitance of the capacitor, which capacitance varies as the sensor means is touched.

19. A method according to claim 18, wherein the sensor means includes a plurality of IFCs for sensing different commands.

20. A ' method according to claim 18 or 19, comprising steps of distinguishing between finger touches of different durations and/ or pressures.

21. A method according to claim 18, 19 or 20, wherein the sensor means comprises a printed circuit board on which the IFC is formed.

22. A method according to claim 18, 19 or 20, wherein the sensor means comprises a microfabricated IFC.

23. An input device for controlling electronic equipment, comprising at least one interdigitated finger capacitor (IFC) sensor and means for generating a signal depending on the capacitance of the capacitor, which capacitance varies as±he sensor is touched.

24. An input device according to claim 23, wherein a plurality of IFC sensors is provided, and the input device comprises a cursor control device, each sensor being arranged to move a cursor of the equipment in a particular direction.

25. An input device according to claim 24, wherein four IFC sensors are provided.