JANUARY 1998 COVER STORY

SENSORS

Creating a sensor of achievement

Tom Shelley reports on a novel way of sensing presence or absence of objects which is a huge improvement over existing methods

Almost any solid object - such as a potted plant - can be turned into a sensor by careful monitoring of its capacitance.

The trick to the new technique, which was developed in the US, is to ensure that small capacitance changes are not swamped by huge background levels. It also ignores changes in leakage caused by variations in humidity and automatically allows for gradual drift or sudden step change.

The technique is appropriate for an almost infinite range of security, measurement and automated system control tasks. The idea originally arose as a method for controlling automatic taps in toilets, in such a way that the taps could be used as sensors regardless of water films or dirt.

Hal Philipp of Pittsburgh-based Quantum Research Group explained to Eureka that the method depends on regularly charging up the capacitance of the system being studied (Cx), then periodically connecting to another much larger capacitor (Cs). Connections are made and broken using MOSFET switches.

The capacitance of a human hand that is 300mm from an object under scrutiny is around 100 femtoFarad (1 fF is 10 -15 F). Capacitance of a potted plant to ground, as used in a demonstration performed for Eureka at the offices of Sensor Technology, is likely to be tens of picoFarads (1 pF is 10 -12 F).

Referring to the diagram, the effect of making the periodic connection is that charge is transferred, packet by packet, from Cx to Cs, raising its voltage in a series of steps. Cs has a typical value of 100 nanoFarads (1nF is 10-9). The final value is then passed to an operational amplifier and to an A/D converter to be digitised.

One effect of charging the sampling capacitor in a series of very short steps is that the measurement capacitance has very little time to leak. A whole sequence of 32 or 64 samplings takes only around 100Fs. Another effect is to perform 'boxcar averaging' of the measured value of Cx. A further advantage is that the whole sampling process need only be undertaken now and then, say every 100ms, reducing power consumption.

If periodic interference is a problem, such as that produced by fluorescent lights, randomising the measurement interval means that the measurement bursts will sometimes be in phase with the interference and sometimes will not. Passing the digital output through a median filter digital signal processing operation then separates the interfered measurements from the non interfered measurements.

A further refinement is to remove charge from Cs in known amounts so as to bring it back towards to zero. This prevents the possibility of Cs being filled and improves sensitivity and accuracy. It is always easier to measure a small change against a small background amount than a small change against a large background amount.

The emptying is achieved in two ways: in a small PCB-based unit, Cs is connect to another capacitor, Cz, the other side of which is connected to a controlled voltage source which ramps voltage down as the capacitor is charged up, reducing the measured voltage exactly to zero; in a chip based unit, four capacitors, Cz, are initially charged up by the reference voltage and then grounded one by one. The net effect is that the final measured voltage is low.

Hal Philipp and David Hampden-Smith performed a series of demonstrations and experiments for Eureka to clarify the particular strengths of the technology. In all cases the sensor module was used as part of an evaluation board kit, monitored by a laptop PC.

When an electrode was placed in the soil in a potted plant, it was quite easy to turn the plant into a sensor which could detect the presence of people and turn on a light. Using an autolearn facility, the plant and sensing system could be triggered when somebody was within 0.3m, some distance away, or only when touching the plant. The system could also be triggered when a person was within a certain distance range (proximity trap), or when they moved rather than stood still.

If the moisture content in the soil were to change, or the plant were to wilt, the system can be set up to constantly readjust its zero set point in response to slow change. If a major change should occur - perhaps the plant were pulled out by its roots - the new conditions may be taken to be the new zero set point.

Such a facility is invaluable if used to monitor sensors which have suddenly accumulated a lump of foreign matter, whether by accident or deliberate vandalism. All settings and logical procedures initiated by the PC keyboard during setup may be downloaded and stored in an EEPROM on the board, or transferred to a module unit using a previously programmed EEPROM.

An application demonstrated to Eureka that is under active discussion is the monitoring of drops of chemical from an applicator onto test dishes. If one electrode wire is attached to the applicator tube, the system detects the change in capacitance between drop and plate as the drop leaves the end of the tube. Another application switches off emissions from ground radar sets when the units are no longer in contact with the ground. In gamma ray medical scanners, the sensing system turns off the machine and raises an alarm if a patient becomes distressed and starts to move about or press their limbs against the machine.

Units have already been tested for measuring fruit ripeness by measuring water content; there is also considerable interest in using them to measure soil humidity for automated horticulture. The same system could also be used to sense water film on a car windscreen to start windscreen wipers automatically.

Security alarm applications are many and obvious, while the automatically controlled taps are still a valid use. If an electrode strip is run up the outside of a container of fluid, the system can be used to measure fluid level. This was tested in front of Eureka and found to work best if an earthing plate was placed beneath the container. Capacitative sensors are already used to measure levels of difficult fluids such as wet concrete, but none are as low cost as this system.

In chip form and 10,000-off quantities, the product can be supplied for less than 10 pounds per unit; one-off prices start at 30 pounds each. In large quantities, the chip design can be customised: one output might indicate that the sensing electrode is too near something, while another could indicate that it is too far away, for example.

Module units are priced at 127 pounds with switched output, 150 pounds with a 0 to 4V analogue output, and 180 pounds with a switched output and heartbeat indication. In this case the heartbeat consists of 5 microsecond pulses every 10ms, at opposite polarity toe the normal output. The heartbeat stops if there is a problem. An E2S evaluation board costs 450 pounds.

Sensor Consultants

Design Pointers

Low cost capacitative sensing system allows any object to be turned into a sensor, and can reliably respond to very small changes. Changes in charge leakage conditions, such as those caused by variations in ambient humidity, have almost no effect

Calibration can be made self adjusting to allow both for slow drifts in conditions and for sudden, permanent step changes. Systems can be made auto learning

Output can be discrete or used for analogue measurement

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