Charge transfer sensing technique is ripe for picking

Water taps and fruit are some of the applications opening up for the QProx sensor

By Sue Stuckey

Scientists working in the sunshine state of Florida are pioneering the commercial development of electrical principles first discovered in England in the 1740s. They hope to develop a gadget which shoppers can use to tell if fruit is ripe enough to eat.

At the heart of the development is the QProx sensor which works by measuring miniscule quantities of stored electrical charge transferred between two capacitors.

Charge transfer sensing, as the technique is called, works on the principle discovered by Englishman William Watson and American statesman Benjamin Franklin.

Franklin’s potentially lethal experiment of flying a kite in a thunderstorm helped to confirm the theory that electrical energy stored in one capacitor – a sandwich like arrangement of conductors and insulators – could be transferred without loss to a second capacitor.

In the QProx, modern ultrasensitive electronics measure the charge transferred between the two storage capacitors. The charge is tiny, just a fraction of a semtofarad (10-15 farads) but will vary according to the proximity of an external trigger.

Every object possesses capacitance, the larger the body the larger its capacitance. In humans, the total body capacitance is around 300 picofarads (10-12 farads). If a hand is used as a trigger, then as it approaches another capacitor some capacitance or charge will transfer into the circuit.

If the first capacitor is made to be the sensing device, then the second capacitor, which resides in the QProx circuitry, becomes the detector. A small power source such as a battery, or even solar energy, energises the first capacitor or sensing device. Split-second switching takes place back and forth between the capacitors.

The amount of charge transferred with each switch depends on the additional charge introduced by the external body which can be measured and interpreted in various ways.

Because ripe fruit has a different capacitance to unripe fruit the difference can be detected.

Capacitance sensing is not new. But the developers claim that this device `which makes overt use of the principle of charge conservation and transference’ is more sensitive and stable.

Hal Philipp, technical director of Intercole Quantum, the UK technology licensee, says the applications are almost limitless. He is the inventor and likens his work, which took five years and around £0.5m of personal funds, to the development of Velcro. `Once you invent something like that you find it can go into all kinds of different places,’ he says.

Interest in the technology is not limited to fruit ripeness detection. A leading computer manufacturer wants to use it in laptop machines.

Here the entire keyboard could be turned into a computer mouse saving the space taken by a conventional ball or tracker pad. Depressing one key activates the mouse which is then controlled by sweeping the fingers over the keyboard.

Four capacitors embedded in a metal matrix under the keyboard detect hand movement above the keys. Two capacitors sense movement in x and y axes.

Physical contact is not strictly necessary. The sensor will work with up to a 1cm gap which is useful where contact would cause damage to, say, fruit.

Low cost makes the QProx an appealing alternative to more expensive sensing systems which use photoelectric, acoustic or optical imaging techniques. A unit price of £6 is quoted for the keyboard.

Another area of interest is in water taps. The metal of the tap becomes the capacitance sensor which can be used to turn the water on and off. The cost in this type of application is around £150 because of the extra cost of the electrically operated valves.

Any tap can be fitted with QProx technology, reducing manufacturing and maintenance costs over an existing system suchas infrared which needs a special window to be moulded in the tap housing. Over time the window will get scratched, reducing the effectiveness of this type of sensor.

Intelligent software on QProx can be programmed in the case of a tap application to control the water temperature. Two finger taps on the tap housing, say, will warm the water, while three will make it hot. It is also possible to control water flow.

The software allows automatic calibration of the sensor so that it maintains its sensitivity even when battery power starts to fail.

Other potential applications for QProx are lighting control, moisture or parts sensing, material imaging and even short range data transmission.