Adding proximity sensing to your next embedded design - Embedded.com

Adding proximity sensing to your next embedded design

Consumer electronic devices have now become a part of everyday life.Gone are the days when mobile phones were a rarity and PDAs could beafforded by a minority. With these embedded electronic devices becomingubiquitous, manufacturers of such devices are finding new ways ofpacking more features and functionality to enhance their usability andbe more competitive in the market.

One such feature that is finding its way into a number of embeddeddevices is proximity sensing. Proximity sensing in the worldof embedded devices is the detection of the presence of a human fingeror hand near an electronic gadget. This feature of proximity sensing isbeing adopted by numerous industries for a variety of applications.

Use of Proximity Sensing
Proximity sensing has found immediate adopters in the hand-held,battery- powered devices industry. The feature is seen in the latestmobile phones and music players. The primary function of proximitysensing in these devices is power saving.

The LCD display on the music player or phone remains off as long asit does not detect the presence of a human finger or hand near it. As aresult of the display staying off, only the CPU draws power from thebattery and power is not drawn to drive the LCD display.

As soon as the device detects the presence of the human hand, thedisplay illuminates, showing either the present song being played orcaller information. The average power consumed is thus reduced, therebyincreasing battery life.

Proximity sensing has become a common feature in home automation. Itis used to turn on table lamps by waving a hand in front of it. Waterfaucets turn on when the hand is placed under it.

Microwaves and refrigerators with displays on them remain blank aslong as a user is not in its vicinity. When a user nears it, thedisplay and lights turn on. New home automation systems are alsoadopting this feature. Touch screen displays used for lighting andappliance control are configured to behave as a digital photo frameunder normal circumstances.

As soon as someone nears it, it changes its display to its controlpanel and illuminating hidden touch sensor buttons. Proximity sensingcan also be used for security by setting off an alarm if it detects anunauthorized presence either near a door knob or a window pane. Extendthis idea to building smarter utensils and appliances.

Utensils that could indicate the temperature of the liquids insideit by changing surface color when a hand neared it. This could savechildren from nasty burns around the kitchen. Bath tubs that could warnyou that the water is too cold even before you put a foot inside it.

Proximity sensing can be adopted by the automobile sector to buildsmarter cars. Envision a car that sensed a driver in its vicinity andunlocked the door for him as he approached the handle. Apart from this,it would turn on the ignition and the AC or heater based on thetemperature inside the car. The car would warn the driver if the handleor the shifters were too hot when his hand approached it.

It could also instruct the driver what a knob or a button in the cardid if it detected the presence of his hand near it. Just when youthought cars could not get any smarter, emerging technology such asproximity sensing is stretching the limits.

Proximity sensing is used to display hidden touch sense buttons inkeyboards. Mice remain in sleep mode until it detects the presence of ahand. It then wakes up, turns on its radio to pair with the PC and isready for operation. This allows for better battery life. Proximitybased buttons could replace mechanical buttons to open laptops whenshut. This could be extended to include refrigerator doors, microwavesand cell phones.

Methods of proximity sensing
A number of technologies exist to carry out proximity sensingincluding, resistive, inductive, capacitive, optical, acoustic andvisual methods. Each of these technologies has its advantages anddisadvantages. Picking one against the other depends on the specificapplication, cost and usability. This article will discuss how toimplement a capacitiveproximity sensor.

Figure1: Capacitive Sensing Element

To understand how capacitive proximity sensing works, consider thecross section of a capacitive sensing element as shown (Figure 1 above ). The conductivecopper areas and conductive sensors are below an overlay material. Twoconductive elements in close proximity of each other create acapacitance called parasitic capacitance.

The parasitic capacitance is created by the coupling of the sensorpad and the ground plane. The parasitic capacitance is usually in theorder of 10pF to 300 pF. The proximity of the sensor and the groundplanes also creates a fringe electric field that passes through theoverlay.

When a conductive object like the human finger is brought near thefringing electric fields (Figure 2,below ), it adds conductive surface area to the capacitivesystem. This change in the overall capacitance of the system is used todetect the presence of the finger near the capacitive sensor element.

Figure2: When finger is brought into proximity, there is a change in overallcapacitance of sensor

Detecting and measuring thecapacitance change
The accuracy and reliability of the proximity sensor depends onaccurate measurement of the changes in the capacitance of the system. Anumber of methods exist for the same. The commonly used ones are ChargeTransfer, Successive Approximation, Sigma-Delta and Mutual Capacitancemeasurements, each having its advantages and disadvantages.

The more commonly adopted techniques on present capacitive sensingICs are Successive Approximation (CSA) and Sigma-Delta (CSD). Bothmethods use switched capacitor circuitry and use an externalmodification capacitor (CMod).

In the CSA method (Figure 3, below ),the switched capacitor network charges CMod. The voltage on CMod isthen routed through a low-pass filter into a comparator, where it iscompared to a reference voltage. A counter clocked by an oscillator isgated by the output of the comparator, the output of which is processedto determine the status of the sensor. CSA requires very few externalcomponents. It is also not affected by power-supply transients.

Figure3: CSA block diagram

In the CSD method (Figure 4, below) ,the switched capacitor input stage contains the sensor capacitor CX.The switched capacitor network is between VDD and the voltage at CMod.A Pseudo-Random Generator controls the switching frequency of theswitched capacitor network. CMod continuously charges and discharges.

When the comparator trips, the bleed resistor switch closes,discharging CMod until a new value is stored in the synchronizationlatch. The bit stream output of the latch is then 'ANDed' with the PWMand enables a counter.

Figure4: CSD block diagram

The output of the counter is processed to determine the status ofthe sensor. CSD is ideally suited for white goods, industrial andautomotive applications since it is least susceptible toelectromagnetic interference and radiated emissions.

Capacitive Proximity Sensing:Pluses and Minuses
Some of the advantages of capacitive proximity sensing over othersensing methods include:

1) Range of finger presencedetection is fairly high (30 cm+ with an external antenna sensor and 10cm+ without an antenna).
2) Sensitivity is higher thansensing methods like resistive or inductive.
3) Inexpensive as sensors canbe constructed from different media, such as copper, external wiresensor, ITO and printed ink.
4) Minimum element size
5) Temperature stability
6) Design flexibility.
7) Resistant to environmentalfactors: water, temperature, humidity.
8) Works with a variety ofnon-conductive overlay materials like glass, with varying thicknesses.
9) High reliability anddurability since they act as replacements to their mechanicalcounterparts.

But there are some potential drawbacks that should be considered,including:

1) The sensed element has tobe conductive. This makes it work well to detect human tissue presencenear it. However, it might not detect a hand approach it when the userhas rubber gloves on.

2) When the conductiveobject remains in the vicinity of the sensor, the system couldeventually recalculate its parasitic capacitance to include thatinduced by the hand in its vicinity. This could result in future falsedetects. This can however easily be remedied in software.

3) The detection range ofcapacitive proximity sensing decreases drastically in the presence ofmetal objects near the sensor. A way to overcome this issue is detailedbelow.

Increasing detection distance inthe presence of metal objects
In the presence of metal objects, the range of capacitive proximitydetection can decrease almost 15-fold. The primary reason for this isthat the metal object increases the sensor stray capacitance.

Such a stray capacitance reduces the proximity response value byproviding a higher full scale range. This often requires reduction inoperating frequency, in turn, decreasing detection distance. Thegrounded metal plane also catches part of the sensor electric field andreduces the added finger capacitance.

Since most white goods have metal casings, this feature can bedetrimental to such designs. However, the influence of a metal surfaceon a sensor is decreased by placing a shield electrode between theproximity detection sensor and the metal object, with the shieldelectrode having the same potential as the sensor. This is a fairlyeffective solution to the problem.

Conclusion
The use of Proximity sensing in consumer electronic devices has provento add a “coolness” factor to the device apart from being functionallyuseful. It might be a worthwhile consideration to give your new designsa competitive edge in the market.

Viren Ranjan is an ApplicationsEngineer at Cypress Semiconductor Corp., involved with the developmentand testing of Programmable System-on-Chip (PSoC)-based embeddeddesigns, including Capsense devices and the PSoC designer IDE, whichprovides a setup wizard for easy routing of capacitive sensors to GPIOpins, as well as a tuner to set the threshold and noise parametersbased on the specific sensor layout. He can be reached atviren.ranjan@cypress.com

References
1) ProximityDetection in the Presence of Metal Objects (AN42851) ” VictorKremin, Andriy Ryshtun, Vasyl Mandzij
2) Capacitivesensing techniques and considerations: The basics.”on Embedded.com

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