Capacitive sensors technology is based on capacitive coupling which detects human body capacitance. The capacitive touch screen consists of an electric insulator such as glass, which is plated with an electrically conductive material. When human touches the glass, it creates a change in the electric field that can be detected as capacitance change by the sensor. In Figure 1 we can see a typical capacitive sensor.
Figure 1. iPhone touchscreen sensor
A good example about capacitive sensors is the touch screen of iPhone. It has a circuitry which can detect changes at each point along the grid. Every point has its own unique signal when touched and relays that to the iPhone’s processor. This way of detecting human touch gives iPhone the ability to determine the location and movement of simultaneous touches in multiple locations on the phone’s touch screen.
Because of iPhone’s reliance of this capacitive material, iPhone’s touch screen doesn’t detect with stylus or non-conductive gloves. In Figure 2 we can see inside iPhone’s touch screen.
Figure 2. Layers of iPhone
How does capacitive sensor detect touch?
Figure 3. Pre-processing circuit for capacitance measurement
Capacitive sensor coverts measured capacitance into a digital value called Raw Counts. With Raw Counts we can detect is the finger on or near the sensor since the sensor can detect change of capacitance even if you are not touching the screen. Figure 3 is a block diagram which shows capacitive touch sensing pre-processing circuit.
- 2. Larry K. Baxter (1996). Capacitive Sensors. John Wiley and Sons. p. 138. ISBN978-0-7803-5351-0.
- http://www.capacitive-sensing.com/ More about capacitive sensors. Math and applications.
Physical Principles of Sensing -Capacitive sensors
Touch Switch D5C
Figure 1. D5C sensor
D5C sensor is mainly used in the industry.
Figure 2. Tempeture characteristics DC and AC
In figure 2 there is one comparison between DC and AC models. In these pictures is sensitivity when temperature is changing. In these pictures we can see comparison with DC and AC models. AC models are more sensitive but they can change less sensitive when temperature increases.
Below is all the information I could find about D5C sensor.
|Degree of protection||Equivalent to IP67|
|Mechanical durability||10,000,000 operations min. (at rated overtravel value)|
|Supply voltage (operating voltage)||12 to 24 VDC (10 to 30 VDC), (ripple: 10% max.)||100 to 240 VAC (45 to 264 VAC), 50/60 Hz|
|Rated frequency||---||50/60 Hz|
|Sensitivity setting range||30 to 100 pF|
|Current consumption||17 mA max.||---|
|Leakage current (Circuit/Antenna)||------|1 mA max.||2 mA max. |1 mA max.|
|Response time||2 ms max||8 ms max|
|Output current||200 mA max. (resistive load)|
|Insulation resistance||50 MΩ min. (at 500 VDC) between lead wires and case|
|Dielectric strength||1,000 VAC, 50/60 Hz for 1 min between current-carrying metal parts and non-current-carrying metal parts||2,000 VAC, 50/60 Hz for 1 min|
|Rated insulation voltage (Ui)||1,000 VAC|
|Pollution degree (operating environment)||3 (IEC947-5-1)|
|Protection against electric shock||Class II|
|Proof tracking index (PTI)||175|
|Switch category||D (IEC335)|
|Vibration resistance||10 to 55 Hz, 1.5-mm double amplitude|
|Shock resistance||1,000 m/s2 min.|
|Ambient temperature||Operating: −25°C to +70°C (with no icing)|
|Ambient humidity||35% to 95%RH|
|Weight||Approx. 110 g (in case of D5C-1DS0)||Approx. 120 g (in case of D5C-1AS0)|
Interface electronic circuits
AD7142: PROGRAMMABLE CONTROLLER FOR CAPACITANCE TOUCH SENSORS
AD7142 is connected to the sensor with excitation source and CIN wires. Sensor itself is integrated to the circuit board.
Figure 1. AD7142 circuit board
How the input signal is conditioned, e.g. how the signal from the sensor is bring up to the format compatible with the load device?
Figure 2. When is the sensor avtive and not active
The practical response from the sensor is defined by the converter’s output change when the sensor goes from inactive to active. When the sensor is not active the AD714x sensors measures the capacitance value as ambient value as seen from Figure 3. As previously explained in Position sensor page, the capacitive increases when the finger is touching screen or a button and it decrease when finger is moving away of the screen or button.
Figure 3. Ambient cpacitive value
What is the electrical input impedance of the sensor?
Maximum Output Load 250 pF(Capacitance load on source to ground).
On which parameters does it depend on? e.g. frequency, temperature, etc.
Capacitance sensor output levels are sensitive to temperature, humidity, and in some cases, dirt.
Does the signal need amplification?
Figure 4. Grumpy cat
Does the sensor need excitation current or voltage? How much?
It uses 250kHZ excitation source, but there is no mention of current or voltage.
What other analog electronics are needed for the signal conditioning?
It has two DACs(Digital-to-analog converter) to null any capacitance sensor offsets.
How the signal is converted to digital format?
It uses capacitance-to-digital(CDC) converter which is Σ-Δ architecture with 16-bit resolution. It has a sampling frequency of 250 kHz.
Does the A/D conversion happen already in the sensor or is it typically converted in PC or embedded system?
It happens on the circuit board. It is possible to program two DACs.
How much there is noise? And what kind of?
It has parasitic capacitance of 40 pF.
How is the sensor calibrated?
It has on-chip calibration logic to account for changes in the ambient environment. Calibration works when sensor is not touched. The calibration sequence is performed automatically and at continuous intervals. By this way the calibration ensures that there are no false or nonregistering touches on the external sensors due to different environment changes.
Figure 6. Sensor calibration
In Figure 6 we can see how exactly calibration works. It goes offline, when user touches sensor, and enables itself after user leaves sensor area and small delay.
- http://www.analog.com/en/analog-to-digital-converters/capacitance-to-digital-converters/ad7142/products/product.html AD7142, PROGRAMMABLE CONTROLLER FOR CAPACITANCE TOUCH SENSORS
- http://www.analog.com/static/imported-files/data_sheets/AD7142.pdf AD7142 datasheet
Materials used in capacitive sensors
Capacitive sensor material has to be conductive. It doesn't really matter from what material sensor is made, so long it's conductive. Common conductor capacitive sensing materials include copper, ITO (Indium tin oxide), glass, acrylic and silver ink. The resistance is also a big factor when choosing the correct material.
ITO for example has a high resistance, which might need a PreScaler, which is used to slow down frequency in the CSD User Module for Capsense to facilitate optimum performance. ITO is broadly used in touchscreens and handling. In manufacturing ITO has some disadvantages. Its materials are fragile and heavy, and the manufacturing process is labor intensive and expensive. Touchscreens today use 1, 2 or 3 layers of ITO depending on the specific product design considerations and the touch panel supplier technical capabilities. More information about touchscreen layers can be read below.
In this article, we will be studying touchscreens of iPhone and Samsung Galaxy. Newest iPhone and Samsung Galaxy touch screens have been constructed using several layers of materials. This is called 'stackup' which can be seen in Figure 1.
Figure 1. Typical stack sensor with multiple layer technology
The top layer is a protective layer, which is made from glass with an anti-scratch coating, or PMMA (polymethyl methacrylate), which is also called acrylic or plexiglas. Directly underneath the surface layer is a layer of thin adhesive and then the electrically conductive layers for touch sensing. In Figure 2 we can see 3 different touchscreen examples. They show just how thick each layer actually is.
Figure 2: Example of layering and thickness differences in sensor designs
Figure 3: Typical touchscreen manufacturing process flow
In figure 3, we can see an example of a typical process flow for manufacturing ITO-based sensors. Steps include sputtering ITO powder over glass, thermal baking the ITO to its melting point and creating a conductive layer and then etching the sensing circuit topology on the conductive layer by use of laser lithography or photo. Every one of these steps adds cost as a result of materials cost, manufacturing time, and yield loss.
- 2010, Materials used for capacitive sensing http://www.cypress.com/?id=4&rID=36843
- 2013, Trevor Davis, Cypress Semiconductor, Reducing capacitive touchscreen cost in mobile phones http://www.embedded.com/design/system-integration/4407698/Reducing-capacitive-touchscreen-cost-in-mobile-phones-
Ultrasonic sensors emit a high frequency sound waves and evaluet the echo which is received back by the sensor. The principle in ultrasonic sensors is pretty much the same as on radars and sonars, after sending the sound wave, the sensor calculates the time it takes between sending the signal and receiving the echo to determine how far the object is from the source. Capacitive sensors are also called as transceivers/transducers, and this is because the sensor sends and receives at the same time.
The formula for calculating how far the object is, is really simple, basically you just measure the time it takes for the pulse to travel, multiply it with the speed of sound which is 341m/s in air, and divide the whole thing with 2, since the pulse travels from the sensor to the object and back.
One of the drawbacks is that the ultrasonic sensor is that there is no way of telling the difference between small and large objects because the pulse that's emitted is cone shaped, because of the shape, an echo will be returned by all objects the pulse comes into contact with. One way to work around this is to use rotating sensor or multiple sensors to find out the shape and objects size.
Figure 4 (Ultrasonic pulse from the transducer sensor)
In industry, ultrasonic sensors are used to detect movement of targets and to measure the distance to targets in many automated factories. As for example, they can be used to sense the edge of material as part of a web guiding system.