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Capacitive sensing for specific apps (Part 2)

Posted: 22 Dec 2011 ?? ?Print Version ?Bookmark and Share

Keywords:user interface? capacitive-sensing? tuning?

Figure 1 provides the typical design flow for implementing capacitive sensing. Firmware development, tuning and production fine tuning are the critical phases in the life cycle of a capacitive-sensing user interface design.

Firmware devt
In a broader sense, firmware implements the functionality required for the specific application; i.e., number of buttons, additional features like PWM, ADC, DAC, etc. From a capacitive sensing standpoint, the firmware does the job of scanning sensors (i.e. measuring the sensor capacitance) as well as other associated functions like processing feedback based on the sensor ON/OFF status.

Figure 1: Capacitive-sensing interface design flow.

For systems implementing only capacitive sensing, devices with configurable options are available. Registers are configured through serial communication protocols (like I2C) for specific sensor functions and no firmware development is required. Implementing capacitive sensing using a programmable device provides flexibility to meet varying user needs as well as perform sensor scanning and processing.

Tuning
Tuning is the process of determining the optimum values for set of capacitive-sensing parameters for robust and reliable performance under various environmental conditions and for different mechanical constructions of the interface. This demands a thorough understanding of how a capacitive sensing system behaves under various conditions.

The key things to be considered while tuning are

???Signal-to-noise ratio (SNR)of sensor
???Sensor scan time
???Finger threshold settings

SNR of sensor: One of the main goals of tuning a capacitive sensing system is to reliably discriminate between TOUCH and NO TOUCH sensor states. In an SNR calculation, the signal is the change in the sensor response when a finger is placed on the sensor.

Noise is the peak-to-peak variation in sensor response when a finger is not present. For reliable capacitive sensing performance, signal strength needs to be significantly larger than noise; the general recommendation is that the signal should be at least five times the noise, for a minimum recommended SNR of 5:1.

Sensor scan time: Sensor scan time is the amount of time the counter counts, as described in the capacitance measuring system section above. Shorter sensor scan times lead to lower SNR. Higher scan times lead to delayed response time and higher power consumption. Thus, based on the sensor parasitic capacitance (CP), the sensor scan time needs to be optimized for SNR, response time, and power consumption.

Finger threshold setting: The Finger Threshold is set to indicate a finger touch. This Finger threshold should be set carefully to avoid false triggering because of noise and atmospheric changes. The general recommendation is that the finger threshold should be set to 75% of the signal strength for reliable touch detection.

Figure 2: Typical tuning flow for a capacitive sensing design.

Figure 2 shows that tuning is a time consuming, laborious, and repetitive process that has to be repeated whenever the PCB or overlay is changed during development.

Production fine tuning
Capacitive sensing performance depends on the physical properties of the capacitive sensors and environmental conditions. The parasitic capacitance for a sensor varies when there is vendor change, process variation, or variation in environment such as humidity or temperature. This requires fine tuning through statistical analysis on samples during production in order to minimize yield loss due to failure. As we can see, there are many steps and issues which one needs to address before releasing a design for mass production.

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