Performance versus power in capacitive touch sensing designs
Capacitive sensing has replaced mechanical buttons in a wide range of consumer appliances, including washing machines, music players, and mobile phones, to name a few.
This has been possible largely because capacitive sensors are durable, more reliable, and provide better aesthetics through their simple yet elegant user interface that supports multiple functionalities at literally the tip of a finger.
The quality of a capacitive sensing implementation, however, depends upon its response time and power consumption. Highly responsive systems give a real-time feel while reducing power consumption improves operating life in the case of battery-based devices.
In this article, we discuss some standard capacitive sensing elements and applications with a focus on balancing power consumption and response time for embedded engineers developing capacitive sensing interfaces.
Capacitive sensing: basic principles
Capacitive sensing can sense the movement of any conductor. The human body being a conductor, capacitive sensing can be used to detect the presence and absence of a finger or trace the movement of a finger. This gives an opportunity for designers to have human finger driven GUI (Graphical User Interface) menus, adding aesthetic value to their designs.
The change in capacitance due to the presence of a finger, or any conducting material for that matter, is measured using a capacitive sensor. A capacitance sensor would typically be a metal pad separated from the ground as shown in Figure 1 below. The Electric Field lines follow the path of least resistance which start from the sensor and end in the ground plane.
Figure 1: Electric Field Distribution in presence and absence of a finger.
As shown in Figure 2 below, the introduction of a finger would form a capacitor, the finger being one plate, the metal pad being the other and air serving as the dielectric. It can be termed as finger capacitance CF. Before the introduction of the finger there would be some parasitic capacitance CP between the metal pad and ground. The finger capacitance adds in parallel to CP and net capacitance becomes CP + CF.
Figure 2. Capacitance Model
Capacitive Sensing: Basic methods
There are many capacitive sensing controllers available on the market today using various sensing algorithms such as the relaxation oscillator method, methods based on switched capacitor(SC) front-ends, mutual capacitance sensing, phase delay and amplitude measurement, etc.
One important aspect for overcoming noise in capacitive systems is to have a robust sensing method which has low input impedance. The two most prevalent methods are:
1. Relaxation oscillator method (RO method) and its variants – The RO method contracts the oscillator using the capacitance of sensor and the variation in frequency is converted into counts.
2. Switched capacitance front-end (SC method) – The SC method converts the capacitance to counts by charging or discharging a reference capacitor with the sensor capacitance at a high frequency. Switched capacitor methods have a lower input impedance than relaxation oscillator methods and so provide better noise immunity.