Sensor Ganging – Optimize power in a capacitive sensing system – Part 1
In many electronics products such as consumer, home appliance, automotive, industrial etc., capacitive touch buttons are fast replacing traditional mechanical buttons. Although capacitive buttons have many advantages over mechanical buttons, there are certain parameters that need to be considered by a system design engineer while building a capacitive sensing system. These parameters are:
- Signal-to-noise ration (SNR)
- Response time
- Power consumption
SNR is critical to ensure the robust performance of a capacitive sensing system as capacitive sensors are vulnerable to noise that is internal and external to the controller. The focus of this article is on the remaining two parameters.
Response time indicates how fast a capacitive sensor responds to a touch. There is often a tradeoff between power consumption and response time. In this article, we will discuss response time considerations that a designer has to make while trying to optimize power consumption.
Capacitive sensors need to be scanned for specific time (called scan time), based on such sensor characteristics as parasitic capacitance and sensitivity to touch. Scan time is a major contributor to the power consumption of a capacitive sensing controller. Power consumption optimization is especially critical for battery-operated devices like mobile phones and wearable devices including heart rate monitors. There are various methods to optimize power consumption, which include optimizing scan time as well as the rate at which sensors are scanned. In this article we will introduce and explain one of the salient methods called sensor ganging for optimizing the power consumption of a capacitive sensing system.
Optimizing power consumption
The important factors deciding power consumption are the scan time of sensors and the rate at which the sensors are scanned. Sleep current values are typically much less compared to active current values. Thus, when a capacitive sensing system is not being used, the capacitive sensing controller can be put into sleep mode so that the average current consumption is reduced. In order to optimize power in capacitive sensing system, a common technique of scan-sleep-scan-sleep is used (See Figure 1). With this technique, all the sensors are scanned and then the controller is put into a low power sleep mode. This is one cycle and this cycle is repeated. One cycle of scan-sleep is called one ‘refresh interval’. Each refresh interval is comprised of active time and sleep time. The ‘active time’ includes scan time, sensor data processing, and post sensor scan activities such as controlling feedback mechanisms like LEDs and a buzzer. The scan time of sensors forms a major portion of the active time.
Power consumption can be reduced by:
a) Shortening the active time i.e., by reducing the scan time or processing time post sensor scan b) Reducing the active current for a given active time
c) Increasing the sleep time
Sensor ganging
Sensor ganging is a techniques that reduces the active time of a capacitive sensing controller and hence reduces the power consumption of the controller. As the number of capacitive sensors increases, for a constant refresh interval, power consumption increases for the same number of sensors, if the refresh interval is decreased then power consumption increases. For a given number of sensors, in order to achieve low power, it may be necessary to increase the refresh interval. However, this action may affect the response time of the sensors. To achieve a good balance between response time and power consumption, you can combine all the sensors and scan them as a single sensor. This is called sensor ganging. The gang of sensors is considered as a single sensor and the capacitive sensing algorithm scans the ganged sensor as one sensor when individual sensors are ganged. Once a touch is detected and confirmed, the sensors are disconnected and scanned individually.
Implementing sensor ganging is possible in devices such as Cypress’ PSoC in which the individual sensors can be connected to a global analog mux bus. In a mixed-signal device like PSoC 4, an internal analog mux bus can be used to connect more than one sensors to the internal CapSense block just in the firmware. A reference Design Guide covering the analog mux bus, and how capacitive sensors are connected to analog mux bus is available at the end of this article.
Sensor ganging use cases
1) Ganging buttons/sliders
In an application where there are only buttons or a slider, we can gang all buttons or all slider segments and scan them as a single sensor until the user touches any of the buttons or sliders. In order to have good system response time, the ganged sensor can be tuned as a proximity sensor by setting the sensitivity to a very high value. Sensitivity of a sensor denotes the smallest change in capacitance caused by a touch that can be detected by the sensor. Because of proximity sensing, the system can respond when a user approaches the device even before the user has touched the buttons for actual functionality, thereby decreasing the response time of the system.
For example, backlight LEDs enhancing the visibility for buttons can be kept OFF when there is no activity. When the user approaches the device, the proximity sensor can detect the approaching hand and light the backlight LEDs, thus aiding the user to operate the appropriate buttons. However, as the proximity sensor is highly sensitive, it needs to be scanned for a longer time, increasing power consumption. In order to reduce power further, the ganged sensor can be tuned for lower sensitivity so that it acts as a button. This means the ganged sensor detects only the user touches and once the user touches, all the sensors are scanned individually. This method will have higher system response time than the method in which ganged sensors are tuned as proximity sensors.
2) Ganging proximity
When there are multiple proximity sensors in an application such as gesture recognition, all the proximity sensors can be ganged together and scanned as one proximity sensor to detect the proximity of a human hand in the Z-direction(away from the board). Once the hand is detected, all the proximity sensors are scanned individually for the gesture detection in X and Y direction. This will also have an advantage of quick response of the system to human hand approach because when the proximity sensors are ganged it increases the proximity detection distance and hence the human hand can be detected further when compared to the scenario when the proximity sensors are scanned individually.
3) Ganging rows/columns in a matrix or trackpad design
In an application where there are matrix buttons or a trackpad, either all the rows or all the columns can be ganged and scanned as one sensor until the user touches a matrix button or the trackpad. Ganging both rows and columns together is not required because:
a) It increases the parasitic capacitance of the ganged sensor. Parasitic capacitance must be within the maximum limit supported by the capacitive sensing controller
b) The trackpad or the matrix layout is such that just ganging rows or just columns can detect any touch on the entire sensor area
4) Ganging of hybrid sensors
Let’s take an example application where there is one proximity loop around four buttons. In this case the proximity sensor and the buttons can be ganged and scanned as one sensor. By doing this, the proximity detection range will be better than when the proximity sensor is scanned individually. This approach can be used when there is a constraint on the board size and hence the proximity sensor size cannot be increased.
In Part 1, we have briefly covered how to optimize power consumption in capacitive sensing system, utilizing sensor ganging with a capacitive sensing controller, and use cases of sensor ganging. Next, in part 2, we will cover trade-offs between response time and power consumption, as well as other issues resolved by a ganged sensor.
Resources:
AN85951 – PSoC® 4 CapSense® Design Guide
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