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Understanding sensor ganging (Part 2)

Posted: 20 Jul 2015     Print Version  Bookmark and Share

Keywords:sensor ganging  power consumption  touch sensors  proximity sensor  LED 

In Part 1, we briefly tackled how to optimise power consumption in capacitive sensing system, utilising sensor ganging with a capacitive sensing controller, and use cases of sensor ganging. In part 2, we will cover trade-offs between response time and power consumption, as well as other issues resolved by a ganged sensor.

Response time vs. power consumption
Before discussing the advantages of sensor ganging, let us discuss the trade-offs between response time and power consumption in a capacitive sensing system. The response time of a system is defined as the time required by a system to give a valid output from the time the corresponding input is received.

In capacitive sensing applications, it is not easy to optimise both power consumption response time as it takes a considerable amount of time to scan sensors. The refresh interval can be varied to optimise either power consumption or response time. But it is not always possible to get the desired level of optimisation by controlling the refresh interval alone. If the device is optimised for power consumption, then response time will be slower, and the opposite is also true.

Figure 6: Typical response time vs. current consumption curve.

Another technique is used to optimise response time and power consumption. When a user interacts with the device, the capacitive sensing system can be optimised to respond quickly and when the user does not touch the buttons, the device can be optimised for power consumption.

To achieve the optimisation of these two key parameters – power consumption and response time – the refresh interval needs to be adjusted. For optimising response time, the sensors need to be scanned at a high rate and hence the refresh interval needs to be small. Let us call this quick scan mode. For optimising power consumption, the sensors need to be scanned at a lower rate as the device needs to sleep for more time, hence the refresh interval needs to be larger. Let us call this slow scan mode.

Power consumption varies between these two modes. The scan modes can be switched between the two modes dynamically to optimise power consumption and response time. When the user does not touch buttons, (i.e., when the UI is idle) the capacitive sensing controller is put into slow scan mode thus optimising the power. When the user has touched the buttons the mode is changed to quick scan mode to optimise response time. In this section of this article, we will see how much power can be optimised with ganged sensors when combined with this technique of switching between quick scan mode and slow scan mode.

Problems solved by a ganged sensor
Power consumption
Now let us take an example application and see how sensor ganging helps in power optimisation. We will also use dynamic scan mode switching as discussed in the previous section to optimise both response time and power consumption. Here is the procedure for average power consumption calculation:

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