Chapter 6:
Basics of controllers for capacitive touch sensors
6.3 Common tuning parameters of capacitive touch MCUs.
So far, we’ve performed extensive analysis of the charge transfer method. As was mentioned earlier, the charge transfer method is simply one of (albeit, the most popular) the methods of operation of capacitive MCUs.
Performing similar analysis for other methods of operation would be a challenging task, not only due to the complexity of certain MCUs, but also because of the secretive nature of the details regarding the operation of some of them.
However, despite the complexity and the differences between the various methods, certain patterns emerge when it comes to the tuning process of a controller. Specifically, there are 4 common parameters that one should expect to encounter (in some form or another) when undertaking such a task. These are:
1. Drive signal time
2. Drive voltage
3. Sense signal gain, and
4. Touch threshold.
Let’s break down each of these, to give you a taste of what you should expect from the tuning process of a capacitive MCU.
Drive signal time
As described in previous chapters, in capacitive sensors, there is a drive electrode and a sense electrode. The drive electrode is energized by the controller, before the sense electrode is measured.
However, if you are familiar with electromagnetics, you know that the drive pulse does not propagate instantaneously from one side of the electrode to the other. Some delay is introduced, called propagation delay. Propagation delay is the time it takes for the pulse to reach its destination (which in our case is the other side of the electrode). Without getting in too much detail, propagation delay depends heavily on the length and the resistance of the electrode.
So, we can expect propagation delay to differ from case to case. In cases where we have small sensors or sensors with little electrode resistance, the propagation delay will be low. The opposite is true for larger sensors or sensors with high resistance electrodes.
A reasonable approach seems to be setting the drive signal time high enough for the pulse to reach the other end of the electrode even in the worst case scenarios, without optimizing for each sensor. This would be bad practice for two reasons:
1.The power consumption would increase, and
2. More noise would be injected into the electrode measurements.
While the first point seems straightforward, the second does not. To measure the capacitance of the sense electrode, we use a circuit component called the integrator. As its name suggests, the integrator integrates the voltage of the sense electrode. So, the longer the drive electrode is energized for, the longer the sense electrode will be integrated for. That will cause more noise to be injected into the measurement.
Thus, a fine tuned system should excite the drive electrode for exactly as long as it takes for the pulse to reach at the end of the drive electrode.
Drive voltage
In most cases, controllers offer some settings regarding their voltage levels. Touch controller voltage levels vary depending on the controller.
Some controllers use their core voltages to drive the electrodes, which in most cases is 1.8V or 3.3V. In many cases controllers can achieve higher voltage levels, either by using voltage doublers or tripplers, or by using external power sources. In controllers that use such components, voltage levels can reach up to 10V or 40V respectively.
Using higher voltage levels in capacitive sensing can have both pros and cons. The main advantage of using a higher voltage level is the improved SNR that it can help achieve. Increasing the voltage level makes the sense signal stronger, and therefore SNR higher. Another advantage of higher voltage levels is their better performance in applications where thick front panels are used, or users wearing gloves are expected to interact with the device. Higher voltages cause the range of the E-field to expand, enabling the sensing of conductive materials that are further away from the sensor.
But using higher voltage levels does not come without its drawbacks, with the main drawback being that of higher power consumption. The power consumption of the controller might be a cause for concern in battery-powered devices.
So, when it comes to tuning the drive voltage level a compromise has to be made between the range and robustness of touch detection and the power consumption of the MCU.
Sense signal gain
The signal that the sense electrode receives is very low, typically less than 50 mV. This is below the dynamic range of the measurement circuit. The dynamic range in this case can be defined as the ratio between the maximum detectable level of sense voltage, to the minimum detectable voltage of the same parameter.
In order for the receive signal to fall within the dynamic range of the measurement circuit, it needs to be amplified, so it goes through an amplifier with a setable gain. The sense signal gain parameter expresses this level of internal gain on sense measurements applied by the controller’s amplifier.
But why isn’t sense signal gain a tunable parameter and not fixed?
This has to do with the touch sensors themselves. Different touch sensors will cause different sense signal levels. To make matters even more complicated, the same sensor can cause a different sense signal level depending on its mechanical configuration and various environmental factors. For example, a change in the thickness of material of the front panel of a touch sensor can have a huge impact on the sense signal. A thicker cover glass will result in lower sense signal gain.
By using an amplifier with a settable gain, the controller becomes more versatile. Having a settable gain makes the controller compatible with more touch sensor designs, as you can tune it to make sense signals fall within its dynamic range.
Touch threshold
The above parameters make sure that the signal that is received by the controller is accurate and strong enough to be useful. The controller uses this signal to figure out if a touch event has occurred or not.
To perform this task, controllers use a cut-off point to compare against the received signal. Below the cut-off point, it is considered that no touch event has occurred. Above it, it is considered that a touch event occurs.
The touch threshold is the parameter that determines how high or low that cut-off point is. Touch threshold can also be referred to as detection threshold or sensitivity.
Though it is certain that most controllers use these four parameters internally in some form, they might not always be available for tuning. This is especially true for more sophisticated sensors that tend to offer parameters that do not directly correspond to the ones discussed above.
Key takeaways from this section:
Even though different controllers offer different tuning parameters, the most common ones are:
1. Drive signal time
2. Drive voltage
3. Sense signal time
4. Touch threshold
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