How to configure Charge Phase in Capacitive Sensing
One of the parameters that engineers have to know in order to properly configure the controller is called ‘Charge Phase’. Tuning the Charge Phase parameter can be done in different ways. In this post, we will show how simulation can help identify the ‘Charge Time’ values of the sensor that are used to properly tune the Charge Phase parameter value.
How a touch system works
Initially, it is important to take a look on how a touch system works. In Figure 1, we show a touch system which in its basic form consists of:
In a touch sensor one set of electrodes may serve as Receivers and the electrodes in the other set may serve as Transmitters. The Traces connect the two sets of electrodes to the controller (IC). A voltage with a specific waveform (usually square wave pulse) is applied to the Transmitting electrodes from the IC and then the voltage response is measured through the Receiving electrodes by the Measurement Circuit inside the IC. This method is called Charge-Transfer modulation and it is one of the most used methods of measuring changes in the capacitance of a touch sensor.
To get into more detail, this method works by charging a large sampling capacitor (Cs) in several steps using the charge that is stored in the sensor capacitances as well. The sensor capacitors are smaller and will be charged much quicker than the sampling one. Sensor capacitors will, then, be discharged to the sampling capacitor. This process is performed many times until the sampling capacitor also reaches a charged threshold.
As shown in Figure 2, the time needed for the voltage response of the Receivers to reach the steady state – approximately 90% of Vout – is called ‘Charge Time’ (tc). The period that a charge is applied to the sensor is called ‘Charge Phase’ (Cph), followed by the ‘Transfer Phase’ (Tph) which is the period of the discharge to the large capacitor (Cs). Charge Time values are defined in the nodes of the Receiver and Transmitter electrodes of the sensor. For example, in a touch sensor that uses 20 Receivers and 30 Transmitters, the sensor will have 600 individual Charge Time values. By identifying the Charge Time values of the sensor, the engineer will be able to determine the proper Charge Phase input value in the IC firmware, as well as, to understand potential mistakes in the design of the touch sensor.
How to tune Charge Phase in touch sensing
In practice, engineers choose two ways of tuning the Charge Phases of a touch sensor:
The first way is the ‘firmware-tuning way’, in which the engineer configures the duration of the Charge Phases through firmware and then measures if the sensor(s) is fully charged and discharged. If the measured voltage reaches the desired voltage, then the charge transfer is ideal (Figure 4) and it works.
If the measured voltage does not reach the desired voltage, the charge transfer is non-ideal (Figure 3) and the Charge Transfer period needs to be adjusted, potentially together with other parameters as well. Although the above process is considered a trusted way for engineers to configure the controller, it creates a lengthy procedure with multiple lab measurements that might result in configuring the controller in a functional but suboptimal way.
The second way is the ‘simulation way’, where the engineers identify the Charge Time values of the touch sensor by simulating the operational parameters of a touch system that determine its performance. Knowing the Charge Time values, it will allow the engineer to determine the Charge Phase parameter in controller’s firmware. Below we will explain the process for identifying the Charge Time in a touch sensor using simulation. To identify the Charge Time of a touch sensor, the following steps are needed:
How to extract R,C Values
The first step is to extract the resistance and capacitance values of a single cell (1 Transmitter and 1 Receiver) as shown in figure 5 below:
Figure 5. R, C values in a single cell
- Rx/2 is the half resistance of X electrode in this single cell
- Ry/2 is the half resistance of Y electrode in this single cell
- C_mutual is the mutual capacitance between X and Y electrodes
- Cx_self is the self capacitance of X electrode
- Cy_self is the self capacitance of Y electrode
Using Fieldscale SENSE you can easily design your touch sensor or import any DXF or Gerber file of your touch sensor. Then you can calculate the Resistance and Capacitance values with prototype-level accuracy. Figure 6 below shows a single cell of a Double Diamond sensor in the environment of SENSE.
Figure 6. A single cell of a Double Diamond sensor in SENSE
How to create an RC equivalent circuit of the whole sensor
Equivalent circuit refers to a realistic representation of the components of an electrical circuit and its aim is to represent the signal flow. Fieldscale SENSE is the only simulation software that provides the ability to export a Netlist file with the RC equivalent circuit of a sensor by automatically analysing and combining the R and C values of the whole sensor.
If you want to get a better understanding on schematic diagrams on touch sensors, in our ‘Guide to Capacitive Touch Sensors: Chapter 3’, we provide a guide to equivalent schematic diagrams for touch button and touch screen applications.
Run SPICE analysis (circuit simulation)
Figure 7. Spice simulation for Charge Time
As a next step, a SPICE analysis for the whole circuit is needed to get results that will allow you to calculate the Charge Time. In order to run a SPICE analysis, it is required to know the following information:
a) The RC equivalent circuit. This is described in the Netlist file as extracted from SENSE in the previous step.
b) The controller’s (IC) functionality in the circuit. For Charge Time estimation, the controller’s functionality can be simplified by the voltage value (ex. 5V) that the controller applies to the touch sensor. In order to obtain stable results it is important that the voltage input is allowed to settle properly and hence transfer all the charge into the electrodes’ capacitances.
If you want to get a better understanding on touch controllers, you can download our free eBook ‘An introduction to Controllers for Capacitive Touch Sensors’ where we describe a few Common Tuning Parameters including Voltage and Signal values.
How to calculate the Charge Time
The final step is to calculate the Charge Time values of the touch sensor. The goal here is to find the X,Y node with the highest Charge Time value and use it as a reference point to determine the Charge Phase parameter in the configuration of the touch controller. The most common practice for the engineer is to configure the Charge Phase value, is to set a value that is a little bit higher than the highest Charge Time value that we found. This way the engineer is confident that the touch sensor will operate properly after the integration in the touch system where other systems might affect the C,R values of the sensor. For example, noise sources.
Figure 8 below shows an example of a Charge time map with Charge Time values for every node of the sensor by processing the output of SENSE. Building a Charge time map is a great way for the engineer to quickly identify the touch sensor’s performance and quickly identify flaws or areas of improvement.
Figure 8. Charge Time map
At this point we have identified the highest Charge Time value of the sensor that is needed to properly configure the Charge Phase value. Does that mean we are all set? Well almost, the process above will provide the Charge Time values of the touch sensor in ‘stand-by’ mode. Now we need to consider the effect of a touch event on the touch sensor.
How touch events affect the Charge Time?
Simulation software SENSE provides the functionality of simulating the influence of a finger on the capacitance values of a touch sensor. It provides an easy and quick way to measure the C values on the nodes of interest of the touch sensor under a touch event by a pointer, either it is a testing probe, a finger, a stylus or finger in a glove. Figure 10 shows the Charge Time difference on selected nodes of a Double Diamond touch sensor, as extracted by processing the outputs of SENSE.
Figure 10. Charge Time with touch / no touch
Proper configuration of the touch controller is not simply inputting values in hopes that it will work. Touch applications are becoming more and more complex these days, demanding engineers to work with new IC vendors and/or new touch technologies in order to meet customer specifications. Identifying the Charge Time of a touch sensor requires an in-depth understanding of how the touch sensor performs and using the right simulation tools can provide a cost-effective way of properly configuring your touch controller.
Try out SENSE, the only simulation software for complete touch systems. Get your SENSE trial and start using it now!