Patent Application
20070034423
The present invention relates to touch screens and to reducing RF susceptibility in touch screens.
Touch input devices can provide a convenient and intuitive way for a user to interact with an electronic system such as a computer, video game, public information kiosk, automotive navigation system, and so forth. Many of these systems include displays positioned behind the touch sensor and in close proximity so that the display is viewed through the touch sensor. The various touch sensing technologies include those known in the art as capacitive, projected capacitive, resistive, surface acoustic wave, and infrared.
The present application discloses a touch screen system that includes a touch sensitive layer disposed on a touch screen substrate, controller electronics configured to apply substantially equivalent signals to a plurality of touch signal drives and an auxiliary drive, where each touch signal drive is coupled to the auxiliary drive through an associated capacitor. The controller electronics are further configured to determine touch input position based on signals received in response to a touch coupled to the touch sensitive layer. A plurality of touch signal lines couple the touch signal drives to the touch sensitive layer so that touch signals can be communicated to the controller. The associated capacitors can be selected to reduce susceptibility to errors in touch position determination due to RF interference, and in certain embodiments can be located on the touch screen substrate, on an electrical tail connecting the controller electronics to the touch sensitive layer, on a circuit board separate from the controller electronics, on a circuit board associated with the controller electronics, and so forth. In addition to reducing RF susceptibility in touch screens lacking a rear shield, the capacitors can function to further reduce RF susceptibility in touch screens that include a rear shield.
Methods are also disclosed for reducing susceptibility to errors in touch position determinations due to RF interference in a touch screen system. The methods include steps of providing a touch sensor of the touch screen system, the touch sensor including a touch sensitive layer disposed on a substrate and lacking a rear shield, providing controller electronics coupled to the touch sensor, the controller electronics configured to apply substantially equivalent signals to a plurality of touch signal drives and an auxiliary drive, and further configured to determine touch input position based on signals received in response to a touch coupled to the touch sensitive layer, and individually connecting each touch signal drive to the auxiliary drive through associated capacitors.
Also disclosed are methods for simulating effects of a rear shield in a capacitive touch sensor lacking a rear shield, the methods including connecting each of a plurality of touch signal drive lines to an auxiliary drive line, each connection made through a capacitor, and applying substantially equivalent signals to the signal drives lines and auxiliary drive line.
The above summary is not intended to be limiting, or to describe each embodiment or every implementation of the claimed invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.
While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It is to be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.
The teachings of the present application can be used to reduce susceptibility of touch input systems to radiated radio frequency (RF) interference, particularly in touch input systems incorporating a capacitive touch sensor that lacks a shielding layer that would otherwise help reduce RF susceptibility, but also to further reduce RF susceptibility in touch input systems that include a rear shield. The effects of a rear shield that tend to reduce RF susceptibility can be simulated, for example, by providing capacitors that couple the rear shield drive (or auxiliary drive for a touch screen system that lacks a rear shield) to the sensing layer touch signal drives to simulate the capacitive coupling that takes place between the sensing layer and rear shield and acts to cancel at least a portion of the RF signal. Reducing RF susceptibility reduces touch position errors that can be caused by the RF interference.
Touch screen controllers can be susceptible to RF conducted into the power cables, video cables, serial cable, or USB cables, and to RF radiated into the touch screen. In particular, it has been observed that certain capacitive touch screen controllers connected to capacitive sensors without a rear shield layer can be on the order of twice as susceptible to RF as compared to the same controllers connected to capacitive sensors having a rear shield layer. For the sake of clarity, and without the loss of generality, the example of capacitive touch screens will be used in the description that follows, but the reader will understand that the teachings can be applied to other types of touch screens having susceptibility to errors caused by RF interference.
The rear shield produces several nano-Farads (nF) of capacitance to the touch sensitive surface of a capacitive touch screen. The rear shield is generally provided as a transparent conductive coating on the back (display facing) surface of a touch screen. The rear shield is connected to a rear shield drive on the touch screen controller. The rear shield drive duplicates the signals sent on the corner (touch signal) drives to the touch sensitive layer. Because the rear shield and corner drive signals are equivalent (i.e., same waveform, same voltage), the controller corner drive does not experience any extra loading from the rear shield. The capacitance is only detected when an external noise signal is injected into the touch screen, and the detection of such can be used to help reduce any undesirable effects of the external noise.
Without a rear shield, an RF signal can couple directly into the four corner inputs of a capacitive touch screen controller. The RF signal acts equally on all four inputs, and as such is termed a “common mode” signal. Such common mode signals are generally not seen by the controller as noise, and therefore can be undesirably included in the touch location determination, introducing error. When a user touches a capacitive touch screen, the user becomes a load to the RF signal and draws current from each corner of the touch screen. Depending on the location of the touch, if not directly in the center, the user will draw more current from the corner or corners nearest to the touch location, causing a differential RF signal to appear at the controller. This RF signal causes an offset in the touch position depending on the signal voltage level and specific frequency.
The RF susceptibility can be reduced substantially by adding filter capacitors between the controller's rear shield drive (auxiliary drive when no rear shield exists) and each touch signal drive. By doing so, even though a rear shield may not exist on the touch sensor, the added capacitors can act to simulate the capacitance that a touch screen rear shield would have otherwise produced.
A Conducted Radio Frequency Immunity (CRFI) test can be conducted to determine RF interference susceptibility. In the CRFI test, controlled levels of RF voltage (3 V/M as per commercial specifications) at various frequencies over a range (e.g., 150 KHZ to 80 MHZ) are injected into the controller power and signal lines individually and simultaneously. The RF can also be injected into the display's video and power cables, because the controller is mounted within the display device housing. The touch screen is then operated by a user that draws continuous vertical and horizontal lines. As the frequency is stepped through its range, for example in 0.1% increments, any deviation in the line position is monitored. Failure conditions can be defined based on deviations of the lines by more than an acceptable amount, for example by more than +/−1% of the screen diagonal. If a sensitive frequency point is noted, the frequency is adjusted by hand to find a peak deviation.
Referring again to
Some capacitive touch screens do not utilize a rear shield, even though they may still utilize electronics platforms that support a rear shield drive. In such touch screens, all of the RF signal is seen by the corner drives so that a relatively large touch position offset may be observed. The offset is largest when touching or tracing at or near the edges of the screen, and is reduced near the center. According to the present invention, capacitors can be added externally to the controller that individually connect the rear shield drive to each of the corner drives to simulate the back shield capacitance on the touch screen. These capacitors provide a low impedance path for the RF signal, thereby reducing the RF induced current into the touch screen corners and correspondingly reducing the touch position offset.
By way of example, for a 17-inch diagonal capacitive touch screen without a rear shield connected to touch screen controller electronics such as the electronics available from 3M Touch Systems, Inc. under the trade designation EX II, adding capacitors between the corner drives and the rear shield drive having capacitances of about 1000 pF to 3000 pF, preferably about 1500 pF, can sufficiently simulate the capacitance that would otherwise be provided by a rear shield. The extra capacitance is not seen as a load by the controller if it is driven with identical, or near identical, waveforms between the corner drive and the auxiliary (i.e., rear shield) drive, and as such no extra power is required, and only the load from the touch is driven by the added capacitors. A 17-inch capacitive touch screen available from 3M Touch Systems, Inc. under the trade designation MicroTouch™ Cleartek II, having no rear shield and utilizing 1500 pF capacitors between the corner drives and the rear shield drive of an EX II controller was tested at up to double the normal RF testing voltage (i.e., 6 V/M), and it passed the CRFI test. Such performance is better than the performance of a typical rear shield equipped capacitive touch screen, and as such adding capacitors as described in the present invention can also be used to improve the performance of touch screen systems including a rear shield.
The EX II controller uses a narrow band signal to measure capacitance and is thus susceptible to harmonics of the fundamental controller operating frequency. The RF susceptibility test steps through the RF frequency band in 0.1% increments so most harmonics of the controller are found. The test requires the RF frequency to be adjusted to maximize the effect when a resonance is found. Several circuit components are included in the EX II controller to reduce such effects, such components including a decoupling capacitor connected to ground and a series connected ferrite element on each corner drive. These circuit elements help reduce the RF signal before it is received in the EX II ASIC. These circuits are sufficient when used with capacitive touch screens having a rear shield.
The foregoing description of the various embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
