Touch sensor Precautions During Use

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Detection Sensitivity

When determining the detection sensitivity of the sensor, be sure to fully understand the following:

(1)　When Sensor sensitivity ≦ Capacity of Touch Electrode Cp + Capacity of Electrode Wiring Cw,
the sensor keeps operating before being touched, which disables detection.
(2)　When Sensor sensitivity ≦ Capacity of Touch Electrode Cp + Capacity of Electrode Wiring Cw + Touch Capacity Co,
the sensor enables detection.
(3)　When Sensor sensitivity > Capacity of Touch Electrode Cp + Capacity of Electrode Wiring Cw + Capacity of Electrode Wiring Co,
the sensor does not operate.

Reference values for the touch capacity:

When an adult sits in a chair, and touches the palm to the touch electrode with the feet contacted on the floor : Approx.200 pF
When an adult is floating approximately 10 cm from the floor (this means for example, a person seated on a non-conducting chair and feet not touching the floor), and he then touches the touch electrode with the palm of his hand when there are no other objects nearby that are less than 10 cm away : Approx. 90 pF
When touching the electrode of 15 mm x 15 mm with a finger, on which vinyl of 50 μm in thickness is placed : Approx. 30 pF
When touching the same electrode above with a finger while wearing cotton gloves of 350 μm in thickness : Approx. 8 pF

Power Supply

(1)　The touch (detection) electrode of the touch sensor is connected to the detection circuit.
Therefore, it is necessary to ensure a sufficient withstand voltage and insulation between the touch sensor power supply and the power supply whose a large voltage enough to harm to a human body like the commercial AC power supply and Automotive battery to ensure safety.
(2)　When the coupling capacity between the primary side and the secondary side is 1000 pF or less in the DC to DC converter power supply,
couple the 0 V line of the primary side and the 0 V line of the secondary side with a capacitance of approximately 1000 pF.
In this case, assure a sufficient withstand voltage for the capacitor.
(3)　The touch sensor detects the electrostatic capacitance to ground (earth).
Accordingly, when a battery is used as a power source, connect the positive (+) or the negative (-) side directly to the housing or other with a strong electrostatic capacitance with the ground (earth),or connect it with the ground (earth) via a capacitor of approximately 1000 pF
(4)　When the sensor is located near a broadcasting station of large output, or when the DC power supply has a large common mode noise by using the power for a large inverter device, eliminate the noise to the FG side by adding a capacitor of 0. 1 μF or more on the 0 V line.

Static Electricity

In the dry environment of the winter season, the human body generates static electricity of thousands or tens of thousands of volts, and is influenced by synthetic fibers or wool carpets.
If such a person then touches a touch electrode, a static discharge occurs.
Our touch sensor can withstand static electricity discharges of 15 kV to 20 kV (under 500 pF, 500 Ω) without connecting any part to externals.
All the static electricity discharged to the touch electrode is diverted to the GND line. Accordingly, the GND line needs to be as short as possible, or the impedance needs to be as small as possible .
This anti-static characteristic depends on the wiring condition and circuit connections. Please check by using the actual product in its intended environment.
It is more effective if the GND line is routed to the F.G. side via a capacitor.

Interference due to high frequency emissions from surrounding devices

The touch sensor uses a low power high frequency signal.
For the interference due to high frequencies emissions from surrounding devices, fully consider in advance before using the sensor.

How to use the touch sensor

(1)　Detecting touch with a single electrode using a single sensor
Normal usage. No special precaution is needed.
(2)　Detecting touch with two electrodes and a single sensor As the power supply of the sensor, uses a switching regulator or other which has a significantly small coupling capacitance between the primary side and the secondary side such as approximately 15 pF. Use two electrodes, the touch electrode and 0 V electrode, and output the signal via a photo-coupler.
Do not use a coupling capacitor (approximately 1000 pF) as previously stated.

(3)　Detecting touch at two points via two sensors
The touch sensors use a high frequency oscillator circuit.
Accordingly, using two of the same sensor positioned close together causes a mutual interference, which disables accurate detection.
When this happens, use a standard sensor and an alternative frequency version of the sensor in combination to avoid mutual interference.

Usage Environment

For both use and storage of the product, avoid any place which is exposed to water, oil, chemicals or corrosive gases,

abrupt temperature changes, or direct sunlight. Dew condensation or freezing could affect the proper characteristics of the product.

Mutual Interference

When using multiple touch sensors of the same frequency for the single device, and when the same person touches the touch electrodes of these sensors,mutual interference occurs among the touch sensors. At this time, the sensor output is variable.

The mechanism of mutual interference and the variable sensor output is described in the following pages.

To understand the mechanism of mutual interference, it is necessary to know the operation principle of human detection by the touch

sensor.The operating principle of the touch sensor is described here.

Operation Principle of Human Detection

8-1.Configuration of the Touch Sensor

The touch sensors manufactured by SENSATECH (HTS-30Y/HTF/HPG/HLG series) are configured via the circuit blocks shown below

(the HTS-30Z/HTS-30L series have wiring instead of contact terminals of the fittings).

8-2.Operation Principle

(1)　A high frequency sinusoidal voltage generated by the high-frequency oscillator of Fig.1 (hereinafter known as“Oscillator”)
is connected to the touch electrode through the depolarization capacitor, static electricity protective resistance, static electricity elimination circuit, and contact terminals.
(2)　When a person touches the touh electrode, the Oscillator stops (causing 0 V to be read out).
(3)　When the Oscillator stops, no high frequency pulsating voltage is extracted in the detection circuit, and when the dc voltage is smoothed it becomes 0 V.
(4)　The smoothed dc voltage is compared with the threshold voltage set to the constant voltage in the comparator , and when it is below
the threshold value, the switching signal is output from the level discriminator.
(5)　The switching signal is amplified by the power transistor, and the sensor triggers the output signal of the touch detector.

Touch Sensor Oscillator

(1)　The frequency selectivity Q of the touch sensor oscillation circuit is significantly increased. With increased Q, as in the following Fig 2,
The gain peak value of the sine wave oscillation voltage in the oscillating frequency is increased. The larger the gain becomes, the larger the amplitude of the sine wave voltage.……(2-a)
The oscillation frequency band is narrowed.……(2-b)
(2)　The reason why the frequency selectivity Q is designed to be very large is:
・ To realize the high sensitivity of the touch sensor as several tens of pF.
・ By narrowing the oscillation frequency band, to prevent mis-operation of touch sensor due to external noise from touch electrode, power supply, or GND. By this, the frequency shifted even a little from the oscillation frequency of the high frequency oscillator is not accepted,
so the operation is normally performed without being influenced by the shifted frequency.
(3)　An oscillation circuit with a large Q can be a receiving circuit of high sensitivity at the same time. This is due to the same reason stated in (2-a) on the above page,
“The peak gain value of the sine wave voltage at the oscillating frequency is increased. The larger the gain becomes, the larger the amplitude of the sine wave oscillation voltage.”
When noise of exactly the same oscillating frequency as the one specified in the oscillator comes into the circuit from outside the touch sensor, the oscillating circuit greatly amplifies
the oscillating voltage to cause oscillation. This occurs typically with Q increased, which is the cause of“mutual interference” explained in the next paragraph.

Mutual Interference

*2: For example, when both sensors use the standard frequencies or when both sensors are common and use the alternative (B product) but same frequency.

(1)　Touching the touch electrode of the touch sensor ① by a person stops the oscillation of ①, and the direct voltage after detection and smoothing process becomes 0 V.
This value is lower than the threshold value of the discriminator, and the sensor outputs the output signal showing touch detection, allowing the sensor to be in output operation status.
(2)　With the condition of (1) above retained, when the same person approaches the touch electrode of the touch sensor②,
the weak oscillation of the touch sensor② enters the oscillator circuit of the touch sensor ① via the person’s body.
This will induce oscillation in the touch sensor oscillator ① from outside, which has stopped the oscillation, and the touch sensor ① returns to the restoration status where no output signal is output even when touched by the person.
(3)　By touching the touch electrode of the touch sensor ②, the touch sensor② tries to stop the internal oscillation.
But the oscillation by the touch sensor ① is additionally input to the oscillation circuit of the touch sensor ② via the person touching, and the oscillator voltage is prevented from decreasing,
and so remains higher than the threshold value inside the discrimination circuit, causing the touch sensor remain in reset status.
This condition, where both of the sensors ①② do not output any operation, but remain in reset status even after touch action, is the typical result caused by “mutual interference”.
(4)　The sensor has been designed so that the frequency selectivity Q becomes as large as possible.
This is why sometimes the oscillation frequencies of the sensors ① and ② are different (*3) due to the reason stated in the section [Touch Sensor Oscillator](2-b) “The oscillation frequency band is narrowed”.
When this happens, no mutual interference occurs but the sensors ① and ② can properly operate.
*3：Even the models with the same frequency can have varied oscillation frequency.
This is because of the general variations in the values of electronic component parts.
The variations are within the range of the variation of the constant. Also, the constant parameters of each part can slightly fluctuate due to the ambient temperature.
(5)　When a person touches only the touch electrode of a touch sensor ①, sometimes the touch sensor ② has an erroneous operation.
This is because of the influences of the power source and GND commonly connected to both sensors ① and ②. Via the same power source and GND, the weak high-frequency
oscillating current transmits from ① to ②, where the frequency interferes with the touch sensor ② when both frequencies coincide.
(6)　When a person touches the touch electrode, a slight variation in the oscillation frequency occurs due to the influence of human capacitance.At this time, the interference can occur although its level or method depends on which frequency is higher or how different the frequency difference.
(7)　When two sine wave oscillation voltages (expressed by sin 2 π f 1 and sin 2 π f 2) are mixed due to mutual interference, the following equation can be obtained :
sin (2 π f 1) + sin (2 π f 2) = sin (2 π (f 1 + f 2) /2) ＋ cos (2 π (f 1－f 2) /2)
*For clarification, amplitude and phase are not expressed here. f 1, f 2 are the oscillation frequencies.
A beat of low frequency (f 1-f 2) /2 (beat frequency) occurs, which can influence the sensor output due to mutual interference.
(8)　In summary, when using multiple touch sensors of the same frequency by a single device, be aware of the following:

(8-1)　When the sensors have the same frequency, the oscillation frequency caused can be exactly the same.
If the same person touches the both touch electrodes, the oscillation is transmitted to the sensors via the human body, resulting in mutual interference.
(8-2)　Even when the sensors have the same frequency, the oscillation frequency is variable in a certain range.
Additionally the frequency selectivity Q is quite large, which means the oscillation frequencies of the sensors are not always the same.
(8-3)　The constants of the electronic parts which determine the oscillation frequency will vary according to temperature fluctuations.
This is why the oscillation frequency is not always fixed to the same value.
(8-4)　Even without a person present, both sensors can be influenced by weak high-frequency oscillation current due to a common power supply or GND.
(8-5)　By touching the touch electrode, the oscillation frequency slightly fluctuates. When the oscillation frequencies match after fluctuation, mutual interference can occur.
(8-6)　The oscillation frequencies are various, and the gaps are various depending on the combination of frequencies.
(8-7)　When two oscillation voltages are added under mutual interference, it causes the beat frequency which size is half of the difference between the frequencies before the interference occurs, which influences the inner circuitry of the sensor, resulting in fluctuation in the output operation.

[Conclusion]

When using multiple touch sensors of the same frequency by a single device, basically the output operation of the touch

sensor due to mutual interference releases the detecting signal from the touch sensor contacted

(the sensor output is not triggered even after the action of touching).

However, due to various conditions as stated in the above 1) to 7), the status with the interference is varied, making the sensor output and oscillation condition variable.

Countermeasures against Mutual Interference

Mutual interference can be completely prevented. We have designed the circuit so that the oscillation frequency does not match between the sensors of different frequencies.

We have a variety of products and parts using different frequencies, so consider using them when multiple touch sensors need to be installed in a single device or machine.

If necessary, we can change the connector colors, which is helpful in controlling your touch sensor inventory for frequency type or in discriminating between them in their appearance in your assembly process.