Multifunctional Sensors:
A New Concept

Temperature, pressure, and humidity sensors based on semiconductive polymers exhibit curves of the same type and slope regardless of the property the sensor is measuring.

David R. Crotzer and Eric C. Cho, IRDAM

ircuit designers are frequently required to incorporate sensors based on a variety of sensing element types into their specific circuit/system applications. But the market offers only a limited supply of sensors capable of providing the same output and/or linear curves. For example, an NTC ceramic thermistor exhibits a negative slope of resistance to temperature; a PZT ceramic transducer changes capacitance with the input of an applied voltage. To use both sensors in a particular application, the circuit designer is faced with the task of devising a different type of circuit for each device.

This article introduces a new sensing element constructed of a special, semiconductive polymeric compound. Semiconductive polymers are electrically located between conductive and insulative polymers in the resistance range of 100 to 1 M. The sensing element exhibits the same type and slope of output curve regardless of the property it is detecting: a positive change in resistance to a positive change in humidity, pressure, and/or temperature. (The sensors could also be designed to provide a negative change in resistance in response to a positive change in the measured property.)

To achieve this objective, we minimized measurement variations from one sensor type to another by directly depositing all the sensor elements onto one sensor substrate. Pins on the substrate are designed for insertion into mating sockets. On both sides of the sensor substrate are multiple Cu traces to handle additional sensing elements. Multiple pins attached to the substrate permit subsequent interconnection by means of insertion into either a plastic module with sockets in a probe or into a protective plastic case connected to a mating PCB.

The similarity of the sensors used for a given application has the effect of reducing the total power requirements. The reason is that only minimal

Figure 1. The curves and Table 1 provide the positive corresponding linear or parabolic change in resistance (H-1 or H-2) to the positive change in humidity. The values provided were measured at standard pressure and ambient temperature.

current excitation is needed to power a combination of elements, as opposed to individually exciting several different sensors. The excitation voltage required to read resistance is 1 mA per sensor element; for three elements on the same substrate (e.g., two front, one back), the voltage is 3 mA total.

In sensor control applications, the switch circuit is provided in either fixed (one preset humidity, pressure, and/or temperature trip point) or adjustable threshold (three preset humidity, pressure, and/or temperature trip points) packages. The sensing elements are placed directly on a populated PCB for this application.

Figure 2. The curves and Table 2 provide the positive corresponding linear or parabolic change in resistance (P-1 or P-2) to the positive change in pressure. The values provided were measured at 33% RH and ambient temperature.

Development of the Technology
Humidity, Pressure, and Temperature. Our first step toward creating multifunctional sensing elements was to develop semiconductive polymer sensor materials that exhibited a change in resistance corresponding to either a positive or negative change in humidity, temperature, or pressure. Several formulations were developed having the following attributes:

• Comparable curves that provide interchangeability between one semiconductive sensor material and another (see Figures 1 and 2, and Figure 3)
• Capability of volume production

  Figure 3. The curves and Table 3 provide the positive corresponding linear or parabolic change in resistance (T-1 or T-2) to the positive change in temperature. The values provided were measured at 33% RH and standard pressure.

• Flexibility of being developed into different curves for specific applications, including:
  –A linear curve that mimimizes the components in a signal conditioning circuit design
  –An exponential curve that provides a greater slope for sensor control circuits
  –Ability to function with minimal current excitation
  –Selected nominal resistance value for the sensing element of ±20%, ±10%, ±5%, or ±1%
  

The sensor substrate was designed to incorporate each sensor element onto an FR-4-based board with traces that would permit attachment to a mating PCB. FR-4 is the standard flame-retardant Class 4 circuit board material, consisting of an s-type woven glass in an epoxy with Sn/Pb-plated Cu traces laminated to the surface for component soldering.

• The substrate material minimizes the effects of the measured property, thus keeping each element's performance isolated from the others. For example, the thermal coefficient of expansion of FR-4 is lower than that of a thermoplastic

  Photo 1. The sensor module is shown with its outer protective plastic case and substrate pins for direct board attachment. The design in its current state has from one to four sensing elements on one substrate, enclosed in a single case.

  such as PMMA (polymethyl methacrylate). This characteristic, when used as a base substrate for the temperature element, significantly reduces the added effect of the substrate material on the change in resistance of the thermal element during a rise in temperature. The same holds true for humidity and pressure measurements.

• Up to four sensing elements can be deposited, two on each side of the substrate, within a small area.
• Either conductive leads or conductive pins can be used for attachment. The sensor element can be enclosed in a plastic case for protection during soldering or socketing to a platform and during actual use (see Photo 1).

The advantages of the semiconductive polymer's ability to attach directly to the circuit traces and the FR-4 surface of the PCB include:

• Minimizing connections to the PCB
• Facilitating high-volume production

Since we deposit the sensing element(s) onto a small single substrate with three parallel conductive traces, solder with pins, and then enclose the unit in a plastic case, it is a logical step in circuit design to be able to take a standard PCB with soldered components such as resistors and capacitors and deposit the sensing elements next to these components on the same plane of the board. This eliminates the need to solder a sensor component to the populated PCB. The populated board is now together, with the sensing element acting as the substrate.

Sensor Circuit and Platform Design
We have tested the sensing elements in our environmental chamber to evaluate each element's ability to provide repeatable data when subjected to repeated environmental cycles.

The signal conditioned sensor platforms selected entailed attaching the sensing elements in a protective plastic case onto PCBs connected by conductive Cu traces to a terminal block. The number of contacts in the terminal block

Photo 2. The sensor module, in its case, is mounted on a populated PCB either by direct soldering or by inserting the pins into mating sockets. A terminal block is supplied on the PCB for connecting up to four individual sensor signal outputs. This configuration also permits removal of the sensor case from the board to interchange the sensing elements when required.

is determined by the number of sensor elements included on the sensor substrate (see Photo 2). The platforms are designed to provide information from the sensing element(s) via an output of current, resistance, or voltage. The range of outputs includes:

• Current = standard 4-20 mA
• Resistance = 1 k to 1 M
• Voltage = analog, 0-5 VDC, 0-10 VDC, or 0-12 VDC

The platform is also designed to incorporate the sensor element's batch tolerance distribution into current, resistance, or voltage between ±20%, ±10%, ±5%, or ±1% of the nominal value selected.

The sensing elements were next incorporated into circuit switching applications. Those investigated included the circuit's ability to switch attached instruments such as a humidifier/dehumidifier, exhaust fan, or other sensor circuit controlled systems. The current capacity is 3 A resistive and 2 A inductive. Among the switches developed are:

• A fixed humidity, pressure, and/or temperature sensor module that provides an output to a controller, based on reaching a preset fixed sensor value. The signal tolerance distribution is designed to be within ±20%,

  Photo 3. This sensor switch, designed to reach a preset fixed sensor value, can handle up to four individual sensor outputs. A terminal block supplied on the PCB transmits each signal to the corresponding device to be controlled.

  ±10%, ±5%, or ±1% of the selected nominal value (see Photo 3). The controlling signal will activate or deactivate an attached instrument. According to the circuit, the signal changes per the curve (either linearly or exponentially) in relation to the corresponding change in the environment. The signal output to the attached instrument is designed within a preset tolerance around a fixed nominal value to provide an acceptable range to respond quickly to a change in the sensing environment. As designed, the module will accept interchangeable sensor elements, eliminating the need for multiple circuit platforms per sensor.

• A variable humidity, pressure, and/or temperature sensor module that outputs a controlling signal based on reaching the threshold value set by a calibration trimpot. At our factory, a shunt resistor is placed into the sensing element's position and the pot is then set at that value. We use three settings on the pot corresponding to the equivalent values of, e.g., humidity, which would equate to humidities set at 40%, 55%, and 70%

  Photo 4. This switch is intended as a direct replacement for an exhaust switch in a wall receptacle. The actuation lever has three positions and an adjustable pot on the faceplate to permit direct adjustment of the sensor threshold value.

  RH, respectively. The circuit also has an internal trimpot, accessed externally through the faceplate with a screwdriver, for adjustment from one nominal value to other nominal values. A calibrated grid on the switch faceplate has visual increments to facilitate setting the trimpot.

The circuit assembly, supplied in an enclosed case with an actuator lever permitting ON, AUTOMATIC, and OFF modes (see Photo 4), is designed as a substitute for a wall socket switch. Its applications include controlling an exhaust fan in an HVAC environment. Placing the sensing element substrate

Photo 5. A probe with an internal socketed sensing element substrate protected by a case can contain up to four individual sensors. The unique socketed design facilitates removal of the sensor substrate for exchange with another, facilitating probe recalibration.

into a stainless steel probe housing yields another application of the multifunctional sensor principle (see Photo 5). Summary
The principles of semiconductive polymer sensor technology have led to the development of multiple sensing elements that can be mounted on a single PCB with no crosstalk. These devices can be mass-produced or customized, depending on the requirements of the application.

Future development entails doubling the number of sensing elements on the substrate. In addition, the selection of sensing elements will be expanded to eight from the current three; these will detect dust, CO, methane, IR, and UV.

By multiplexing the sensing elements and using interchangeable sensor modules, and with circuit component miniaturization, the devices can be manufactured in low-cost packages for HVAC, building construction, and process control applications, to name a few.

Resistance vs. RH
	H-1	H-1	H-2	H-2
% RH	R/R33	R()	R/R33	R()
90	118.309	901,281	7.204	54,881
80	28.693	218,580	3.788	28,858
70	10.659	81,197	2.725	20,761
60	5.054	38,500	2.142	16,342
50	1.853	14,114	1.853	14,114
40	1.307	9956	1.404	10,695
33	1.000	76718	1.000	7618
20	0.711	5418	0.789	6010
10	0.365	2787	0.628	4787

Return to Figure 1.

Resistance vs. Pressure
	P-1	P-1	P-2	P-2
% RH	R/R33	R()	R/R33	R()
90	118.309	901,281	7.204	54,881
80	28.693	218,580	3.788	28,858
70	10.659	81,197	2.725	20,761
60	5.054	38,500	2.142	16,342
50	1.853	14,114	1.853	14,114
40	1.307	9956	1.404	10,695
33	1.000	76718	1.000	7618
20	0.711	5418	0.789	6010
10	0.365	2787	0.628	4787

Return to Figure 2.

Resistance vs. Temperature
	P-1	P-1	P-2	P-2
% RH	R/R33	R()	R/R33	R()
90	118.309	901,281	7.204	54,881
80	28.693	218,580	3.788	28,858
70	10.659	81,197	2.725	20,761
60	5.054	38,500	2.142	16,342
50	1.853	14,114	1.853	14,114
40	1.307	9956	1.404	10,695
33	1.000	76718	1.000	7618
20	0.711	5418	0.789	6010
10	0.365	2787	0.628	4787

Return to Figure 3.

Dave Crotzer is a material scientist and Eric Cho is a system developer. They are cofounders of IRDAM, Northwood Executive Park, 10 Northern Blvd., Unit #3, Amherst, NH 03031; 603-598-6351, fax 603-598-6374, www.irdam.com

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