Sensible Sensors: It’s A Control Thing

Chemical sensors can warn occupants of potential toxic agents in the air in environments ranging from subway stations to concert halls and from factories to office buildings. Demand for such sensors is projected to surpass €5 billion by the year 2012.

The greatest demand is in the industrial and environmental markets in applications such as process control, human comfort control, automotive emission monitoring, home safety alarms and, more recently, national security. The most popular method for sensing toxic agents and oxygen is through the use of electrochemical sensors but suitable control electronics that interface to the sensor are an essential design element if these sensors are to meet power and performance parameters.

WHAT ARE THEY?

Electrochemical sensors are small, self-powered micro fuel cells that produce an output signal as a result of a chemical reaction. Electrochemical sensors are lightweight and power-efficient - features that make them ideally suited for personal monitoring devices that are small enough to fit into a shirt pocket and that can operate continuously for as long as four months without the need to replace the battery.

In automobile emissions control systems electrochemical sensors positioned in the exhaust pipe feed data to the engine management computer. In this case the aim of the sensor is to help the engine run efficiently by detecting oxygen levels in the exhaust flow.  The computer uses the sensor readings to ensure that the engine is being given the right amount of fuel - too much or too little fuel can cause nitrogen-oxide pollutants, and, in some cases, lead to poor performance and even engine damage.

In hospitals, oxygen electrochemical sensors are designed as area monitors that have upper and lower alarm levels so that oxygen enriched atmospheres may also be monitored. Sensor output can be recorded on a strip chart recorder so that a permanent record of the oxygen in the atmosphere may be maintained.

In industrial applications such as oil drilling, oil refining and wastewater treatment, hydrogen sulfide (H2S) is a common hazard. Electrochemical sensors are used in these industries in portable and fixed systems to give an early and reliable warning of the presence of H2S.

SENSOR OPERATION

Electrochemical sensors (Fig. 1) use a porous membrane to control the amount of gas molecules that diffuses into a cell containing liquid electrolyte and three electrodes. The three electrodes are made of noble metal and stacked parallel to each other.

Depending on the design of the sensor, all three electrodes can be made of different material to complete the cell reaction.  The thin layer of electrolyte facilitates the cell reaction and carries the ionic charge across the electrodes efficiently. This chemical interaction generates a small current proportional to the concentration of the gas. Because of the current generated in this process the electrochemical sensor is often described as an amperometric gas sensor or a micro fuel cell. In some sensors, a scrubber filter is installed in front of the sensor to filter out unwanted gases. The most commonly used filter medium is activated charcoal (Fig. 1, again).

When gas comes in contact with the sensor, it passes through the membrane to reach and react at the surface of the working electrode (WE). The working electrode is where the potential is controlled and a current flow is generated that is proportional to the gas concentration.

The performance of the sensor deteriorates over time due to the continuous electrochemical reaction of the changes in potential occurring on the electrode. To reduce the deterioration while maintaining a constant sensitivity with a good linearity, a reference electrode (RE) is placed close to the working electrode.  The purpose of the reference electrode is to anchor the working electrode at the correct potential. In order for the reference electrode to maintain a constant potential, no current should flow through it.

The counter electrode (CE) is a conductor that completes the cell circuit. Current that flows into the solution via the working electrode leaves the solution via the counter electrode.

When the sensor is exposed to the target gas, such as carbon monoxide, the reaction at the working electrode oxidizes the carbon monoxide to become carbon dioxide, which diffuses out of the sensor. Hydrogen ions and electrons are generated.  The hydrogen ions migrate through the electrolyte towards the counter electrode. This process leaves a negative charge deposited on the working electrode.

The electrons flow out of the working electrode and can then be measured. The control circuitry that is used to measure the current is referred to as the potentiostatic circuit.  There are several variations in the implementation of potentiostatic circuits, but the function and the outcome are the same.

POTENTIOSTATIC CIRCUIT

The potentiostatic circuit’s main purpose is to maintain a voltage between the reference electrode and the working electrode to control the electrochemical reaction and to deliver an output signal proportional to the working electrode current. Figure 2 shows a simplified potentiostatic circuit which comprises three amplifiers and one JFET transistor.

The main purpose of the control loop made up of amplifiers U1 and U2 is to provide the current to the counter electrode to balance the current required by the working electrode. Amplifier U1 provides the current to maintain the working electrode at the same potential as the reference electrode.

The voltage follower (U2) is connected to the reference electrode and cannot draw any current from the reference electrode.

The input bias current of the amplifier is very critical. In the circuit shown in Figure 2, for example, a National Semiconductor PowerWise LMP7721 ultra low input bias current amplifier is used ensure that the reference electrode will maintain constant potential by having less than 3fA of bias current.  The control amplifier (U1) provides the current to the counter electrode to balance the current required by the working electrode.

The current-to-voltage converter (U3) is configured as a trans-impedance amplifier, which will convert the signal current from the working electrode into a voltage proportional to the applied gas concentration.  Output voltage is the sensor current multiplied by the feedback resistor value. The capacitor in the feedback reduces the high frequency noise. Amplifier performance is critical to the performance of the current-to-voltage converter circuit as the amplifier’s input offset voltage and input bias current will add to the sensor bias voltage, generating an error. The LMP7721 amplifier (Fig. 3) combines 3fA of input bias current with a maximum of 180 μV of offset voltage. Also, the low power consumption of this amplifier makes it suitable for portable electrochemical sensors where battery life and heat dissipation are critical.

In the circuit shown the P-type JFET is used as a switch to prevent the sensor from polarising when the circuit has no power. If the sensor is polarised, it will take the sensor a long time to stabilise and come to equilibrium. The JFET is only active when the power is off and at this time it shorts the working and reference electrodes to ensure that the working electrode is maintained at the same potential as the reference electrode. 

TAGS: Automotive
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