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What’s to consider in a sensor design?

Sensors are a crucial part of many areas of our daily lives. They are therefore present in nearly all areas of the electronics industry, such as automotive, industrial, medical or aeronautics. They pick up valuable information about their surrounding and make them machine-readable and interpretable. This enables modern applications to work in a very detailed and specified way. Disruptive technological changes, like the Internet of things (IoT) in the private or industrial sector would not be possible without the monitoring capacities, that sensors provide. There are numerous variables to consider in a sensor design. Therefore we provide you an overview of sensor types, and how to use them in the design of your sensor application.

What exactly is a sensor?

IPO principle sensor

Generally speaking, a sensor is an electronic component, that is able to receive non-electrical input, and translate it into a machine-readable set of data. Non-electrical input could be a physical value, for example, temperature.

They pretty much work like a simplified version of the IPO principle:

A certain input gets processed and leads to an output.

Since there are many different types of input variables, there is a great portfolio of different sensors. The enormous aount of factors you have to consider in a sensor design extend this portfolio even further. Because you will need to pay attention of what is going to be measured, under which conditions, how tolerant can these values be, how will the data be transmitted… the list goes on and on. That’s why we wanted to explain different types of physical input variables and their sensor counterparts, in the infobox below.

Categories of sensors

Sensors can be divided into different categories. Common classifications are analog and digital, or passive and active sensors.

Passive sensors consist of passive components, such as resistors or capacitors. These change their electrical properties according to the physical impact on them. For example, an increase in ambient temperature leads to a change in resistor value. Which is then picked up and processed by another component. To make this possible, an external power source is always necessary.

Active sensors convert mechanical, thermal, or chemical energy into electrical energy. Therefore, these are current creating components and don’t necessarily need an external power source. Piezoelectrical components are an example of active sensors

Acceleration

An often-used method to measure acceleration is the usage of micro-electromechanical systems (MEMS). These measure the movement of a silicon pendulum. The measured shifting indicates an acceleration. Measurement usually is done capacitive.

Another way is to use piezoelectrical sensors. The deformation of a piezo crystal generates a current, which is then used to measure the acceleration. These sensors are called accelerometers. This type of measurement can also be used to quantify torque or force.

Magnetic Field

Hall sensors are used to measure a magnetic field.

These consist of thin semiconductor plates. If a current is applied, and the sensor is placed in a magnetic field, the accumulation of electrons generates a potential difference, which is a current. The so-called hall current is proportional to the strength of the magnetic field the sensor is placed in. In addition to that, sensors based on magnetic resistive (MR) measurement can be used, which measures the resistance value of conductive surfaces in a magnetic field

Angle

Angle or position sensors measure one object’s position relative to the steady sensing object. One way of measuring an angle is to use an optoelectrical scanning of a code disc. The disc has a defined reference position in its’ normal state. Every deviation from this position gets picked up by the optoelectrical element.

Another possibility for angle sensing is the usage of a turning magnetic field. A hall sensor placed in a steady position then picks up the exact position of a turning permanent magnet. This type of sensor is also called gyroscope.

Current

Every conductor creates a magnetic field when electrical energy gets applied to it. The strength of this magnetic field is proportional to the applied current. Therefore, the current can be measured accordingly to the magnetic field.

To measure this, usually, hall sensors or magnetic resistive sensors are used. An advantage that the usage of hall effect sensors provides, is the possibility to measure all kinds of current signals.

Humidity

Humidity sensors are very often capacitive components, due to their reliability and simple structure. They consist of a hygroscopic dielectric layer between two plates of electrodes.

An increase or decrease in humidity causes a shift in the hygroscopic dielectric layer. This causes a change in the capacitance value of the component. To measure the relative humidity present, the change in capacity is used. The same principle is utilised in resistive humidity sensors. Instead of capacity of the sensor, the resistance value is measured. Output voltage of the sensor changes accordingly to an increase or decrease in humidity in the sensors environment.

Proximity

Proximity sensors are used to measure if an object is coming close to the sensor’s surface. They usually work without mechanical parts and are therefore touch-less. Options to measure proximity are optical, capacitive, and inductive proximity switches.

The decision on which parts to use depends on the final application. For detecting a metallic target it is useful to use an inductive proximity sensor. However, these cannot detect other materials.

If the proximity of polymer or plastic targets needs to be measured, a photoelectric sensor is the better choice.

In smartphones, usually a combination of an infrared LED and an IR detector is used. The time span between the emmission of the infrared light and back to the detector determines the distance between smartphone and user. For example, when holding the phone to your ear.

Pressure

There are different types of pressures, for example, stationary pressure, pressure differences, or sound pressure.

Therefore, the type of pressure decides the needed type of sensor. There are piezo-resistive, piezo-electric, hall elements, capacitive, and inductive pressure sensors.

A piezo-electric pressure sensor uses the generated charge of a piezo crystal according to the applied pressure level. Pressure hits a diaphragm, which then transfers the applied force onto the piezo element. The inverse piezo-electric effect makes these sensors unsuitable to measure constant pressure levels. The applied voltage causes the piezo element to shift back to a normal state, even when pressure is present. However, these are very well suited sensors to measure small changes in pressure levels

Temperature

Temperature sensors are available in a variety of variants, depending on the usage in the application.

The range starts at simple temperature sensitive resistors, also called thermistors, and ends with sophisticated semiconductor components with a digital output. Using a thermistor, the working principle is quite similar to the resistance humidity sensor. A change in temperature leads to an increase or decrease in resistance value inside the component. These changes can then be monitored and furtherly

As shown, there is a vast portfolio of different sensor types out there. The depicted criteria were chosen in order to have a categorization of sensor types according to environmental conditions. There are many more possibilities for sorting sensor types. That’s the reason the list doesn’t show special types of sensors, such as liquid flow or vibration sensors. In addition to the described conditions above, there are always several varying sensor types based on different physical effects.

Topics to consider in the design of your sensor application

The final choice for a certain type of sensor is always application-specific. When designing a sensor application it is crucial to take all working conditions into consideration, the sensor and the application itself need to perform in. Otherwise, the sensor could transmit wrong information and cause a malfunction of the finished gadget.

Environmental operation conditions, such as temperature, humidity, or dust can easily affect sensor functionality. Typical consumer applications usually require sensors that can dependably work in moderate temperature ranges of 0 to +50°C. However certain industrial working conditions can expand this range from -20 to +85°C. It is important to consider a potential offset in the data output of the chosen sensor, due to changes in environmental conditions. A pressure sensor, for example, can emit data with an offset of ±0.75 Pa/K.

In addition to the environmental conditions, the electrical properties of the application need to be taken into consideration. When designing a PCB layout, space is often a rare resource. That’s why the sensor’s package size is an important factor.

Depending on peripheral components on the board, there are several given characteristics, such as voltage or data interface. If these vary from the sensor’s operation values, additional components like voltage regulators or converters need to be included in the circuit.

Measuring Range and sensitivity of the chosen component needs to be matched with the operating conditions of the final application. If the product is used in a very wet climate, and the chosen sensor has a low tolerance towards humidity, the given data might lead to a complete failure of the entire application. The topic of sensors and sensing applications is a very vast field of science. This also explains the variety of key factors to consider in a sensor design.

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