Sensors. Sensors can be transducers. A transducer is a device converts one form of energy into a corresponding signal or different energy form.

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Beverly Hood
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1 Sensing

2 Sensors A sensor is a device that detects/measures a signal or stimulus. It acquires information from the real world. A sensor converts various forms of physical energy into electrical energy, which a micro controllers can interpret therefore learning about changes in the physical world. Sensors can be transducers. A transducer is a device converts one form of energy into a corresponding signal or different energy form. Primary Energy Forms: mechanical, thermal, electromagnetic, optical, chemical, etc. Typically interested in electronic sensor which converts desired parameter and primary form of energy into electrical energy/electrically measurable signal. For example?

changes real world parameter into electrical signal Secondary transducer (micro, ADC,

3 Transducers sensor Micro-controller (signal processing & communication) Input from the real world Sensor data Processed data (networked or wired) Primary transducer (sensor): changes real world parameter into electrical signal Secondary transducer (micro, ADC, amp.): converts electrical signal into analog or digital values, into usable information

4 Sensor types Resistive Sensors A resistive sensor is a transducer or electromechanical device that converts a mechanical change into an electrical signal that can be monitored after conditioning. They work by converting the energy they read into a changing electrical resistance by using a variably resistive material at their heart. The theoretical background for all these sensors is the theory of resistivity. In order to read changes in resistance, you typically place these sensors in a voltage divider circuit, which converts the resistance change into a changing voltage. Resistive sensors are among the most common in instrumentation. The simplest resistive sensor is the potentiometer. Other examples?

5 Sensor types Inductive Sensors This sensor consists of an induction loop and as electric current goes through it, it generates a magnetic field. Common applications of inductive sensors include metal detectors, traffic lights, car washes, and a other tools for automated industrial processes. Because the sensor does not require physical contact it is particularly useful for applications where access presents challenges. Proximity Sensors Many sensors measure movement or distance indirectly, by sending out a pulse of light or sound and reading the reflected signal when it bounces off a target. These are called ranging sensors because they return a range of distance, for example IR range sensors or Ultrasonic range finders.

6 Sensor types Capacitive Sensors In this basic technology, only one side of the insulator is coated with conductive material. A small voltage is applied to this layer, resulting in a uniform electrostatic field. When a conductor, such as a human finger, touches the uncoated surface, a capacitor is dynamically formed. With no moving parts, it is moderately durable, but has low resolution, is prone to false signals and needs calibration. It is mostly used in simple applications such as industrial controls and interactive kiosks. Piezoelectric Sensors A piezoelectric sensor is a device that can generates an electric charge in response to applied mechanical stress. The prefix piezo- is Greek for 'press' or 'squeeze'. The voltage at the source is directly proportional to the applied force, pressure, or strain. Two main groups of materials are used for piezoelectric sensors: piezoelectric ceramics and single crystal materials.

both sides by electrical contacts. Because the conductive mass is parallel to the outer contacts, a capacitance builds up between the contacts.

7 Sensor types MEMS (Micro-Electro-Mechanical System) MEMS sensors work by converting the energy they read into a change in capacitance. For example, accelerometers and other miniaturized electromechanical sensors typically have a tiny moving conductive mass at their core, suspended on tiny springs and surrounded on both sides by electrical contacts. Because the conductive mass is parallel to the outer contacts, a capacitance builds up between the contacts. When the mass is moved, the capacitance changes, effectively creating a variable capacitor. That variable capacitor is then placed in a resistor-capacitor circuit to convert the change in capacitance into a changing voltage which can be read by the micro-controller.

8 Sensor types Optical Sensors Light is used in many sensors in a variety of ways. Light-emitting diodes, lightdependent resistors, and phototransistors can be combined to sense dust, measure distance, or determine reflected color. Light is also used in some ranging sensors to determine distance from a sensor to a target. Analog Sensors Most sensors you buy are integrated circuits that include the resistance-to-voltage or capacitance-to-voltage circuit and provide you with an analog voltage output. Some will even include an analog-to-digital converter and provide you with a serial data interface (I2C, SPI, or asynchronous serial) so that you can connect the sensor to the serial ports of your micro-controller. Others will provide a changing pulse output, or PWM output, where the width of the pulse represents the sensor value.

9 Data Sheets Data Sheets The data sheet will usually include the following essential facts: a text description of the sensor and its operation; a pin diagram to tell you what pins perform what functions; a table of electrical characteristics that tells you what the supply voltage is, what the operating current is, and what the output is; a graph or conversion formula that relates the input energy to the output energy; a mechanical description of the sensor itself. Some data sheets might also include application circuits, reliability data, and more. Don t exceed the maximum or minimum operating voltage, and make sure to supply adequate current to operate the sensor. Avoid supplying too much current, because the excess current will get turned into heat, which often changes the sensor s operating characteristics. Understand whether it s an analog electrical property like resistance, capacitance or voltage, or a digital serial data interface. Look for typical application circuits and microcontroller code samples online if you can find them.

10 Aref AREF means Analog REFerence and configures the reference voltage used for analog input. This is important as the accuracy of any analogread() values will be affected by not having a true 5V. If you don t have this option, you can use math or remapping to compensate for the drop in voltage. For example, if your voltage is 4.8V the analogread() range of 0~1023 will relate to 0~4.8V and not 0~5V. This may sound trivial, but if you re using a sensor that returns a value as a voltage (e.g. the TMP36 temperature sensor) the calculated value will be wrong. When your Arduino takes an analog reading, it compares the voltage measured at the analog pin being used against the reference voltage. In normal analogread use, the reference voltage is the operating voltage of the board. An external AREF can come from a regulated power supply, or if you need 3.3V you can get it from the Arduino s 3.3V pin. If you are using an external power supply, be sure to connect the GND to the Arduino s GND pin. Very important note when using an external voltage reference, you must set the analog reference to EXTERNAL analogreference(external); before using analogread(). This will prevent you from shorting the active internal reference voltage and the AREF pin, which can damage the microcontroller.

Temperature Resistance Temperature Detectors (RTDs) Platinum, Nickel, Copper metals are typically used. Coiled wire wrapped around ceramic or glass core.

11 Temperature Resistance Temperature Detectors (RTDs) Platinum, Nickel, Copper metals are typically used. Coiled wire wrapped around ceramic or glass core. Thermistors (Thermally Sensitive Resistor) Affordable, easy to use, very popular for basic temperature control. Thermocouples No electronics, simply made by welding together two different metals. Analog or Digital Sensors Temperature sensors that are interface already to be used as an analog input or digital input.

12 Temperature Thermistor RTDs Thermocouple Digital Temp Sensor Analog Temp Sensor

13 Trouble Sensors can exhibit non-ideal effects (deviations) Offset or bias: nominal output nominal parameter value (the output signal is not zero when the property measured is zero) Nonlinearity: sensitivity is not constant over the range of the sensor Drift: output signal slowly changes independent of the measured property Cross parameter sensitivity: The sensor may to some extent be sensitive to properties other than the property being measured (for example temperature) Noise is a random deviation of the signal that varies in time Calibration = adjusting output to match parameter Analog signal conditioning Look-up table Digital calibration Compensation Remove secondary sensitivities Must have sensitivities characterized Polynomial evaluation

Microcontroller to Sensor Interfacing Techniques

Microcontroller to Sensor Interfacing Techniques to Sensor Interfacing Techniques Document Revision: 1.01 Date: 3rd February, 2006 16301 Blue Ridge Road, Missouri City, Texas 77489 Telephone: 1-713-283-9970 Fax: 1-281-416-2806 E-mail: info@bipom.com