Sensors

Prof. Prabhat Ranjan

References

1. Sensor Terminology - http://mechatron.me.wisc.edu/Courses/me601/terms/sensorterm.html

2. Designing Embedded Hardware – John Catsoulis

Sensors

● A sensor is a device that responds to a physical stimulus (as heat, light, sound, pressure, magnetism, or a particular motion) and transmits a resulting impulse (a signal relating to the quantity being measured)

● For example, certain sensors convert temperature into a change in resistance

● A transducer is a device that is actuated by power from one system and supplies power usually in another form to a second system

● For example a loudspeaker is a transducer that transforms electrical signals into sound energy

● Often the words transducer and sensor are used synonymously

● For our purposes, a sensor is a device which converts a physical phenomena into an electrical signal.

● As such, sensors represent part of the interface between the physical world and the world of electrical devices, such as computers.

● The other part of this interface is represented by Actuators, which convert electrical signals into physical phenomena

Biological Sensor

● All living organisms contain biological sensors with functions similar to those of the mechanical devices described. Most of these are specialized cells that are sensitive to:– light, motion, temperature, magnetic fields, gravity,

humidity, vibration, pressure, electrical fields, sound, and other physical aspects of the external environment;

● Artificial sensors that mimic biological sensors by using a biological sensitive component, are called biosensors.

Transfer Function

● The functional relationship between physical input signal and electrical output signal

● Example from ADXL50A

Sensitivity

● The sensitivity is the relationship indicating how much output to get per unit input

● The sensitivity is generally the ratio between a small change in electrical signal to a small change in physical signal

● For the device ADXL50A, the sensitivity is 19 mV/g.

Span or Dynamic Range

● The limits between which the input can vary● The range of input physical signals which may be

converted to electrical signals by sensor ● Signals outside of this range are expected to cause

unacceptably large inaccuracy. ● Usually specified by the sensor supplier as the

range over which other performance characteristics described in the data sheets are expected to apply.

● Example: A load cell may measure forces in the range 0 to 50kN.

Error

● The difference between the true value of the quantity being measured and the result of the measurement

● Units are those of the quantity being measured

Accuracy

● Generally defined as the largest expected error between actual and ideal output signals

● Accuracy is the extent to which the value indicated by the measurement system might be wrong

● Accuracy is the sum of all possible errors that are likely to occur, in addition to the accuracy to which the sensor has been calibrated

● Accuracy is often expressed as the percentage of the full range output

Accuracy ...

● Thus if a sensor is specified as having 5% accuracy of full range output and the range of the sensor is 0 to 2000kN then the measurement can be expected to be within + or -100kN of the true reading.

Hysteresis

● A sensor may give a different reading when measuring the same quantity depending on what ``direction" the value has been approached from

● Such sensors do not return to the same output value when the input stimulus is cycled up or down

● The maximum width of the expected error in terms of the measured quantity is defined as the hysteresis

Nonlinearity or linearity

Noise

● All sensors produce some output noise in addition to the output signal

● In some cases, the noise of the sensor is less than the noise of the next element in the electronics, or less than the fluctuations in the physical signal, in which case it is not important

● Many other cases exist in which the noise of the sensor limits the performance of the system based on the sensor

Noise

● Noise is generally distributed across the frequency spectrum

● Many common noise sources produce a white noise distribution, which is to say that the spectral noise density is the same at all frequencies

Resolution

● The resolution of a sensor is defined as the minimum detectable signal fluctuation

● Since fluctuations are temporal phenomena, there is some relationship between the timescale for the fluctuation and the minimum detectable amplitude

● Therefore, the definition of resolution must include some information about the nature of the measurement being carried out

Repeatability

● The ability of the sensor to give the same output for repeated applications of the same input (with all other factors in the environment held constant), without the sensor being disconnected from the input

● The error is typically expressed as the percentage of full range

Reproducibility

● The ability of the sensor to give the same output when measuring a constant input, measured on a number of occasions (i.e. with the sensor being disconnected between measurements)

● The error is typically expressed as the percentage of full range

Stability

● The ability of the sensor to give the same output when measuring a constant input, measured over a period of time

● The term drift is often used to describe the change that occurs

● The term zero drift is used to describe the change that occurs when there is zero input

Dead Band

● This is a region for which the sensor input-output relationship has a small or zero slope

● This region causes the quantization levels of the output voltage to be mapped back to unacceptable inaccuracies of the measured value

● For example, a flow meter using a rotor with bearing friction might mean that there is no output until the input has reached a particular velocity threshold

Dynamic Characteristics

● Response Time● Time constant● Rise Time● Settling Time● Bandwidth

Bandwidth

● All sensors have finite response times to an instantaneous change in physical signal

● In addition, many sensors have decay times, which would represent the time after a step change in physical signal for the sensor output to decay to its original value

● The reciprocal of these times correspond to the upper and lower cutoff frequencies, respectively

● The bandwidth of a sensor is the frequency range between these two frequencies

Commonly used sensors for our projects

● Accelerometer● Temperature Sensor● Light Sensor● Gyrosensor● Magnetic Sensor● Image Sensor (CMOS)● Proximity Sensor● Pressure● GPS● Humidity● Rain Gauge

● PIR Sensor● Acoustic

Sensor(Microphone)●

Accelerometer: Applications

● Accelerometer is a device that measures non-gravitational acceleration

● Single or multi-axis possible● Can be used to sense orientation, vibration and

shocks● can be used to measure vehicle acceleration● can be used to measure vibration on cars,

machines, buildings, process control systems and safety installations

● can also be used to measure seismic activity, inclination

● Used for machinery health monitoring of rotating equipment such as pumps,fans, rollers, compressors, and cooling towers. Vibration monitoring programs are proven to save money, reduce downtime, and improve safety in plants worldwide by detecting conditions such as shaft misalignment, rotor imbalance, gear failure or bearing fault which can lead to costly repairs. Accelerometer vibration data allows the user to monitor machines and detect these faults before the rotating equipment fails.

● Within the last several years, Nike, Polar and other companies have produced and marketed sports watches for runners that include footpods, containing accelerometers to help determine the speed and distance for the runner wearing the unit.

● used to detect apogee in rocketry● One of the most common uses for MEMS

accelerometers is in airbag deployment systems for modern automobiles

● Many laptops feature an accelerometer, such as Lenovo's (formerly IBM's) Active Protection System, and Apple's Sudden Motion Sensor, which is used to detect drops. If a drop is detected, the heads of the hard disk are parked to avoid data loss by the ensuing shock

● Many digital camera use this

Gyroscope

● Angular rotation

3-d motion of air-craft

Inertial motion sensor

● Combines Accelerometer, Gyroscope and Magnetic sensor to provide full information about motion

Connecting sensor to uC

● Basic Concepts– Amplifiers– Analog-to-Digital Conversion– Sampling rates

Interfacing an external ADC(Maxim)

Temperature Sensor (AD22100/03)

Light Sensor

Light sensor sensitivity

Light sensor amplification

Accelerometer (ADXL150 – Analog Devices)

Pressure Sensor (Motorola)

Digital Pressure Sensor (Infineon)

Magnetic Field Sensor

Digital-to-Analog Conversion

PWM – Pulse width modulation

Motor Control - H-Bridge