Fundamentals of Mechatronics Engineering (2)

Mechatronics A Multidisciplinary Approach

William Bolton

MCH 104 Dr.Eng. Abdüllatif BABA

2017 - 2018

Measured Quantity

Sensor

Signal Temperature

Pressure Distance Moisture Wieght .. etc.

Analog contains information that changes in a continuous way. It is not sampled or quantified.

Digital Sequence of ones/zeros (on/off) .

• Helpful information to evaluate the measured variable (Quantity) • Convenient fro computational environment and to design control systems Ex : For thickness measurments some sensors are capable of giving (analog or digital outputs ( 4-20 mA analog, or RS-485 digital comunication)

Smart sensor

Memory

Sensor

Input

(Calibration data + Signal Conditioning)

Output

Microprocessor

All this combination may be called Smart sensor Self calibration & Self fault diagnosis Adapt itself to random changes in the environment

Terminology

Sensor Range

200

Input

Sensor

Output

5 The sensor may be called also “transducer” This term is normally used to devices that convert the signal form one form to another

Sensor Range : the limits between which the input can vary, Ex. [5

200]

Sensor Span : Max. – Min. = 200 – 5 = 195 Error : e = Measured Val. – Real val. Ex. Temprature Reading: A sensor might give the reading as a resistance change of 10.2 Ω when the true change should have been10.5 Ω. e = 10.2 – 10.5 = -0.3 Ω.

Input

Sensor

Output

Accuracy : Is the extent to which the measured value might be around the real value. Ex. A temperature-measuring instrument might, for example, be specified as having an accuracy of ±2°C.

But, if the accuracy is expressed as a percentage of the full output range ? Ex. If a sensor was specified as having an accuracy of ± 5% and its output range is [0 200°C]

Then : the accuracy of the measured reading is ±10°C of the true reading. Sensitivity : The sensitivity is the relationship indicating how much output there is per unit input. Ex . A pressure sensor may have a temperature sensitivity of ± 0.1% of the reading for each 1 °C as a change in temperature.

Hysteresis error : Transducers can give different outputs from the same value of quantity being measured according to whether that value has been reached by a continuously increasing change or a continuously decreasing change

Nonlinearity error : For many transducers a linear relationship between the input and output is assumed over the working range

Repeatability / Reproducibility :

Describes the ability of the transducer to give the same output for repeated applications of the same input value.

repeatability 

er



max  min *100 full output range 200.3  200.1 *100  0.09% 220

Best Repeatability; er ---> 0 Stability :

The stability of a transducer is its ability to give the same output when used to measure a constant input over a period of time. •

•

The term (drift) is often used to describe the change in the output that occurs over time. The term (zero drift) is used for the changes that occur in the output when there is zero input.

Dead band The dead band or dead space of a transducer is the range of input values for which there is no output. Output

Dead band

Threshold

Input

Resolution : The resolution is the smallest change in the input value that will produce an observable change in the output.

Example :