Sensors Selection Ppt

Mechatronics System Design

Sensors Selection

Dr. Ashraf Saleem

Displacement Sensors Selection Procedure (1/8)Displacement Sensors Selection Procedure (1/8)

Th f ll i l f h i di lThe following steps are general steps for choosing a displacement sensor for a particular application:The first parameter to address in any application is whether the p y pptransducer can physically touch the object being monitored. If your application is sensitive to outside influences, a noncontact transducer may be the most appropriate Otherwise a contacttransducer may be the most appropriate. Otherwise, a contact sensor might offer advantages not found in a noncontact sensor. Another parameter to consider early is whether you need to

li t t U i bl itimeasure linear or rotary movement. Using cable position transducers, cams, pulleys, levers, electronics, software, and other methods can enable a rotary transducer to measure linear motion, and vice versa. Lack of space, cost, and ease of mounting are a few reasons for doing this.

Dr. Ashraf Saleem/ Philadelphia University


Next, determine if you are monitoring one-dimensional or multidimensional motion. If the motion is multidimensional, decide if

d t i lti l di i if th bj t iyou need to measure in multiple dimensions or if the object is moving in multiple dimensions and you only have to measure one of them. Often, multidimensional motion is measured with multiple one-dimensional transducers.Also, think about the type of signal you need to obtain. If you need a signal that specifies a unique position be sure to specify asignal that specifies a unique position, be sure to specify a transducer with absolute output. However, if all you need is relative position from a prior position or a simple on/off indicator, then incremental or threshold technology is more appropriateincremental or threshold technology is more appropriate.



The range of many precision transducers is limited to 10 inches or less. If your application needs to operate on the International Space St ti th i d i ht iti l tf dStation or some other size- and weight-sensitive platform, you need to specify the maximum values for the dimensions and weight of your transducer.The operating environment of the application can have a large impact on your technology choice as well. You need to determine what operating and storage temperatures the device will be in andwhat operating and storage temperatures the device will be in and whether you need to meet commercial, industrial, or military environmental requirements. Also consider whether excessive humidity moisture shock vibration or EMF will be encounteredhumidity, moisture, shock, vibration, or EMF will be encountered. See if your environment has other unique aspects, such as high or low pressure or the presence of hazardous or corrosive chemicals.



An often-overlooked parameter is the method and time required for p qtransducer installation and mounting. For testing applications, this parameter may not be so important. However, large-volume applications often require simple installation and removal to reduceapplications often require simple installation and removal to reduce labor costs and enable easy maintenance.Study the device accuracy. Typically, various components of accuracy linearity repeatability resolution and hysteresis areaccuracy - linearity, repeatability, resolution, and hysteresis - are quoted for vendor convenience or per user requirements.In the early stages of transducer specification, product cost sometimes does not even make the list. More often than not, this parameter gains importance as the project moves forward. When you are determining costs, make sure to look at the initial acquisition y g , qcost as well as the cost over the life of the product. For example, are special signal-conditioning electronics, power supplies, electrical connectors housings installation tools or mounting fixtures


connectors, housings, installation tools, or mounting fixtures required?


Another parameter that is occasionally overlooked is the time it p ytakes the product to be delivered to you after you order it. The custom nature of some transducers, combined with production processes and manufacturing economics, requires lead times ofprocesses and manufacturing economics, requires lead times of eight weeks or more. This delivery schedule might be acceptable now, but six months from now you may need extra quantities or a spare part Evaluate whether you can afford to be without a part forspare part. Evaluate whether you can afford to be without a part for an extended period of time.Obviously, the transducer is going to be a part of a system.

f fTherefore, determine your preferred electrical input and output requirements. Common output choices include analog AC and DC voltage, resistive, current (4-20 mA), digital, and visual (meter). Increasingly, outputs using sensor bus protocols are being offered. Most position transducers require 50 V or less, and some are self-powered.


p


Finally, for fast-moving applications, determine the maximum y g ppvelocity or acceleration that needs to be monitored. Ensure that your data-acquisition or control system has an adequate sampling rate to record the resulting data stream.record the resulting data stream.



Parameter Choice

Contact Contact Non-contact

Motion Type Linear Rotary

Dimensions One Dimensional Multidimensional

Measurement Type Absolute Incremental Threshold (Proximity)

Range Less than 1" 1"-30" Greater than 30"

Physical Size/Weight Size Restriction _______ Weight Restriction ______

Environmental Protection Humidity Vibration Temperature Moistureature

Installation/Mounting Removable Fixed Installation Time _____

Accuracy Linearity Resolution Repeatability

Lifetime Cycles _______ Hours of continues operation _______

Cost Less than $50 $50-$500 More than $500

Delivery Less than 1 week 1-4 weeks More than 4 weeksDelivery Less than 1 week 1 4 weeks More than 4 weeks

Output Voltage Current Digital Visual

Frequency Response Less than 5 Hz 5-50 Hz More than 50 Hz


Temperature Sensors Selection ProcedureTemperature Sensors Selection Procedure

Temperature Limitations for RTD’s An RTD consists of a sensing element, wires to connect the sensing

element to the measurement instrument and some kind of support to position the sensing element in the process Each of these materialsposition the sensing element in the process. Each of these materials sets limits on the temperature that the RTD can be exposed to.



Temperature Limitations for RTD’s

The wires which connect the sensing element to the readout or control instrumentation are usually made of materials such as nickel, nickelinstrumentation are usually made of materials such as nickel, nickel alloys, tined copper, silver plated copper or nickel plated copper. The wire insulation used also directly influences the temperature the RTD can be exposed to Table 2 contains the commonly used wire andcan be exposed to. Table 2 contains the commonly used wire and insulation materials and their maximum usage temperatures.



Temperature Limitations for Thermocouples• Thermocouple materials are available in Types E, J, K, N, T, R, S

and B.


Th l A li tiThermocouple Applications


RTD StrengthsRTD StrengthsRTD’s are commonly used in applications where repeatability and accuracy are important considerations. Properly constructed y p p yPlatinum RTD’s have very repeatable resistance vs. temperature characteristics over time. If a process will be run at a specific temperature the specificIf a process will be run at a specific temperature, the specific resistance of the RTD at that temperature can be determined in the laboratory and it will not vary significantly over time. RTD’s also allow for easier interchangeability since their original variation is much lower than that of thermocouples. For example, a Type K thermocouple used at 400°F has a standard limit of error of ± 4°F. A 100 Ohm platinum RTD has an interchangeability of ±2.2°F at this same temperature.RTD’s also can be used with standard instrumentation cable forRTD s also can be used with standard instrumentation cable for connection to display or control equipment where thermocouples must have matching thermocouple wire to obtain an accurate measurement


measurement.

RTD WeaknessesRTD Weaknesses

In the same configuration you can expect to pay from 4 to 10 timesIn the same configuration, you can expect to pay from 4 to 10 times more for an RTD than for a base metal thermocouple. RTD’s are more expensive than thermocouples because there is more

t ti i d t k th RTD i l di f t f thconstruction required to make the RTD including manufacture of the sensing element, the hooking up of extension wires and assembly of the sensor.RTD’s do not do as well as thermocouples in high vibration and mechanical shock environments due to the construction of the sensing elementsensing element. RTD’s are also limited in temperature to approximately 1200°F where thermocouples can be used as high as 3100°F


Thermocouple StrengthsThermocouple Strengths

Th l b d hi h 3100°FThermocouples can be used to temperatures as high as 3100°F, generally cost less than RTD’s and they can be made smaller in size (down to approximately .020” dia) to allow for faster response to temperature. Thermocouples are also more robust than RTD’s and can therefore be used in high vibration and shock applicationsbe used in high vibration and shock applications.


Thermocouple WeaknessesThermocouple Weaknesses

Th l l bl h RTD’ h dThermocouples are less stable than RTD’s when exposed to moderate or high temperature conditions. In critical applications, thermocouples should be removed and tested pp , punder controlled conditions in order to verify performance. Thermocouple extension wire must be used in hooking up thermocouple sensors to thermocouple instrument or controlthermocouple sensors to thermocouple instrument or control equipment. Use of instrumentation wire (plated copper) will result in errors when ambient temperatures change.


Sensors Selection Ppt

Documents

Transcript of Sensors Selection Ppt