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Transcript
What is Instrumentation? Seth Price Department of Chemical Engineering New Mexico Tech Rev. 1/6/16 Simple Instrumentation • Collecting data • Manual measurements • Slow sampling rates (1/10 Hz) • Not controlling a process http://3.bp.blogspot.com/-hVVw6rZDBn4/ToFRnKN2sxI/AAAAAAAAATU/1ZfrvzQeUEQ/s1600/thermometer.jpg Advanced Instrumentation • Automated measurements • Faster sampling rates • Required for automated process control http://digital.natinst.com/public.nsf/$CXIV/ATTACH-AEEE-78KKB8/$FILE/Open%20Circuit.bmp Simple Control Loop http://i-cdn.apartmenttherapy.com/uimages/re-nest/3-13-2008shower.jpg Advanced Control Loop http://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Change_with_Kd.png/320px-Change_with_Kd.png http://www.aboutplcs.com/directlogic/images/dl205_app_pid.jpg Back to the Shower Example…. • Sampling Rate: – How often measurements are taken – Expressed as Hz • Dynamic Range: – High measurement – low measurement • System Response: – How long it takes for a change to take effect http://epicthings.net/wp-content/uploads/2011/12/LED-temperature-shower-head.jpg Back to the Shower Example… • Linearity: – Does the response change correspond linearly to the control change? (y = mx + b) • Measurement Error: – Measurement does not represent truth • Disturbance/Impulse: – Unexpected step change http://epicthings.net/wp-content/uploads/2011/12/LED-temperature-shower-head.jpg The Magic of Automation • Physical phenomena are converted to voltage or current through a transducer (input) • Control voltages or currents are converted into physical phenomena www.futurlec.com Besides Voltage: • Voltage In: – – – – – Temperature Pressure Humidity Detection of light Etc. www.auberins.com • Voltage Out: – Warning Lights – Motors – Voltage controlled switches – Etc. http://www.globalspec.com Control Loop • Measurements + Logic = Control Loop – Compares current condition to set point – Adjusts to bring current condition to set point Fictional Scenario • You are in charge of controlling the temperature in a heat exchanger. – The output temperature of the hot side must be below a setpoint, TH. – You can control the flow rates of the cold side, but nothing else. What Can We Do? • In the past, the company has just ran “lots” of cold water through the cold side to keep the temperature down below the setpoint, TH. • How can you improve this? Iterative Improvements • How can we make this process more efficient? • How can we make this process more accurate? • How can we make this process safer? Increasing Efficiency • If the temperature gets close to setpoint, TH, increase the cold side flow rate. – Now we have a way to increase the flow rate, but no way to decrease it! • Define a second setpoint, TC, which is the minimum temperature. – Decrease the flow rate when the temperature approaches TC Increasing Accuracy • Sample temperature in several places instead of just one, such that: 𝑇𝐴𝑉𝐺 𝑇1 + 𝑇2 + 𝑇3 = 3 Increasing Safety • Add leak/fault detection: if the cold side flow rate drops below a flow set point, FC: – Light a warning LED – Operator checks for warning LEDs periodically Keep Iterating and Expanding http://www.snboiler.com/uploads/images/201209/18/CFB_boilers_(1).jpg …Integrating the system http://embeddedsystemnews.com/images/embedded/2009/08/commonwireless-sensor-networkarchitecture.jpg …Fully Integrated Control System http://www.jenriks.de/jenriks/wp-content/photos/jh1065_power_plant.jpg …but it all starts here