APPARATUS DESIGN PHGN 384
Colorado School of Mines
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This 31 page Class Notes was uploaded by Donato Hoeger Jr. on Monday October 5, 2015. The Class Notes belongs to PHGN 384 at Colorado School of Mines taught by Staff in Fall. Since its upload, it has received 18 views. For similar materials see /class/219612/phgn-384-colorado-school-of-mines in Engineering Physics at Colorado School of Mines.
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Date Created: 10/05/15
Labview Day 2 Temperature Control Objectives Use the Analog Out and Analog In features of our Data Acquisition Systems to control temperature Understand the physical basis of temperature measurements and control Understand that measurements and excitation involve electronic conversion of analog and digital signals Labview Day 2 Outline Outline Analog In and Out How does the DAC and ADC work Temperature measurements and Control Open Loop system Temperature Control Closed Loop Data Acquisition in the Labview Environment Uses DAQ cards or controls instruments via a standard bus such as the RS232 ports USB parallel ports or GPIB Virtual instrumentation combined hardware and software to develop exible used de ned computer based instruments The computer behaves similar to a standard instrument in having controls Front Panel and display and storage of data Parts of Data Acquisition with a virtual instrument Data Acquisition and Analysis Hardware Signal Conditioning Personal Software Computer Typical computer based data acquisition system from Application Note ANOO7 National Instruments Signal Conditioning This is the treatment of analog signals prior to digitization For sensors of different types there are many forms that is true of the output signals as well since a voltage may need to be converted to a current for instance Ampli cation for low level signals ampli cation near the source is preferable Linearization transducers such as thermistors can be corrected 1n hardware or software Many are bu11t1nto Labv1ew Excitation an external voltage or current is often required for a transducer such as a stra1n gauge or a phototrans1stor Isolation separation of voltage spikes and isolation to reduce n01se are common Filtering antialiasing is common in addition the use of a 60 Hz filter is required in some cases because the ADC is not an averaging type Virtual Instruments In order to make this seamless between different hardware the software consists of two parts MAX Measurement and Automation Explorer and LabView Measurement amp Automation Explorer MAX provides access to all your National Instruments DAQ GPIB and VISA devices With Measurement amp Automation Explorer you can 0 Con gure your National Instruments hardware and software 0 Add new channels interfaces and virtual instruments 0 Execute system diagnostics 0 View devices and instruments connected to your system The use of MAX is not needed with the new express VIs DACs and ADCs Digital D1g1tal Out In DAC Analog Out Summing DAC Enabled counter R2R ladder application Successive approximation converter SAC R2R DAC ladder The output for this circuit is a voltage DACs are typically very fast limited only be the switch speeds and the slew rate of the op amp maX rate of change of the output in response to a change of the input DACSOO Diagram 5 2 0 To T0 To TO o 30 0 F0 Bl NE39ITWURK ffT i alour RRE VHEF H 15 VREFl39l V 0 0 menu 15 3 R This circuit has a current output which makes it faster It has a very clever arrangement of resistors that contribute amounts of current that are summed The small range of resistance values and the duplicated circuits make this very precise Successive approximation SA ADC Reset SA Register 4r Switch largest untested bit on DAC outputs value s DAC greater than Vi Reset bit try next one Binary search with timing and comparison A decision has to be made for each bit so the time doesn t depend on the value Can be very fast Leave bit set try next one Dual Slope converter Measure this part it is proportional to Vin Basic Operation of a Dual Slope ADC Relies on CTRL circuit that allows the capacitor C to charge with rate given be ViRC for a xed time T 0 this is typically a xed number of clock cycles N0 Then the capacitor discharges for a time T1 at a xed rate VreJRC or for N1 clock cycles until it reaches zero If T is measured T 0 is known then Vin T IT 0 Vref N INa Vref Advantages doesn t require Vc precise components so inexpensive It also can be designed to average out noise particularly line noise Disadvantage slow if the output is 10 bits typical display on DMM it requires 210 clock cycles Conversion time usually few ms Pinout of 68 pin Connector for PCI6035E In addition to the 25 pins associated With analog in 4 With analog out and 20 associated With the 8 D10 lines there are a number of other lines as well some inputs some outputs some both More on these next semester in PHGN317 Analog Inputs A DAQ card can have analog inputs With different characteristics added The input is almost always a voltage Our system has a single successive approximation converter With 16 bits Characteristics Resolution given by number of bits or minimum step size 16 bits smallest resolution Speed maximum rate data can be read 200 kSs Number and Type of Inputs single ended differential 16 SE or 8 D1 Range can be bipolar or unipolar range is software selectable 50 mV to 10 V Input impedance 100 G9 in parallel with 100 pF Differential measurements are normally used if the signal is small or the lines are long and may be noisy and to avoid potential ground loops We have the E series cards they have 8 differential inputs or 16 single ended SC 2075 ANALOG IN Analog Input Numlicr i ul cliunncls If Series h39 lil39l39crcnliul IICJIII Series dil39l39crcnliul I mmsclnr 1 lies nnd Signals 1i Series 1200 Serial 302075 mrneclmv Signal Name Signal i amc Signal Name Binding Pm 39L39li39Hn in ll M Hr Binding l nsis 39llil fCHrJnI liL39llH l 1 HM CII CHI AL39H J quoti ll 3 BNL L 2CHJI A 1 lil I il lll 5 BN39 TRICII EXTTRIG l39l39lll 39l39RlUI iRliI liX l39Tth I pring 39lcrminnls CHMHere AL39llS lib pring 39l39crminnls 39H3 CHi l MI ll 1 39l 7 pring 39lLrininnls CH4Vill MCI H N A Spring l39crminnls gm main AL39llll N Spring 39I39crininnls CHMNil L39lli N pring 39bi ininuls CHE Nil AL I N A Spring 39l39crminnls H rfriquotl g39l ih N Spring 39l39crminnls HG WU rtl39l I H N A printI 39l39crminnls CH7quot vquotll iL39l 17 N A Spring 39l39crininnls m m vL39ll 5 N A Analog Outputs The output is normally a voltage but can be con gured to be a current in some cards Characteristics Resolution given by number of bits or minimum step size 12 bits smallest resolution Speed maximum rate data can be written The data output is dependent on the size or existence of a buffer 10 kSs With DMA unbuffered Number and Type of Inputs single ended differential 2 SE Range can be bipolar or unipolar range is software selectable i10 V Output impedance 01 Q can drive or source 5 mA max SC 2075 ANALOG OUT The important thing to note here is that the output is only 10 volts positive or negative Anaiug Output 39n lnncclw lypca mil signals 511721175 E Series 1213 Series Connector ignal Name Signal Name Signal Name BNCL39 CHWC39HHJ MTII i39 llquotl i l BNL39 CHII39T39HN L39KL L39I ilg ul39lilil39i39l Seebeck Effect Thermocouples Intrinsic to material doesn t correspond to surface effect like work function Imagine conducting material with ends at different temperatures What happens to the number of carriers If no current ows builds up an electric field that balances the diffusion current Electric Field E S VT S is called the thermopower If have two different materials the measured voltage is related to the difference in Seebeck coefficients for the material pair a thermocouple Thermocouples Important laws law of intermediate metals solder UK law of successive metals V ACV ABVBC can use reference metals to use any combination law of successive temperatures if Va developed between T1 and T2 and Vb developed between T2 and T3 then Vc Va Vb developed between T1 and T3 The latter is important because it means that need only tabular values for one reference temperature All others can be determined It also points out that you need a reference temperature Peltier Effect Thermoelectric Coolers There are two other related thermoelectric effects the Peltier effect 1834 and the Thomson effect 1854 both involve passing current through a junction or a homogeneous material respectively don t forget good old Joule Peltier changing the direction of current ow can result in either heating or cooling at a junction with the work performed by the current source Thompson if there eXists a thermal gradient across a material then passing a current can result in heating or cooling H eatPel tier ABTSASB 01471 01AJAT dSA dT 7Z39ABJ HeatT hom son Peltier Effect Simple diode To demonstrate basic idea imagine that you have two metal plates separated by vacuum both at the same temperature initially Each has a distribution of electrons that is identical Now heat one slightly it has more electrons so initially there is a net ow of electrons away until equilibrium is reached again Now return to same temperature and apply a potential resulting in a current The one that has current leaving is cooled IC Temperature Sensors Note that the circuit is simple with the addition of a resistor at the terminal this looks at VCE even though we treat is like a diode with a temnerature denendent voltage b 339 50k ling lp s I ml r 1 hd k bli k 01a b at pann I v A a x A L b llA E ELI E 20 a 200 I h H 4 L1 m V E In h m we I 10 m 7 U 2 IC Temperature Sensors Based on dependence of diode equation on the absolute temperature of the junction 8 a w o 0 25 050 075 V 55 Volts Base hitter Valtage Hm VCE Volts 15 20 5 1 CollectorEmitter Voltage bidirectional current driver You have already designed and tested a circuit that can control a Peltier device Designed to provide large bidirectional current ow necessary to power both the heating and cooling of the thermoelectric device You will build the circuit and ultimately use LabVieW to provide control input and feedback System Schematic Vin TIP 120 470 Ohm GND Data sheets for components 1 HF are in binders TIP 125 398 VGND GND Controlling and measuring As the rst stage of this test connect the Peltier device and measure the current and temperature as a function of applied voltage using the Agilent supplies Make a table and plot this in your notebooks Then using the Analog output capabilities drive the input stage of your pushpull to control the output You should place a limit on the voltage to protect the device Construct your VI to measure and control the system Try to incorporate subVIs Open vsCosed Loop Control As with Op Amp circuits these mean exactly the same no feedback is open with feedback is closed More complex control theory expresses the response function in terms of Laplace transforms we will focus on a timedomain only View 10 Tct System Fx1x2 Closed Loop Control Tset ea i0 S t Tea 4 gt F3 31 1 i0 Tset Ct S ystem FX1X2 I Kl Ietdt Tct Closed Loop Control The use of correction terms that are P I and D allows for a stable control of Temperature it T t System C FX19X29 Closed Loop Control selecting the constants At first just play A more systematic approach is based on the lag time L and the maximum rate of change R Start by looking at response of system with NO feedback Change the voltage some amount instantly D watch how long the lag is and the maximum rise Finally look at What temperature the system stabilizes at the change in controlling parameter is D K23 KZKpL P LR 2L D 2 Since it is not continuous all of this is in loops with the time per step about 110th of the time constant of the system Closed Loop Control selecting the constants 2 An alternative way to do this with a closed loop system is called the ZieglerNichols method Start by using only P control and nd when it oscillates with period T OSC this gain we will call Kpcm Then the gains are Kliril 2Kliril K Kliriljgsc 1 2 T D 8 OSC K The main problems with this mode is that the system will overshoot and it may be difficult to get an oscillation the gain may not be set high enough
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