kick-off meeting - european commission :...

Love Wave Fully Integrated Lab-on-chip Platform for Food Pathogen Detection - LOVE-FOOD

(Contract No 317742 – Starting Date: 1 September 2012)

Deliverable 5.3

Temperature control unit and related report

Due date: 28.02.2014Date of submission: 18.03.2014Author: Chatzandroulis StavrosDocument history: -

DELIVERABLE SUMMARY SHEET

Project Number : 317742Project Acronym : LOVE-FOODTitle : LOVE Wave Fully Integrated Lab-on-Chip Platform for FOOD-Pathogen

DetectionDeliverable : 5.3Partners Contributed : NCSRDAuthors : S. Chatzandroulis, E. Kappos, D. Moschou, D. PapageorgiouClassification : RE

DOCUMENT HISTORY

Date Version Description29.01.2014 0.0 draft03.02.2014 1.0 First complete version06.03.2014 1.1 Final17.03.2014 1.2 Submission to coordinator

18.03.2014 1.3 Coordinator approves and submits to EC officer

FP7-ICT-2011.3.2 – Contract No 317742 – Deliverable No ……/30

Table of Contents

Executive Summary 4Main Text 5

1. PCR temperature control specifications 52. Design of a 3-Channel Temperature Controller for Polymerase Chain Reaction (PCR) Control 63. Microcontroller firmware 104. PCR thermal cycler control software 105. Microheater Temperature Control Testing (R0 range 15-25 Ohm) 15

Conclusions 23APPENDIX 24


Executive Summary

The present document describes the design and implementation of the temperature control unit which has been developed within Task 5.3 “Temperature control system for micro- and isothermal PCR” (M12-18, NCSR-D). This unit is a necessary component of the system under development within Love-Food as it is responsible for the control of the microheaters of the µPCR. The latter consists of the microfluidic circuit and 3 embedded resistive heating elements (microheaters) fabricated on a common substrate. The layout of the 3 heating elements forms 3 planar temperature zones and the controller is designed to keep the temperature variation within each zone within ±1°C.

The circuit is built around a micro-controller (for the PID control loop and communication) and 16-bit external DACs and ADCs for driving and sensing the micro-heaters. Temperature control is achieved by controlling the voltage across the resistive heaters whilst a small sensing resistor is used to measure the current flowing through them. Communication with a host PC is done using a USB serial port in order to set the operating parameters of the PCR (e.g. zone temperatures, calibration data and control coefficients) as well as log data during its operation. The prototype board has been implemented and tests have been conducted utilizing aluminum (Al) and copper (Cu) resistors fabricated on polyimide and/or PCB substrates. Their resistance values ranged from 15 to 50 Ohms. Tests involved target temperatures useful for the PCR protocol; that is, 95oC for denaturation, 72oC for extension and 55oC for annealing. The controller successfully reached the target values within a few seconds and kept the temperature constant with an error of ±0.2oC, an error acceptable for PCR experiments.

The board was also tested with the flexible Cu resistors placed in a PMMA chip holder. In this case, the controller needs longer times to reach the target temperatures, as expected, due to the increased thermal mass of the system. It is, however, able to effectively drive the resistors to the target temperatures and from there on keep them within the same error (±0.2oC) as in the previous case. Tests with larger resistors (of the order of 50 Ohms) led to a slight redesign and a second version of the temperature controller which allowed it to supply more power to the microheaters and thus is able to effectively drive the µPCR module.


Main Text

1. PCR temperature control specifications

The Polymerase Chain Reaction (PCR) microsystem under development in the project consists of microchannels and microheaters which are responsible for maintaining the fluid temperature in the microchannel stable. Below (Fig. 1 and 2), an example of a PCR device with three microheaters fabricated from Cu on Pyralux poly-imide (PI) flexible substrate is shown.

Fig. 1 Schematic of meandering microheaters Fig. 2. Image of part of the fabricated Cu resistors on one side of the Pyralux substrate

Each Cu microheater resistance is about 15-20 Ohm at room temperature. In order to facilitate the device fabrication, the same three microheaters are to be used both as temperature sensors and as thermal elements. According to device simulations (see D5.1), the total power required for the three resistors to reach the three temperatures (65, 72, 95 ⁰C) was determined to be about 1.1 W. However, in the case that higher thermal mass is added, as in the case of the μPCR unit attached on a chip holder to facilitate fluidic interconnects, a higher total power would be required.

Similar microheaters were developed from aluminum (Al) instead of copper. As a result of that, the room temperature resistances are of the order of 50 Ohm. Temperature control for both types of devices is desired, therefore the resistance range for the microheaters to be controlled is 15-50 Ohm.

The temperature control circuit to be developed should handle devices with either Cu or Al microheaters. The overall specs are shown in the table below:

Cu heaters Al heatersNumber of Heaters per device 3Heater Resistance 15 – 25 Ohm ~ 50 OhmPower required 2 W 2 WOperating Temperature of heaters 65, 72, 95 oC

Temperature stability for each heater ± 1 oC


2. Design of a 3-Channel Temperature Controller for Polymerase Chain Reaction (PCR) Control

i. PCR device and controller description

The µPCR microsystem accommodates three microheaters (one for each of the reactors) which are used as heating elements to regulate temperature at the µPCR zones. It is designed in a way that allows two types of resistive elements (copper (Cu) and aluminum (Al)) to be used depending on the µPCR implementation and requirements.

A key feature of the µPCR and controller design is the use of microheaters not only as heating elements, but also as temperature sensors. The controller measures in real time the resistance of each microheater to derive its operating temperature, and the resulting value is used in a proportional-integral (PI) feedback control loop to stabilize the temperature of each microheater at a certain set value.

ii. Analysis of requirements

Initial experiments with prototype Cu microheaters showed that each element is expected to consume no more than 1.8W over its entire operating temperature range, i.e. from 60°C to 120°C. The resistance of the Cu microheaters in these tests ranged from 15Ω to 25Ω at room temperature (20°C).

In a purely resistive microheater, electrical power P is converted entirely into heat Q raising its temperature above ambient by T. Assuming a thermal resistance Rθ between the mass being heated and ambient (air), under nominal load and steady state heat flow conditions, T = Rθ P. The same applies to incremental changes in heat, i.e. dT = Rθ dP. A complication may arise if temperature variations in one microheater are affected by the temperature of its two adjacent ones. This is because they all share a common substrate in close proximity, thus the heat from one source could potentially influence an adjacent element affecting its temperature. This situation has of course been foreseen in the design of the µPCR microsystem and studied extensively using thermal simulations. In the unlikely event, however, that there is an influence between heaters, this is handled automatically by the feedback control mechanism, since it does not affect the measurement of individual temperatures (i.e. resistances). To achieve the specified temperature stability of ±1°C for the reactor, the design aimed to achieve a relative accuracy of 0.1°C to 0.5°C in temperature measurement and a maximum of ±0.5°C in absolute accuracy.

Temperature measurement relies on the accurate measurement in real time of the resistance of each microheater. The relation between temperature and resistance is given by the equation below:

R = R0 (1+ α(T – T0))

where the temperature coefficient of resistivity α is equal to 0.0043/°C for Cu at 20°C and 0.00429/°C for Al at 20 °C and are constant over the entire operating temperature range. Therefore, if R0 = 15 Ω at T0 = 20 °C, then the resistance R of the microheater when operating in its nominal operating range at 60°C is 17.58Ω, which means that the resistance changes by 2.58Ω @ 60°C and by 6.45 Ω @ 120 °C.


In order to track temperature changes with 0.1°C relative accuracy over the entire range of 60°C to 120 °C (i.e. 1 part per 600 over a 60°C range), one needs to track resistance changes by 2.58 Ω to 6.45 Ω with the same accuracy, i.e. 1 part per 600. This implies that the resistance measurement method must have a worst case resolution of 2.58Ω/600=4.30 mΩ. Similarly, for 0.5 °C relative accuracy, the measurement resolution must be at least 5 x 4.30 mΩ = 21.50 mΩ. Alternatively, a 10mΩ resolution corresponds to a temperature accuracy of (10/4.30)x0.1°C ≈ 0.2°C.

One way to improve relative accuracy is by increasing the minimum incremental change to be detected, i.e. choosing a higher value resistance for the heating elements. This way the incremental change from ambient to the entry point of the operating range (e.g. 60°C) is increased in direct proportion, and becomes easier to measure precisely. For instance, if R0 = 2 x 15 Ω = 30 Ω, the incremental change will now be 2 x 2.58 Ω=5.16 Ω @ 60°C, and 0.1°C accuracy can be achieved providing 8.60mΩ resolution in resistance measurement.

Power is fed to the microheaters by three low impedance voltage sources with voltage levels set by the microcontroller firmware (the control algorithm). Each power source, with adequate heat sinking, can supply maximum 10V @ 1A. This design choice allows for maximum flexibility in the choice of microheater resistance, in order to optimize the sensitivity of temperature sensing. For instance, a 45Ω @ 120°C (31.5Ω @ 20°C) microheater can be controlled with supply voltage set at V=9V and current I=9V/45Ω=200mA to provide heating power P=I2R=V2/R=1.8W. Alternatively, a 10Ω @ 20°C microheater (=14.3Ω @ 120°C) supplied by V=4.24V at I=V/R=424mA, provides heating power P=I2R=V2/R=1.8W.

iii. Resistance measurement

The resistance measurement method implemented in the prototype controller is illustrated in Fig. 3. This is a commonly used method in commercial DMM Ohmmeters, using either a 2-wire or a 4-wire (Kelvin) connection to the resistance under measurement1.

Figure 3: Ratiometric resistance measurement.

A current sensing resistance RS is inserted in series between a voltage source VP and the heater resistance RH. The current flowing through both resistances is the same, thus

I=U S

RS=U H

RH

By measuring the voltage drops US and UH, the heater resistance may then be calculated by

RH=UH

U SRS

1 KEITHLEY, Low Level Measurements – Precision DC Current, Voltage and Resistance Measurements, 5th Edition


By choosing as voltage source the power source VP of the heating element, the resistance measurement can be performed without interrupting the heat supply to the element. This, however, implies the use of a low resistance RS to keep power dissipation on the reference resistor low. Low RS, however, results in low voltage drop US across the sensing resistor, which necessitates amplification in order to bring it within the measurement range of the analog-to-digital converter (ADC) - typically 0V to 2.5V or 5V.

Taking into account that the microheater current ranges from 0.100A to 0.500A, U S ranges from 10mV to 50mV. A current sense amplifier with fixed gain GS=100 is used to bring the sensed voltage within the 5V range of the ADC. The RS tolerance is ±0.1% which results to ±0.001RH

estimation error for RH, assuming no errors in the measurement of UH and US. Note that besides the initial tolerance of 0.1%, the drift over temperature for the specific RS choice2 and temperature range is much lower than 0.1% (its temperature coefficient of resistance is ±15ppm/°C max, for -55°C to +125°C) and as a first approximation is ignored.

iv. Controller architecture

A key part of the system is its analog to digital converter (ADC). To choose the appropriate ADC resolution the practical case of a microheater with maximum resistance of 30 Ω over its entire operating range is assumed. Then, if the desired measurement resolution is 5 mΩ, the ADC full scale count would be NADC=30Ω/5mΩ=6000 and the required resolution is log2NADC = log106000 / log102 = 12.5 bits. Thus the ADC resolution should be better than 12.5 bits.

Since the ADC full scale input is 5V, an attenuator by GH=0.4 is used to bring the voltage on the RH

terminals within range, as in certain cases a VP higher than 5V is required to supply the required power to the heater element. This way, the maximum VP of 10V can be used. The overall architecture of the prototype controller is shown in Fig. 4 (the complete schematic may be found in the appendix). The amplifiers convert the differential voltage drops along RS and RH to single-ended signals in the range 0V to 5V. As discussed previously a gain of 100 is required for the sensing resistors. The differential signals are filtered by a common mode filter as well as two differential filters to the ground plane (one for each wire), to reduce any EMI interference (these filters are not shown in the figure). The single-ended signal between the multiplexer (MUX) and ADC is also filtered. The ribbon cable receptacles on the circuit board are also protected against electrostatic discharges (not shown in the figure). The final assembled board of the temperature controller is shown in the photo of Fig. 5.

2 Vishay foil resistors, CSM Series, CSM2512, Part No Y14870R10000B9R.


Figure 4. Architecture of the prototype 3-channel controller

Figure 5. Final assembled temperature controller prototype.


3. Microcontroller firmware

At power up the microcontroller initializes the input-output ports, interrupt flags etc. and then establishes a USB connection to a host PC. Subsequently, the firmware enters a polling loop waiting for a USB packet to arrive from the host PC. USB packets are 64-bytes long and consist of a tag byte, which identifies a command to be executed, followed by data bytes for updating DAC values etc.

Upon receipt of a USB packet from the host PC, the firmware decodes its tag byte and executes the command (e.g. update DACs, read ADC etc.). In case the command requests data or status information, a reply packet is formed containing the requested information and forwarded to the PC via the USB connection. After executing the requested command, the firmware enters again the USB polling loop.

A command to set one DAC channel to a 16-bit value takes 155µs for the firmware to execute. The 3 DAC channels can therefore be updated in 465µs. Similarly, a command to read the 16-bit ADC takes 12µs for the firmware to execute. Averaging ADC measurements by 16 to reduce random noise means that each ADC reading takes 192µs. For 6 channels (3 for the current sensing resistors and 3 for the heaters), with 400µs delay between channel switching for any transitive effects to settle, the reading of all 6 channels takes 6(400+192) µs = 3.5 ms, which is well within the PID control sampling period.

4. PCR thermal cycler control software

After launching, the control application is initialized as follows:

a. Read in the thermal cycling script (see sample in Fig. 8) from a text file.b. The user enters the number of cycles (repetitions) the basic thermal sequence in the script is

to be performed (Fig. 6).c. Read in the parameters R0 , T 0 ,a of each heater element from a text file. The user may also

input and save (pressing “Update”) these parameters interactively (Fig. 6).d. Calculate auxiliary variable β, according to the parameters supplied in step c.e. Reset the sampling clock (period is 20ms) counter.f. Set the drive for each heater either to zero or some (small) default value which corresponds to

a heater temperature in the range of 35-45°C. Reset the integral term of the PI control.

After initialization, the thermal cycling control algorithm starts execution when the user presses the control button “Run” (Fig. 7) as shown below:

1. Wait for the 20ms clock interrupt. Update the clock counter.2. Apply any changes to the temperature set points which are specified for the current value of

the clock according to the thermal cycling profile script.3. Send a command packet to the controller board to obtain the current values N H , NS of the

voltage drops across the current sensing resistors and the heater elements. Note that the values returned are the result of averaging 16 consecutive readings by the microcontroller firmware, to reduce the effects of random noise.


4. Calculate R ,∆ R ,∆T ,T , P∧I for each heater element.5. Calculate the temperature error e=T sp−T for each heater element.6. Calculate the PI plant drive values u=up+ui for each heater element and update the

cumulative sum of the integral terms ui.7. Send a command packet to the controller board to set the precision regulated power source of

each heater element to the calculated PI control value u.8. Perform data logging functions as specified (typically keep a record of the values calculated in

steps 4, 5 and 6, or a subset thereof as per specified logging interval, e.g. 1sec).9. Update the information on the user interface display as per specified screen update interval

(e.g. 1sec).10. Check the exit condition and if false go to step 1.

When the number of cycles specified interactively by the user (Fig. 6) is completed, the above loop exits. Upon exit the following shut down procedure is performed.a. Set the drive for each heater to the same default startup (idle) value as in step f of the

initialization procedure described above.b. Write and close all data logging files.

Figure 6. Control application: setting the parameters for a trial run. Script repeated 2 times.

Figure 7. Control application: running


Figure 8. Example of a thermal cycling script

Figure 9. Result of executing 4 times the thermal cycling script of Fig. 7.

Syntax of the thermal cycling scriptThe cycling script is a text file consisting of a sequence of command lines i=0,1,2,…N. Each command line i has the format: [t i, T1i, T2i, T3i], where ti indicates the instant in time (expressed in units of sec, as an integer number) where the temperature set point of each heater element will assume the value indicated by T1i, T2i, T3i respectively (expressed in units of °C, as a number in integer or decimal format). The numbers within a line must be separated by at least one space character.

If a number of repetitions, M > 1, is specified by the user (e.g. as in Fig. 8), after executing the first line (i=0) at t0, the sequence of script lines [1… N] is repeated M times. The time instants t N+1, tN+2, … , for the 2nd repetition are calculated as:

tN+1 = tN + t1, tN+2 = tN + t2 , etc. tN+N = tN + tN = 2 tN

Similarly for the 3rd repetition the times are:


1 50.0 50.0 50.010 90.0 80.0 70.020 80.0 70.0 60.0

t2N+1 = t2N + t1, t2N+2 = t2N + t2 , etc. t2N+N = t2N + tN = 3 tN

The times for the last (M) repetition are:

t(M-1)N+1 = t(M-1)N + t1, t(M-1)N+2 = t(M-1)N + t2 , etc.t(M-1)N+N = t(M-1)N + tN = M tN

The first script line [t0, T10, T20, T30] is used only to setup the heaters in a known initial state at the beginning of the first cycle at t1, and is not part of the repetition cycles.

Parameter definitionsThe following definitions apply for each of the heating elements 1, 2 and 3.

R denotes the resistance of a heating element.R= R(T) resistance depends on element temperature.R=R0(1+α (T−T 0 )) a first order linear approximation is assumed.∆ R=α R0∆T

∆T= ∆ Rα R0

=β∆ R , β= 1α R0

T=T 0+∆T=T 0+ β ∆R

Real time feedback control of temperature is based on measuring T for each heating element with a 20ms sampling rate. This is achieved indirectly by measuring ∆ R. For each heating element the parameters R0 , T 0 ,a are fixed and given in advance. They are derived by means of a calibration procedure. Resistance increment ∆ R=R−R0 is derived by measuring R using a series current sensing resistance R s (100mΩ) between the power (voltage) source and R, and measuring the voltage drops on both.

T heater temperature, T=T(R).Tsp temperature set point (i.e. target) for a heater.RS current sensing resistance, 100mΩ nominal value, 0.1% accuracy.US (differential) voltage drop across RS.VS (single-ended) voltage at current sensing amp output (gain Gs = 50), measured directly by the ADC with full scale range 5.0V.

I = US / RS = current through the sensing resistor and heater.UH (differential) voltage drop across a heater element R.VH (single-ended) voltage at heater sensing amplifier output (gain GH = 0.3), measured

directly by the ADC with full scale range 5.0V.I = UH / R = current through the sensing resistor and heater.

Since I=U s

R s=UH

R,R=Rs

UH

U s and ∆ R=R−R0


Controller – PC communication

At power up the microcontroller initializes the input-output ports, interrupt flags etc. and then establishes a USB connection to a host PC. Subsequently, the firmware enters a polling loop waiting for a USB packet to arrive from the host PC. The USB packets are 64-bytes long and consist of a tag byte, which identifies a command to be executed, followed by data bytes for updating DAC values etc. The format of the command packets from the host PC to the microcontroller and reply packets from the microcontroller to the host PC is shown in figure below:

Command and reply packet format.

The PC software maintains an input and an output buffer for USB communication with the controller. To send a command, the PC software initializes its output buffer with any parameters in bytes 2..64. Byte 1 specifies the command tag to be sent. To send a reply packet, the microcontroller initializes bytes 1..63 of the output buffer with the requested data from its peripherals (ADC, DAC) or status information and copies the received command tag to byte 0 of the output buffer before sending the packet via the USB port. The command tag is used by the PC software to determine how to unpack the received data as the data is different, depending on the actual command.

The 16-bit DAC and ADC values are placed in two consecutive bytes of a command or reply packet. The most significant byte (high byte) of a 16-bit value is placed in packet byte i, and the least significant (low byte) in packet byte i+1. As an example, the command to set the three DAC values that determine the three voltage (power) sources on the controller board, involves sending three 16-bit values (6 bytes) to the microcontroller. Byte 1 of the command packet is initialized to the tag value CMD_WRITE_DAC. The value of DAC channel 1 is copied into bytes 2 (high byte) and 3 (low byte). DAC channels 2 and 3 follow in buffer bytes 4, 5 and 6, 7 respectively, with their high bytes first. To read the current DAC values, the software sets the tag byte to value CMD_READ_DAC and issues a command packet. The DAC data values are received in a reply packet in the same byte locations as with the write command. Upon receipt, the software unpacks them to form three 16-bit values, representing the voltages of the three power sources.


Command packet (PC software)Reply packet (microcontroller firmware)

Report ID0 1 2 643 4 . . .

0 1 2 633 4 . . .

5. Microheater Temperature Control Testing (R0 range 15-25 Ohm)

A series of tests were conducted to evaluate the temperature control system using Cu resistors fabricated on polyimide substrates. Their resistance values ranged from 20 to 25 Ohms. A first sample was connected and target temperatures were set according to pre-defined PCR protocol temperatures (95oC for denaturation, 72oC for extension and 55oC for annealing), without using a chip holder which was fabricated for facilitating the fluidic interconnects. The system was able to control and record the temperature as shown in Fig. 10. As it can be seen in this graph, the controller successfully reached the target values within a few seconds and kept the temperature constant within an error of ±0.2oC, which is acceptable for PCR experiments.

0 10 20 30 40 50 60 70

30

40

50

60

70

80

90

100

T (o C

)

t (sec)

T1

T2

T3

Figure 10. Heater temperatures for PCR target values, without chip holder.

Next tests involved the use of a PMMA chip holder. This provides an additional thermal mass for the system to control and thus it was expected that power consumption should increase, along with cooling and heating rates of the resistors. Indeed, as seen in Fig. 11 the controller can not reach the target temperatures within 1 min of operation.

So, the system was allowed to run for longer operating time, in order to give it enough time to reach the designated temperatures. As it can be seen in Figs. 12 and 13, for longer operating times all the target temperatures are reached and from there on are kept within the acceptable error of ±0.2oC as in the previous experiment.


0 10 20 30 40 50 60 70

30

40

50

60

70

80

90

100 T1

T2

T3

T (o C

)

t (sec)

Figure 11. Heater temperatures for PCR target values, with the PMMA chip holder. The system was unable to reach steady state within 1 min, as in the case without the holder.

0 20 40 60 80 100 120 140

30

40

50

60

70

80

90

100

T1

T2

T3

T (o C

)

t (sec)

Figure 12. Heater temperatures targeting typical PCR set point values, with the heaters held in a PMMA chip holder, and the controller operating for 2 min.


0 100 200 300 400 500 600

30

40

50

60

70

80

90

100

T1

T2

T3

T (o C

)

t (sec)

Figure 13. Heater temperatures targeting typical PCR values, with the heaters held in a PMMA chip holder, and the controller operating for 10 minutes.

So, even when the thermal mass of the uPCR is increased with the use of the chip holder, the designated temperature values are successfully reached and kept stable within the acceptable error for performing PCR. The longer heating rates observed are not expected to be a problem for flow-through PCR devices like the ones we fabricate (each temperature zone is kept at a constant value throughout the experiment).

Dynamic testsFollowing the evaluation of the temperature controller unit in operating the µPCR microsystem in a static mode (where stable temperature zones are required), experiments were conducted to also test the operation of the device in dynamic mode. In this case, the heating and cooling rates were recorded, while cycling each heater at the designated PCR temperatures. Since, based on our previous measurements, use of a chip holder significantly decreases heating rates, these tests were conducted without it.

As it can be see in Figure 14, thermal cycling in all of the three resistors is successful, with a very fast response. We then conducted a more detailed measurement with a representative microheater to quantify the heating/cooling rates of interest. From the corresponding Figure 15, very fast heating and cooling rates were calculated : from 55oC to 72oC (19.5 oC/s): heating rate and 72oC to 95 oC (12.5 oC/s): heating rate and from 95oC to 55oC (7.2 oC/s): cooling rate

In conclusion, this first prototype controller achieved successfully T-control of microresistors with room temperature resistance of 15-25 Ohms.


0 50 100 150 200 250102030405060708090

100

T1

T2

T3

T (o C

)

t (sec)

Figure 14. PCR temperature cycling for each of the three resistors (R1, R2, R3).

100 110 120 130 140 150 160

50

60

70

80

90

100

EXTENSION

ANNEALING

T (o C

)

t (sec)

DENATURATION

Figure 15. Detailed PCR temperature cycling (between denaturation, annealing, and extension temperatures)for a representative microheater.


6. Microheater Temperature Control Testing (R0 ~50 Ohm)

A second series of tests were conducted involving the efficient control of microheaters with room temperature resistance in the order of 50 Ohms to the target temperature requirements set by the µPCR. These microheaters are mostly made of Al, which is a critical material choice for the microheater design of the µPCR microsystem (see also specifications in section 1).

The following test highlights the necessity of the temperature controller module to be able to deliver suitable power in order to drive larger than 20 Ohms resistors to the targeted temperatures. Fig. 16 shows the temperature response of the first prototype temperature controller (TC1) with resistor values: R1=47.8 Ohms, R2=20.8 Ohms, R3=30.9 Ohms. The results are shown in Fig. 16 where it can be seen that while the heaters with the lower values (R2 and R3) successfully reach their target temperatures, the resistor with a value of 47.8 Ohms cannot rise above around 65oC. The design of the TC1 control system, indeed does not allow the heating of resistors above 50 Ohms. Since we know that the increase in temperature from 23oC to 95 oC corresponds to an increase in resistance by about 5 Ohms, the fact that a resistor of 47.8 Ohms cannot reach the target value (about 53 Ohms) is explained. Consequently, a second version of temperature controller (TC2) was designed that could deliver more power, while keeping a similar accuracy of the temperature set point. TC2 was indeed fabricated and tested.

0 10 20 300

102030405060708090

100

T1

T2

T3

T (o C

)

t (sec)

Figure 16. Heater temperatures for PCR target values utilizing larger value resistors using TC1 temperature controller.

In Fig. 17, a comparative study of the two TC modules (TC1 and TC2) is shown.


Figure 17. Typical temperature responses of TC1 and TC2 for typical PCR temperature target values a. TC1 driving Cu resistors in the order of 20 Ohms. B. TC2 driving Al resistors in the order of 50 Ohms.


Figure 18. a. Validation of the temperature response of TC2 driving Cu resistors with R0 values in the order of 20 Ohms. b. Close up of the temperature fluctuation around the temperature set point (ΔΤerror=±0.1oC)


As seen in Fig. 17 and 18, with the second version of the temperature controller (TC2) it is possible to efficiently drive resistors of the order of 50 Ohms towards the target temperatures and from there on keep them within the same error (±0.1oC) (cf. Fig 18) as in the TC1 case. Minor fluctuations from the targeted value (cf. Fig. 17) are due to Al deposition imperfections, which never occur in the case of Cu resistors.

Thus, with module TC2 the operational window is extended in terms of microheater resistor value range that may be fabricated and be efficiently driven.


Conclusions

A temperature controller circuit for the control of the µPCR device has been designed, assembled and tested. The circuit is built around a micro-controller (for the PID control loop and communication) and 16-bit external DACs and ADCs for driving and sensing the micro-heaters. Temperature control is achieved by controlling the voltage across the resistive heaters whilst a small sensing resistor is used to measure the current flowing through them. Communication with a host PC is done using a USB serial port in order to set the operating parameters of the PCR (e.g. zone temperatures, calibration data and control coefficients) as well as log data during its operation. A first prototype board was implemented and tests were conducted utilizing Cu resistors fabricated on polyimide substrates. Their resistance values ranged from 20 to 25 Ohms. Tests involved target temperatures useful for the PCR protocol; that is 95 oC for denaturation, 72oC for extension and 55oC for annealing. The controller successfully reached the target values within a few seconds and kept the temperature constant with an error of ±0.2oC, an error acceptable for PCR experiments. The board was also tested with the flexible Cu resistors placed in a PMMA chip holder. In this case, the controller needs longer times to reach the target temperatures, as expected, due to the increased thermal mass of the system. It is, however, able to effectively drive the resistors to the target temperatures and from there on keep them within the same error (±0.2oC) as in the previous case. The board was further tested with larger resistor values of the order of 50 Ohms. In this case, the first prototype of the temperature controller required longer times to reach lower and mid PCR temperatures, whilst it failed to reach the higher one of 95oC. This problem was solved with a slight redesign of the temperature controller which allowed it to supply more power to the microheaters. A second prototype was build and tests with 50 Ohms Al microheaters proved that the system is able to reach the target temperature values within the specified accuracy.


APPENDIX


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