Thermal cycler for PCR: Precision improved and time shortened through co-creation among Dynex Technologies and Meerstetter Engineering
This report presents two key advancements aimed at enhancing temperature cycling using simple PID concepts with Peltier devices. The first improvement is achieved by introducing an additional control cascade, which effectively decouples the time constant of the Peltier cells from the overall system under control. The second improvement replaces the physical temperature measurement of the liquid inside a vial with an estimator, allowing for precise temperature control of the liquid within the PCR cycler vial. These improvements demonstrate significant more accurate and optimized temperature transitions between setpoints, leading to better overall performance in maintaining the desired sample temperature. For practitioners this means: The integral difference between vial temperature and target temperature is reduced by a factor of 20, taking into account the last 5 seconds of a plateau. Faster cycle sequences are thus possible and processes become more accurate and cost-effective.
Introduction
Peltier cells are extensively used in analytic temperature cycling applications, such as polymerase chain reaction (PCR), due to their multiple advantages: the ability to heat and cool with the same device, compact design, absence of moving parts, and minimal wear and tear [Temperature Cycling with TEC Controllers (meerstetter.ch)].
Operating Peltier cells in a current-driven mode has shown superior performance compared to PWM [Peltier Element
Efficiency (meerstetter.ch)]. The insertion of a filter coil into each branch of the driver H-bridge allows
for smooth and seamless operation around zero power, enables to operate the H-bridge in two separate half bridges as
well as reduces radiated and conducted electromagnetic emissions.
The direction of temperature change (heating or cooling) and the power level is typically determined by a
TEC
Controller with PID control loop. Advanced approaches, including model-based and
predictive strategies, have also
been explored, as discussed in [1]. However, to maintain high usability, Meerstetter Engineering
has retained the
PID control loop concept while integrating an application-specific I-freeze/I-limit feature and a
thermal model of
the Peltier cell to compensate for non-linearities [Meerstetter
TEC Controller User Manual, chapter: "Temperature Control].
Dynex Technologies is developing a real-time PCR device, focusing on speed and accuracy. To achieve an optimal solution, Dynex Technologies requested that Meerstetter Engineering optimizes the temperature control by leveraging precise temperature overshooting of the sample block. The objective was to optimize the transitions between target temperatures of the sample liquid, achieving precise control of the sample liquid temperature without overshooting and ensuring that the temperature is accurately maintained during hold stages.
Optimizing control concept to reduce cycling time
An improvement of control speed can be obtained in two dimensions: Improvement of the thermoelectric setup to increase heating and cooling capacity OR improving the control algorithm to provide fast yet accurate actions to the Peltier cell. The slowest time constant of the controlled system (plant) defines the maximum reachable bandwidth of the control loop, which is adjusted through the P and I part of the controller. Choosing values for P- and I- parts, which are badly matched to the time constant of the plant or to each other, leads to an unstable control loop [2]. How could the control concept be optimized for reduced cycle time (given the thermoelectric set up is capable to drive the required heat fluxes)?
Cascade control loops
Cascade control loops are a well-known technique for subdividing control systems into more manageable components [cascaded PID], and to accelerate the control action without having to employ regulators relying on derivative action, [3]. Meerstetter decided to integrate an additional control cascade in order to decouple time constants within the control loop with an additional separate Peltier object side temperature control.
Standard PID: The desired vial temperature is actually never achieved and since in most applications, the vial temperature is not measured, this discrepancy remains uncovered.
Cascaded PID: Vial temperature is measured as in intermediate step at a representative vial. By the help of the cascade, the block temperature does overshoot. In the result, gradients in the vial temperature are steeper and the vial temperature does reach and stabilize on the target temperature.
Cascade temperature control for your project
Cascade temperature control provides more precise and stable temperature control. Its optimized structure helps to minimize temperature fluctuations and achieve faster reaction times - ideal for demanding thermal systems.
To use the cascade temperature control feature, the following prerequisite must be met:
- One high-resolution input must be available (it is not recommended using a low-resolution input, as these are filtered slower).
- The associated actuator NTC sensor must be installed close to the Peltier element.
Find out more about cascade temperature control:
Estimated vial temperature
Another significant challenge is that the temperature of the liquid inside the vial is typically not measured directly. For precise control during PCR, where the liquid temperature (not just the sample block temperature) must be maintained accurately for a defined period, controlling this parameter better is crucial. However, measuring the temperature of a representative vial within the vial holder has proven impractical.
Thus, the temperature of the liquid inside the vial was estimated. As a first-order approximation, a PT1 model with heat transfer to the environment was found to yield satisfactory results. Depending on the nature of the thermal object under control, any suitable model can be applied. The model can be described by the following formula:
Where:
α = Heat loss factor
t1 = Time constant of heat transfer from block to vial liquid
The model parameters were determined using a test pattern that separates these two effects. The following diagram displays the temperature test pattern, consisting of a dynamic phase (a), where damping is dominant, and a static phase (b), where heat transfer is dominant.
As optimization criterion for the model parameters, the least mean square difference between the estimated and measured vial temperature was minimized with the side condition, that the maximum difference remains within acceptable limits. The following diagram shows the measured vial temperature (measured only during model parametrization phase) in comparison with the estimated vial temperature. The temperatures are in very good agreement and therefore, the estimated vial temperature will be used as control variable.
Ultimately, the temperature of the PCR solution more closely follows the setpoint temperature, leading to significantly improved chemical analysis results.
Temperature estimator for your project
The estimated target temperature allows more accurate control of the probe temperature without the need for direct measurement. This reduces temperature fluctuations and improves process efficiency.
To use the estimated probe temperature feature, the following prerequisite must be met:
- It must be possible to measure the actual target temperature, at least during the design phase, to experimentally determine the model parameters for the estimator.
Find out more about estimated target temperature:
Conclusion
After all, the integral difference between vial temperature and target temperature has been reduced by a factor of 20 considering the last 5 seconds of a plateau. At the same time, the temperature cycling can be done faster.
At this point, Marc and Martin from Meerstetter Engineering wish to express their gratitude to Tobias and Martin from Dynex Technologies for their outstanding collaboration. Your requests, timely execution of experiments, and constructive feedback during the iteration and validation phases contributed to the development of an easy-to-use, next-level approach for significantly improved temperature cycling. Thank you.
Dynex Technologies thanks Meerstetter Engineering for they excellent support and communication throughout this project. We are impressed by the quality of hardware, software and the competence of all engineers at Meerstetter. All our requests were implemented in very short amount of time and with highest degree of satisfaction from our perspective.
Experience the benefits for yourself now
Accurate temperature control is essential for reliable results. From cascade temperature control to temperature estimator, our solutions help you maximize temperature stability and efficiency. Both features are available exclusively for the TEC controllers from Meerstetter Engineering.
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About Meerstetter Engineering
Meerstetter Engineering is a Swiss company specializing in electronics for laser diodes and Peltier elements. Our products are used in the pharmaceutical and mechanical industries, as well as in research laboratories. With our extensive expertise, we offer development services and collaboration in analog and digital electronics, FPGA, embedded systems, and digital interfaces .
About Dynex Technologies
Dynex Technologies, headquartered in Buštěhrad (Czech Republic), is one of the largest suppliers of laboratory equipment, in-vitro diagnostics and related services in the Czech and Slovak markets. Beyond its own instruments, the company distributes an extensive diagnostics portfolio — covering PCR, LFA and more — from internationally respected brands such as Element Biosciences, Revvity, SOPHiA GENETICS and others, supported by a strong team of MSc/PhD application specialists and a dedicated demo laboratory for hands-on training. Complementing its distribution activities, Dynex designs, develops and manufactures CE-IVD-marked instruments — including fully automated immunoblot processors and microplate PCR systems - in ISO 13485-certified facilities. These products are exported worldwide and backed by a comprehensive lifecycle-support package that includes accredited pipette-calibration services and on-site service provided by more than 100 staff.
Update note: This article has been subsequently updated. The project originally carried out by DIANA Biotechnologies has been transferred to Dynex Technologies. The article has been updated accordingly.
Literature
- [1] .. Qiu, Yuan; "Temperature Control for PCR Thermocyclers Based on Peltier-Effect Thermoelectric" in Proceedings of the 2005 IEEE, Engineering in Medicine and Biology 27th Annual Conference, Shanghai
- [2] .. Ziegler, J.G & Nichols, N. B. (1942). "Optimum settings for automatic controllers" in Transactions of the ASME. 64: 759–768.
- [3] .. Föllinger; “Regelungstechnik: Einführung in die Methoden und ihre Anwendung” in VDE Verlag, 1972