High-Precision Temperature Control for GRADFLEX

Paper number

IAC-06-A2.5.05

Author

Mr. Matthias Boehme, OHB-System AG, Germany

Coauthor

Mr. Juergen Moors-Nitschmann, OHB-System AG, Germany

Coauthor

Mr. Lutz Guenther, OHB-System AG, Germany

Coauthor

Mr. Ralf Greger, Switzerland

Coauthor

Mr. Antonio Verga, European Space Agency (ESA)/ESTEC, The Netherlands

Year

2006

Abstract

The GRAdient Driven FLuctuations EXperiment (GRADFLEX) ¹ will fly on-board the FOTON-M3 spacecraft in 2007 on a 12-day mission. GRADFLEX comprises two fluid physics experiments to study temperature gradient induced fluctuations inside two different fluids under microgravity conditions. Core element of each experimental setup is a fluid cell to which a well defined temperature gradient is applied. The fluctuations are observed by optical means using the shadowgraph technique.
A very precise temperature control is essential for both experiments: The gradients have to be adjusted with 0.07 K accuracy, and must be stable within 0.01 K rms over 12 hours and longer. Heating and cooling are realised mainly by TECs, while one experiment also includes a resistive coating on the optical window as a heater.
The temperature and experiment control is the task of the ECM (Experiment Control Module), a dedicated controller board in the GRADFLEX electronics. Each of these boards (one per experiment) provides 4 serial interface ports (RS-422) and allows for standalone operation, controlled by a PC.
Each ECM board can drive 4 independent loops to control different temperatures in the experiment. The control loops are configurable by software – they can also be coupled to run in parallel and keep the same temperature in an array of 4 TECs, as needed in one of the GRADFLEX experiments.
The control loops are based on commercial TEC driver circuits (current sources), which are completely controlled by a software PID algorithm. In the table driven software concept, all control parameters can be adapted to the specific experiment situation.
Temperatures in GRADFLEX are acquired by individually calibrated 10 kOhm thermistors to achieve a maximum accuracy. The temperature measurement chain eliminates all disturbances as far as possible, e.g. by using a separate 24-bit ADC for each sensor signal, adequate filters on each input, and a reference resistor in each sensor line. A sensitivity below 0.005 K shall be achieved in order to meet the temperature stability requirements of GRADFLEX.
The ECM is a flexible board design that can be adapted to any application needing a very sensitive temperature control. Apart from the temperature control, it has its own microcontroller and flash memory, and offers multiple acquisition channels for HK data as well as timer and trigger services for experiment equipment.
In the conference contribution, the ECM board concept and the test results of the temperature control loops for GRADFLEX will be presented.

---

1

GRADFLEX: A Microgravity Experiment for Gradient Driven Fluctuations, B. Hirtz et al., IAC-05-A2.6.01

Abstract document

IAC-06-A2.5.05.pdf

Manuscript document

IAC-06-A2.5.05.pdf (🔒 authorized access only).

To get the manuscript, please contact IAF Secretariat.