Control of liquid cooling garments: technical control of mean skin temperature and its adjustments to exercise

This paper describes an automatic control concept for liquid cooling garments. The concept consists of an own controller for mean skin temperature whose setpoint is either fixed or adjusted to the metabolic heat production by means of the heart rate signal. The controller for mean skin temperature included both a proportional and an integral signal path (PI-type), the latter being able to eliminate any load error within the control loop. This means that the actual skin temperature will always match the given setpoint irrespective of the amount and the origin of the heat gain at the body shell. Experiments were carried out to test the operation of the skin temperature controller. There the setpoint was fixed while metabolic heat production was changing. After a transient period with deviations, the load error was always eliminated by the skin temperature controller. With this result one can also imagine the controllers ability to compensate changing heat gains from the environment. Despite this behaviour, the amount of heat removal was not high enough to prevent sweating and warm discomfort during all exercise levels. Therefore we draw the conclusion that, in addition, the setpoint of the skin temperature should be adjusted to the metabolic rate/heat production. A convenient physiological signal that reflects the current level of metabolic rate is the heart rate signal. After being filtered the heart rate signal was used during some experiments to change the setpoint of the skin temperature controller. The reason for this filtering (lowpass, time constant = 10 min) was, firstly, the necessity of attenuating the consequences of short-term psychological effects on the heart rate and secondly, the avoidance of vasoconstriction due to too fast changes of the exercise/heart rate induced cooling rate. In the following experiments it became clear that the adjustment of the skin temperature setpoint to the exercise level was an improvement as there was less sweating and the subjects felt more comfortable.