Ultrasonic Air Temperature Sensing for Automatic Climate Control — Vehicle Test

Ultrasonic Air Temperature Sensing for Automatic Climate Control — Vehicle Test

ABSTRACT

An ultrasonic air temperature sensor, intended to help improve automatic climate control (ACC), has been demonstrated in a vehicle. Ideally, ACC should be based on inputs correlated with thermal comfort. Current ACC systems do not measure the air
temperature best correlated to thermal comfort — at breath level in front of an occupant. This limits the thermal comfort that ACC can provide under transient conditions. An ultrasonic sensor measures the bulk air temperature, is transparent to the driver, and can use commercially available components. In a proof-of-concept test, we monitored the thermal transients in a vehicle during cool-down after a hot soak and also during warm-up after a cold soak. The ultrasonic path was along the roof console. The ultrasonic temperature always agreed to ±1 ºC with the air temperature measured by a thermocouple at the midpoint of the ultrasonic path. Compared to breath-level temperature, there was good agreement for the winter test but, for the summer test there was a 5 ºC constant shift. Thermocouple data taken at the two locations showed the same effect. The ultrasonic sensor is rapid and precise, and it is a good candidate for improving ACC.

INTRODUCTION

We recently showed that thermal comfort could be improved by basing control on the air temperature at breath level [1]. We also proposed an unobtrusive ultrasonic method to measure bulk air temperature in vehicles, shown in Figure 1, and showed that it had accuracy of ±0.5 ºC. Ultrasonic pulses, above the range of human hearing, are timed as they move from the source to the detector [2]. The measured delay gives an indication of the average temperature of the air along the path. In this paper we demonstrate an unobtrusive ultrasonic air temperature sensor in a vehicle. To evaluate its performance, two tests were performed: cool down from a hot soak (summer test) and warm up from a cold soak (winter test). The air temperature measured by the ultrasonic sensor is compared to the temperatures measured at various locations in the vehicle by conventional thermocouples.

Most production ACC systems utilize cabin temperature as one input to help control interior temperature. Other commonly used inputs are outside ambient temperature and solar load. Two types of cabin temperature sensors are conventionally used: fan-aspirated “in-car” sensors intended to measure air temperature, and infrared (IR) sensors intended to respond to average temperature of the surfaces in the field of view. Even the best implementations of fan-aspirated in-car temperature sensors for ACC in vehicles suffer from tracking error, time-lag, and drift problems. The in-car temperature sensor is usually located in the
instrument panel, behind a small grille. This sensor picks up air temperature trends, but absolute accuracy is degraded by air stratification, heat storage in the instrument panel, and discharge from nearby heater vents. In vehicle tests, the temperature indicated by the in-car sensor sometimes is as much as 10 ºC different from the air temperature at breath level. The system is sometimes even driven away from desirable conditions. Another problem with the in-car sensor is its response time. The error between the temperature sensor and true breath-level temperature depends on the initial ambient conditions. The in-car temperature sensor tends to overestimate the interior temperature during a cool-down, and underestimate the temperature during a warm-up. This can cause overshoot past the desired control point and uncomfortable conditions.

Delphi supplies an IR system for ACC. It was first used on the 1999 Jeep Grand Cherokee. One or more IR detectors in the instrument panel view the driver, the front seat passenger, or both. The IR system is intended to provide the average surface temperature of the objects within its field of view. However, the temperature of a solid surface in a vehicle’s interior changes slowly in response to a change in air temperature. During a fast transient, a special algorithm is needed to compensate for the sensor output lag compared to the breath-level air temperature.

Thermal comfort is affected by factors besides air temperature. People differ in their individual preferences. Radiant heat exchange and the distribution of air velocity around the occupant [3] are important. Other variables that influence thermal comfort are the solar load, the occupant’s clothing, and the humidity in the passenger compartment. The importance of breath-level air temperature was shown in [1]. Subjective thermal comfort votes from occupants were compared with temperatures from the in-car sensor and from a thermocouple at breath level. In Figs. 2a and 2b, we plot the measured temperatures as a function of comfort votes. To quantify the correlation of the sensor outputs to thermal comfort we use the linear correlation coefficient, r. There is scatter in the comfort votes due to the subjective evaluation of thermal comfort. Despite this, the correlation of comfort votes to breath-level temperature (r = 0.71) is much better than it is to the temperature measured by the in-car sensor (r = 0.28). The data show that perceived thermal comfort could be significantly improved if the input to the ACC control system from the in-car sensor were replaced by input from a sensor that measures the bulk air temperature in
front of the occupant.

In the following, Section II provides background information about ultrasonic temperature measurement and its use in a vehicle. Section III compares ultrasonic measurements of bulk air temperature in a vehicle with thermocouple temperature measurements under various HVAC operating conditions. Finally, Section IV discusses implementation of ultrasonic temperature measurement in a vehicle.

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