## 2 Materials and Methods
### Electronic Nose (eNose)  
Figure 1 shows the eNose used in this work, a custom compact implementation developed at the Robotics Laboratory of the Universitat de Lleida [61]. This eNose consists of an array of sixteen BME680 environmental sensors from Bosch Sensortec [51]. The BME680 is a system-on-a-chip (SoC) device that integrates a temperature sensor, a humidity sensor, a pressure sensor, and a gas sensor. This eNose has been applied to discriminate different gases and odors [62] but this is the first application operating as a mobile sensing device.  
The gas sensor embedded in the BME680 is based on a MOX [52] resistive layer whose ohmic resistance changes in the presence of various volatile compounds. The sensing deposition layer of the MOX gas sensors embedded in the BME680 has particles of different sizes to sense a wide range of gases. This deposition layer configuration provides poor selectivity, making it difficult to identify a particular volatile compound without additional data processing. Additionally, MOX gas sensors present an inherent variability between sensors in the response curve to various gases. This eNose implementation can overcome these individual limitations by processing the readings of its sixteen gas sensors. Additionally, this eNose implementation applies different configuration parameters to each gas sensor to increase sensor variations and enhance its combined detection performance. A detailed explanation of the eNose design and implementation is provided in [61].  
The eNose (Figure 1) uses a standard USB-A male connector for the main power and data connection with the main central processing unit of the robot. The eNose power consumption under normal operation conditions is 900 mW [61] so it can be plugged into any USB 2.0 host device as this specification allows a power output of up to 2.5 W. The device also incorporates a redundant secondary power input that powers the device in case the USB connector is powered off or disconnected. The eNose is automatically activated when it receives power from any of its inputs. In this case, the redundant power is connected directly to the main battery of the robot to keep the eNose activated even when the central processing unit of the robot is switched off.  
The BME680 sensor devices used in the eNose are controlled internally through a shared SPI connector. The registers of the BME680 allow the definition of the target preheating temperature used to heat the sensor resistor of the MOX gas sensor. This preheating temperature affects the response curve to the gas mixtures and therefore must be maintained between calibrations. Following the manufacturer's recommendations, the eNose was powered on, using the auxiliary power supply, at least 24 h before the first experiment to ensure the sensing elements were fully heat soaked and operated in a steady state. In this application, the redundant power of the device is used to keep the eNose active, even when the robot is switched off between the different experiments.  
The BME680 gas sensor's target preheating temperature can be set in a range between 200 \({}^{\circ}\)C and 400 \({}^{\circ}\)C. Table 2 contains the target preheating temperatures defined for the  
Figure 1: Details of the eNose embedded in the mobile delivery robot.  
sixteen eNose gas sensors. As stated before, the target preheating is different for each sensor to enhance the variability between sensors and create a multi-sensor device.