## 1 Introduction
### Sensors  
The current state-of-the-art method for capturing the data necessary for gear condition monitoring is by using acceleration sensors on the bearing blocks or casing [2; 7; 8]. The mounting of the sensors means that vibrations are not measured at their source. These measurements are also called ex situ, as opposed to in situ, which means at the source. As a result, the vibration signal contains vibrations from multiple moving elements along the signal path. Furthermore, the longer the signal path, the more dampened the signal gets [2; 9; 10]. Consequently, the signal quality can be poor, making it difficult to detect mild wear. [PERSON] et al. also emphasized the necessity of monitoring mild wear [6].  
However, the approach of integrating sensors into machine elements of the transmission is not new, as several studies show [7; 11; 12]. In the following, a few such studies are introduced. The design is similar for all of them. The sensors are placed as close as possible to the source of vibration. The sensors are scanned by a central processing unit which temporarily stores or wirelessly transmits the data. Additionally, a power supply module is required. It is not always the case that all of these necessary components are integrated. [PERSON] et al. [11], [PERSON] et al. [7] and [PERSON] et al. [12] all presented accelerometers integrated into a gear or a hollow shaft. They showed that it was possible to detect faults like spalling, tooth fracture or heavy wear with in situ measured data. Although [PERSON] et al. suggested an additional benefit from shortening the signal path [9], the benefit could not be demonstrated. This lack of demonstration could be due to poor sensor data as noted by [PERSON] et al. and [PERSON] et al. [PERSON] et al. integrated the sensor into a hollow shaft, resulting in a longer signal path than the others. These factors may explain why the quality of condition monitoring was not as good as expected. Another problem is the space required by the solutions presented. Due to the design chosen, the sensors were installed separately from the microcontroller. This design required a second module on the shaft, which takes up extra space. For full integration on the gearwheel, the sensor and the necessary accessories such as microcontroller and data transfer unit need to be even smaller. There are a number of publications dealing with the full integration of sensors into the gearwheel [13; 14].  
Besides vibration signals, temperature is one of the common quantities which is used for condition monitoring in general [15]. [PERSON] et al. used an integrated sensor that measured the temperature of the oil bath. It was shown that it is possible to detect tooth breakage due to higher temperatures [15]. This method is only suitable for measuring temperature where oil bath lubrication is used. The method is not suitable for other types of lubrication such as grease or dry running. In addition, it is not possible to differentiate the fault between several gears in a gearbox. [PERSON] et al. conducted a study to classify the abrasive wear of steel gears based on gearbox temperature [16]. Temperature was measured using an infrared camera, and the classification was based solely on features extracted from the camera images. The study did not measure the temperature of individual gears [16]. To overcome the challenge of measuring temperature at individual gears, it would be necessary to integrate the temperature sensor into the gear.  
In conclusion, the condition monitoring of gearboxes based on integrated sensors, namely, acceleration sensors, is part of ongoing research. Other sensors with potential, such as temperature sensors, have received less attention. In addition, the benefits of these integrated acceleration sensors have not yet been experimentally proven, although the benefit of integrated sensors, which would be crucial for monitoring mild wear, has been shown in theory.