## 1 Introduction  
Reliable pipe conveyor operation is every user's basic requirement. Conveyor belts are filled with transported material, and after having been filled, they close into a pipe shape when the belt's edges overlap. One of the options for ensuring the reliable operation of a pipe conveyor is the accurate identification of the pipe conveyor's failures during its operation. Nowadays, during the operation of pipe conveyors, excessive twisting in the closed conveyor belt occurs, significant changes occur in the contact forces on rollers located in the hexagonal idler housings due to a missing roller, and damage to or rupture of the conveyor belt occurs, along with a notable change in the working temperature. Further research and development in this area are aimed at online diagnostics for the mentioned effects. Due to uneven loads, an uneven direction in bends, or otherwise badly set rollers in the idler housings, belts in the pipe conveyors can rotate around their own axis. The rollers used to maintain the closed pipe shape of the belt in the correct position cause damage to the pipe belt [1]. Early detection of unwanted twisting in the conveyor belt will limit, prevent, or minimize damage to the conveyor system and the loss of transported material [2]. The company FLSmidh has created a cost-effective and reliable solution for transporting material using a pipe conveyor, which continuously online monitors twisting in the pipe belt using pairs of light detection sensors and ranging technology (LIDAR) [3]. Based on the detection of excessive pipe belt twisting, with the help of a pair of adjustable idler housings, their position and the rollers are automatically adjusted during operation, thereby correcting the twist. A similar principle was also used by [PERSON] et al. [1], who designed the Industrial Pipe Conveyor Belt Orientation Monitoring System. It consists of a microcontroller connected to LIDAR sensors and a Wi-Fi module. The design of the monitoring system can easily monitor the pipe conveyor belt position from a considerable distance and shut down the conveyor if the level of twisting is exceeded. A pipe conveyor monitoring system was also developed by VVV Most [4]. The device is designed to measure the degree of deflection and deformation in the pipe conveyor belt under operating conditions. The design of the monitoring system allows tracking in most pipe conveyor positions. The device evaluates the location of RFID tags and magnetic markers when passing them around detectors and RFID reader antennas, and magnetic markers also determine the vectors of their magnetic moments. Furthermore, a larger number of detectors allows one to ignore time changes in the magnitude of the mark magnetization. The control system monitors the passages of the said magnetic marker points, and from the values obtained with the detectors, determines the transverse position and spatial orientation of the vulcanized marks, and subsequently the degree of folding and deformation in the conveyor belt. The Pipe Conveyor Health Monitoring System [5] uses laser scanning technology to collect pipe conveyor operation data. Using the real-time data, it detects belt swing, the belt overlap position, cuts/tears/breaks in the conveyor belt, and mechanical joints. The state control algorithm generates alerts in the case of any deviation from the set level on the computer monitor. The system can be configured to stop the conveyor belt. The good technical condition of rollers guarantees minimal costs for the conveyor's operation. Monitoring contact forces on rollers is currently just beginning to be applied in pipe conveyors. It is related to ongoing research in laboratories using test stands [6; 7; 8; 9; 10]. Using a test stand, [PERSON] et al. [11] studied the rotational resistance of the roller under operating load. The authors found a linear increase in the rotational resistance of the roller with an increased radial load of the roller as well as the roller's rotational speed.