## 1 Introduction
A special adapter was designed so that only 12 wires leading from the hand were conducted inside the hollow shafts of the robotic joints. The developed robot was the basis for developing the DLR space robot. Robotics is also one of the fields where strain gauge sensors are used for online monitoring of forces and torques. Often, this involves not only the use of commercial sensors but also various innovative modifications to selected parts of the manipulators. [PERSON] et al. [19] used a custom three-axis sensor to measure torque and forces in the leg of a walking robot. They achieved a maximum deviation of \(\pm 2\%\) higher than other commercial three-axis strain gauge sensors. However, the developed sensor's deviation can be considered a compromise when comparing its mass, dimensions and, especially, manufacturing cost. [PERSON] et al. [20] dealt with developing an ultra-low-cost torque sensor for a measurement range of 1 Nm, 5 Nm and 20 Nm. They showed different approaches to designing sensor geometry using numerical modelling by finite element method. Based on the results obtained, real sensors were manufactured from aluminium 7075-T6 material. The experimental measurement results were in good agreement with the numerical simulations. This paper presents the design and implementation of a low-cost torque sensor as a part of a prototype manipulator designed for manipulation with a load of 10 kg. As a standard practice, commercially available sensors are used for monitoring various parameters, such as acceleration, displacement and the magnitude of a moment. Their dimensions, measuring range and location often must be considered at the initial design stage. The vibration of the manipulator's links in terms of the precision of the exact positioning of the actuators must also be considered, as this directly affects the accuracy and quality of production. In addition, from a mechanical point of view, vibration can be considered a cyclic loading that, in some cases, can cause a reduction in the lifetime of a seemingly minor component in terms of fatigue, which can subsequently cause more significant damage. It may not be interesting for short-term operations, but manipulators are often designed as part of automated systems for long-term use. In such a case, operating parameter diagnostics participate essentially in predicting the lifetime of machines and manipulators. The aim of the design modification of the load-bearing element proposed by the authors is the possibility of immediate identification of the magnitude and orientation of the torque by using the strain gauges applied directly on the mechanical part of the manipulator. The advantage of the presented design over other authors' designs is that it does not require structural interventions in other mechanical parts of the manipulator. The presented results confirm the suitability of the proposed methodology and its practical use not only for testing prototypes but also for long-term diagnostics of operating parameters of various types of manipulators or collaborative robots. The more information we have, the sooner we can assess the influence of the operating parameters not only on the lifetime of the components but also on the overall safe and reliable operation of the manipulator. This is, of course, not only associated with financial savings but more importantly, the knowledge gained can significantly help designers and developers further develop machines and equipment or confirm their correct designs and procedures.