## 1 Introduction  
The computer simulation used by the discrete element method (DEM) has been applied more frequently in studies of processes in agricultural machines, which describes the motion of the grain related to sowing, fertilising, drying, sorting, pneumatic transport, etc. [1, 2, 3]. This method includes direct equation integration of grain system motion under the action of various forces--aerodynamic, electrostatic, and, most importantly, the contact forces that arise between the grains and the surfaces limiting the space of their motion [4]. To make correct calculations, mathematical models of grain contact with motion surfaces are necessary [5, 6]. The creation of such models requires their empirical validation by measuring the course of contact forces at the impact of grains with stiff surfaces, which may be movable or immovable [7]. The investigation into force measurement, the reduction of measurement errors, and the development of possibly the smallest sensors have been described in other works [8, 9]. A rapid increase in researchers' interest in the DEM has led to its application in various types of calculations [10]. [PERSON] [11] was one of the first researchers to measure the impact forces of maize grain against a steel plate. The test was carried out in a laboratory station where a steel plate driven by a crank mechanism hit the motionless grain through a piezoelectric force sensor. This helped to control precisely the impact velocity and place of the grain impact. The literature has also shown research results from the influence of the grain elasticity model on the development of individual signals, where the energy decline was shown to be slower for glass balls than wolfram balls [12]. Research into the sensor response to crop waste in a combine-harvester has also been conducted, but the possibility of reducing sensor measuring errors has not been analysed here [13; 14]. In Poland, such research into the needs of the computer simulation of threshing and sowing processes was undertaken for the first time at the Poznan University of Technology by Professor [PERSON] [15]. A spring-arm thrower was used to give the grain velocity, which allowed velocities of grain impact with the sensor to reach values of up to 10 m\(\cdot\)s\({}^{-1}\). The grain velocity was measured with photoelectric sensors before and after the impact. A piezoelectric PCB companies impact force sensor of 70 kHz natural vibration frequency was used to measure the force. Figure 1 shows a typical course of the impact force recorded in these tests. In the studies by Professor [PERSON] [15], regression relations of the impact force and the impact duration maximum value to moisture and velocity were determined for several selected cultivated seed species. Some interpretation difficulties of the recorded signals from the force sensor were observed while measuring hard, small grains (of rye) with low moisture. The grain, after having bounced off of the surface of the force transducer, had an oscillating fading motion for a significant time after bouncing off. These oscillations resulted from the sensor's own vibrations excited by a sufficiently short force pulse. Similar studies on rapeseed were later performed in the Institute of Agrophysics in Lublin, Poland [16]. In the tests, rapeseed was dropped freely on a piezoelectric force sensor from a height of 0.2 m, which caused an impact velocity of approximately 2 m\(\cdot\)s\({}^{-1}\). Figure 2 shows the results; a clear course of the measuring signal in the form of fading vibrations of approximately 40 kHz, which is characteristic of many piezoelectric force sensors. Figure 1: The measuring signal course of the impact force of rye grain of 14% moisture at the speed of 8 m\(\cdot\)s\({}^{-1}\) in a force sensor with a 70 kHz resonance frequency [15]. This type of testing assumes that an impact occurs with a perfectly stiff surface. In fact, such a material does not exist, so grain hitting a sensor material induces parasitic vibrations, which usually take the form of a high fading sound.