## 3 Results  
As a result of the simulation, if the measurement was correct, the graphs should have shown an overlap between the contact force and force signal from the sensor. Figure 5 shows the course of the contact force (the thick line) and the response of the sensor (the thin line) for an elastic impact of a 5 mm diameter grain assuming a contact stiffness of 1 \(\times\) 10\({}^{4}\) Nm\({}^{-1}\), which corresponds to a soft material, i.e., a grain with a high moisture content. The resonance frequency of the force sensor was \(f\) = 40 kHz. In this case, the contact was approximately 100 \(\upmu\)s, and the course of the measurement signal corresponded to the course of the contact force. Figure 5: The course of the impact force and measurement signal from the sensor for a large moist grain. Contact stiffness \(k_{1}\) = 1 \(\times\) 10\({}^{4}\) Nm\({}^{-1}\), _freq_ = 40 kHz. Figure 6 shows similar courses for a hard grain, i.e., with equivalent contact stiffness increased tenfold to \(1\times 10^{5}\) Nm\({}^{-1}\). In this case, the course of the contact force differed significantly from the course of the measurement signal from the sensor. The time of the first half-period of sensor oscillation, which is often considered in tests as the contact duration, was significantly longer than the actual contact time. Figure 7 shows that the sensor resonance frequency in the simulation increased to 200 kHz. At this point, the course of the measurement signal from the sensor was closer to the course of the contact force. Similar experiments were performed for the second extreme case of the contact force model, i.e., using a perfectly plastic impact model, with a constant value of the contact force (Figures 8 and 9). Figure 6: The course of the impact force and measurement signal from the sensor for a small dry grain. Contact stiffness \(k_{2}=1\times 10^{5}\) Nm\({}^{-1}\), \(f_{\pi 2}=20\) kHz. Figure 7: The course of the impact force and measurement signal from the sensor for a large dry grain. Contact stiffness \(k_{2}=1\times 10^{5}\) Nm\({}^{-1}\), \(f_{\pi 2}=200\) kHz. The validity of the considerations may be confirmed by numerous research works concerning the measurement of the mechanical characteristics of machines and devices, in which the problem of the proper selection of force sensor properties has been repeatedly addressed [28; 29; 30; 31; 32; 33]. However, such considerations have not been undertaken regarding the aspect of the correct assessment of measuring plant material mechanical characteristics, where the diversity of morphological structure poses many problems. The graphs above show the risk of obtaining false measurements of the course of plant grain impact forces against a flat surface. In the majority of the widely available scientific publications on measuring the course of impact forces for vegetable grain, it is not possible to find analyses that take into account the influence of sensor mechanical characteristics. The authors have focused on direct analyses of the obtained courses, in which components after the main input period have been rejected as measuring noise. Figures 5 and 8 show that this may have been significantly false and could have led to a wrong conclusion about the course of the input function. According to the characteristics obtained, the size and moisture content of the investigated samples (caryopsis) had a significant influence on the discrepancies in the results obtained at low resonance frequencies of the sensor. Therefore, conducting research on larger legume seeds such as soya, beans, and peas has enabled the use of simple sensors with a relatively low resonance frequency, confirmed in numerous publications  
Figure 8: The course of the impact force and measurement signal from the sensor for the perfectly plastic model and large soft grains. The contact force value was \(k_{1}=1\times 10^{4}\) Nm\({}^{-1}\), \(f_{rez}=20\) kHz. Figure 9: The course of the impact force and measurement signal from the sensor for small hard grains and the perfectly plastic model.