Design and Applications of a Novel Performance Test Bench for a Single-Knot Knotter

Abstract

The knot-forming motions of knotters are difficult to directly observe, and knotting tests are labor-intensive and time-consuming. To address these issues, a novel test bench motion scheme was proposed to simulate the bundling process of a bale, and a performance test bench for a single-knot knotter was developed under controllable indoor conditions. The bench can verify the working principles of different knotters and assess the knotting success rate. To meet the requirements of unattended and safe operations for long-term continuous knotting tests, an automatic control process for the test bench was constructed to identify the knot-releasing failure and twine breakage fault based on monitoring the pressure of the simulated bale. By establishing the corresponding relationships between knotting actions and image states, a test and analysis method for detecting the motion timing sequences of the knotter was developed. This allowed us to obtain a motion timing diagram of the knotter, which can provide references for its reverse design or performance evaluation. The results of the continuous knotting tests show that the tested knotter achieved a knotting success rate of 99.57%, and the test bench can meet the performance testing requirements of the knotter.

1. Introduction

The square baler is essential for straw storage and forage production, with the knotter as its pivotal component. The knotter must perform tasks such as twine feeding, twine clamping, twine winding, twine hooking, twine cutting, and knot release within one second. Given the brief action cycle and precise timing required for the actuating mechanism, field operations present challenges. Factors such as forage moisture content, heavy straw-dust, bale density, twine pretension, and operation speed are in constant flux. These variations complicate efforts to maintain a consistent load-bearing boundary for the knotter and hinder accurate fault location and action phase tracing through manual observation. As for the development of knotters (including the reverse design and performance evaluation of knotters, or the verification of the working principle of new knotters and the testing of their knotting success rate), due to the complex field environments, they are not directly installed on balers for testing in normal practice. Usually, the knotter test bench is used to complete these processes. The newly developed knotter will only be installed on the baler after completing the test bench testing, and it will then undergo reliability testing under field conditions according to the knotter testing standards to obtain a third-party inspection report. The knotter manufacturer will use it as the basis for producing and selling knotters. The purpose of developing a knotter test bench is precisely to meet the requirements of performance testing and reliability research for various single-knot knotters, and to solve problems such as difficulty in directly observing the knot-forming motions of the knotter installed on the baler, as well as the labor-intensive, time-consuming and costly nature of knotting tests in the field. Therefore, a knotter test bench needs to be developed to replicate the knotting process of the knotter, to collect key load data, and to support the monitoring of knot-releasing failure and twine breakage, as well as action phase tracing under controllable indoor conditions. This is of great significance for the performance evaluation, structural optimization and reliability research regarding knotters.

Extensive testing has been conducted on the working principle, structural optimization and reliability of knotters. Such tests are heavily reliant on the knotter test bench to complete the verification after completing the structural design, theoretical calculations and virtual prototype simulations of the knotter. In terms of the wear and fatigue of key knotter mechanisms, Yin et al. [1] proposed a wear calculation method for the tripping mechanism based on a rigid–flexible coupling dynamic model; meanwhile, Lv et al. [2] carried out anti-wear design and knotting tests for the tripping mechanism. Regarding knotter structures and knot-forming performance, Bao et al. [3] performed optimized design and tests on electric knotters using ADAMS (Automatic Dynamic Analysis of Mechanical Systems). Other authors [4,5,6,7] have conducted research on the improved design and bench tests of the knotter. Cen [8] evaluated the reliability of knotters for square balers from the perspective of failure mechanisms, using a combination of theoretical analysis and bench testing methods. In terms of knotting test methodologies, Yin et al. [9] put forward a continuous knotting test method and developed a knotter fatigue test bench with maximum knot-forming efficiency of 3 knots per minute. Li et al. [10] drew on the motion scheme of the sliding table [9] to develop another knotter test bench, but this test bench could only test a knotter. So far, there are still no commercial knotter test benches available for purchase. Moreover, existing knotting test methods have low knot-forming efficiency—although different motion schemes for test benches have been adopted. The existing test benches are unable to meet the requirements of unattended and safe operation for long-term continuous knotting test because they lack these functions of monitoring knot-releasing failure and twine breakage faults. Moreover, the existing knotter test benches also lack the functions of observing the knot-forming motions to establish motion timing diagrams of the knotter, which is extremely important for the reverse or optimization design of knotters. In addition, the service life of the existing knotter test benches is insufficient.

Focusing on the above-mentioned problems, this study proposes a novel test bench scheme to simulate the bundling process of a bale with compression and rebound characteristics. With reference to the functional requirements of the test bench, we designed its mechanical structure and measurement and control system; the performance test bench for a single-knot knotter was developed to meet the requirements of unattended and safe operation for long-term continuous knotting tests under controllable indoor conditions. Using the test bench, a test and analysis method for detecting the motion timing sequences of the knotter was developed to obtain a motion timing diagram of the knotter. Continuous knotting tests were carried out to measure the knotting success rate of the tested knotter and to verify the effectiveness of the test bench design.

2. Materials and Methods

2.1. Functional Requirements of the Test Bench

To address the difficulty of observing the knotting process during field tests and the time-consuming and labor-intensive nature of knotting experiments, the test bench had to meet the following functional requirements: (1) to perform single-cycle or continuous rapid knotting tests under adjustable twine pretension conditions and automatically calculate the knotting success rate; (2) to simulate the compression–rebound characteristics associated with different bale densities; (3) to monitor knot-releasing failure and twine breakage, and to automatically stop the system and issue an alarm, thereby supporting unattended long-term continuous tests and ensuring safe equipment operation; (4) to synchronously acquire in real time the main-shaft rotation angle, main-shaft torque, twine tension force, and frame images of knotting actions for measuring the motion timing of the knotter; and (5) considering that the service life of the knotter should at least reach 300,000 knots, the time interval between the first mechanical failure of the knotter test bench should be greater than 100,000 knots, and the service life should meet at least 1 million knots.

2.2. Conceptual Design and Scheme of the Test Bench

Based on the above functional requirements, the conceptual design of the test bench was divided into three units: (1) a main-shaft servo drive and knotter main-shaft assembly; (2) two sliding tables for simulating bale motion; and (3) two twine preload adjustment units for simulating the load acting on the twine. In terms of spatial arrangement, these units are organized into an upper–middle–lower layer architecture, as shown in Figure 1.

In conceptual design of the test bench, the key was to adopt the kind of motion scheme of a sliding table to reproduce the successive formation of field bales under indoor, material-free conditions. When bundling in the field, the compressed twine was tensioned to maintain the state required for knotting. After a bale was bundled, it was pushed out of the compression chamber by the following bale, and the process was repeated continuously. In this study, the successive formation of an actual bale was simulated by repeatedly moving a simulated bale. As shown in Figure 2, the simulated bale carried by the sliding table moves in coordination with the needle. The simulated bale reciprocates independently in the transverse and longitudinal directions. Within each knotting cycle, it first moved outward in the transverse direction and then advanced longitudinally to push the twine according to a prescribed sequence. After the knotting was completed, the simulated bale retracted transversely to avoid the twine while simultaneously returning to its longitudinal initial position. In this manner, the repeated motion of a simulated bale replaced the successive formation of an actual bale.

2.3. Structural Design and Working Principle of the Test Bench

According to the above-mentioned test bench scheme, considering the rapid knotting tests and detection accuracy, the main technical parameters of the test bench were determined (see

Table 1. Main technical parameters of the knotter test bench.

Parameter	Value
Knotting efficiency	5–6 knots/min
Main-shaft rotation speed range	10–90 r/min, user settings
Main-shaft torque sensor	Dayang DYN-210 torque sensor (Dayang Sensors Co., Ltd., Bengbu, China), range 0–200 N m, comprehensive accuracy 0.1% F.S., allowable overload 200%, response 1 kHz, maximum speed 5000 r/min
pressure sensors	Dayang S-type pressure sensor (Dayang Sensors Co., Ltd., Bengbu, China, range 0–150 kg, accuracy ±0.1% F.S.
Main-shaft angle encoder	YSK80 hollow rotary encoder, 3600 pulses/rev, angular resolution 0.1°
Main-shaft servo motor	Lw 130 main-shaft servo motor (Xinje Electric Co., Ltd., Wuxi, China), rated power 5 kW, rated speed 2500 rpm, rated torque 20 N m, reduction ratio 20
Longitudinal servo-electric-cylinder units	DLP65-S300B07T10B2 longitudinal servo electric cylinder, equipped with 750 W Xinje servo motor (Wuhan Dailing Automation Technology Co., Ltd., Wuhan, China), 300 mm stroke, 750 W XINJE servo motor, pulse control, multi-turn absolute encoder
Transverse servo-electric-cylinder units	H-25B046V1 transverse servo electric cylinder, equipped with 400 W Inovance servo motor (Inovance Technology Co., Ltd., Shenzhen, China), 100 mm stroke, 400 W Inovance servo motor, pulse control, multi-turn absolute encoder
High-speed camera	WP-GXT050 high-speed camera (Weipusmart Technology Co., Ltd., Shenzhen, China) with 500,000 pixels, resolution 816 × 180 pixels, 3600 frames/s
Control and communication	S7-200 SMART programmable logic controller (PLC; Siemens AG, Munich, Germany), Chongchang IPC, bare TCP Socket communication

). For the selection of motors and the torque sensor, Yin [9] and Li [10] suggest using the motors and torque sensors used in this study and shown in Table 1. The main-shaft rotation speed in the test bench ranged from 10 to 90 r/min, encompassing the typical operating range of single-knot knotters. The sensor ranges and accuracy for main-shaft torque and twine tension force can effectively capture load peaks during critical phases of knotting. The encoder can output the main-shaft rotation angle, providing a high-resolution reference benchmark for synchronizing multi-source signals. The power of the main-shaft servo motor adequately met the load drive requirements during the knotting process.

Figure 3 illustrates that the test bench consisted primarily of a main-shaft servo motor drive, a knotter main-shaft system assembly, a wedge-type clutch mechanism, a twine-feeding mechanism, two sliding tables, two twine preload adjustment devices, and a measurement and control system.

As shown in Figure 3, the two knotters were mounted on the main-shaft system assembly in the upper layer of the bench frame. Power from the main-shaft servo drive system was transmitted through a chain to a clutch sprocket freely mounted on the main-shaft. The clutch mechanism integrated the clutch sprocket with the clutch cam and wedge-block clutch jaw. A metering-wheel motor drove the internal-gear tripod to control the engagement and disengagement of the wedge-block clutch jaw. Once the clutch mechanism was operated to engage, driven by the twine-feeding crank fixed to the main-shaft, the twine-feeding needle guided the twine toward the twine-clamping disc of the knotter, which can clamp one end of the twine. This arrangement enabled the main-shaft servo drive system to drive the knotter to sequentially finish twine-clamping, twine-winding, twine-hooking, twine-cutting and knot-releasing under continuous power input. In the main-shaft system assembly, an encoder and a torque sensor were installed on the main shaft to measure the main-shaft rotation angle and the torque of the tested knotter, respectively. It must be pointed out that, from the perspective of the front view in Figure 3, the torque sensor can only measure the torque of the left knotter. The torque of the right knotter may be obtained by reversing the installation positions of the left knotter and the right knotter. In fact, our original intention in designing the main-shaft system of the knotter was that the left knotter and the right knotter could be knotters provided by different manufacturers, or knotters developed by ourselves, and could be compared and tested for knotting simultaneously.

The sliding tables were arranged in the middle layer of the bench frame and below the main-shaft system assembly, which consisted of left–right symmetric longitudinal and transverse motion units. Each transverse servo electric cylinder was installed on the transverse mounting plate, which was installed on the slider assembled with the short guide rail, and which can move along the short guide rail. The short guide rail was fixed to the longitudinal mounting plate, which can move along the long guide rail. The pressure sensor was connected between the end of the transverse servo electric cylinder and the twine-pushing block. An extension spring was connected between the transverse mounting plate and the longitudinal mounting plate. Before knotting, the transverse servo electric cylinders drove the twine-pushing blocks outward in the transverse direction, while the longitudinal servo electric cylinders advanced the longitudinal mounting plates along the long guide rail toward the twine-feeding needle, pushing the twine into a semi-closed loop, as shown in Figure 4b. Under the action of the extension springs, the transverse motion units can reciprocate relative to the longitudinal motion units, which may simulate the compression–rebound characteristics of bale formation. Thus, the function of the twine-pushing block was equivalent to that of the simulated bale. Due to the complexity of the compression–rebound behavior of straw/grass bales [11,12,13,14,15,16,17], the knotting tests were conducted by replacing extension springs with different stiffness coefficients to determine which stiffness coefficient spring was suitable for this design. In the absence of material compression, the compression–rebound characteristics of grass bales were approximately simulated using extension springs.

The working processes of the test bench are shown in Figure 4.

Figure 4a shows the initial state of each component in the test bench. In Figure 4b, the twine-pushing blocks are moved to their designated position and the clutch mechanism is operated to engage. The horizontal component of the force exerted on the twine is measured by the pressure sensors and marked with the symbol P. Compared to the initial position shown in Figure 4a, the 0° of the main shaft is marked as the reference position for the timing analysis of the knotter. Figure 4c shows that the needle introduced the twine into the twine-clamping disc of the knotter, and a closed loop of twine is formed. In Figure 4d, the twine is indicated by a thick red line. One end of the twine is clamped by the twine-clamping disc. Form Figure 4e–j, the knotter sequentially performs twine-winding, twine-hooking, twine-cutting and knot-releasing operations, and a single knot is ultimately formed [18].

To complete the above-mentioned workflow of the test bench, a measurement and control system was developed [19,20,21,22,23,24,25], as shown in Figure 5.

This system mainly comprised a monitor, an industrial personal computer (IPC), six motor drives, and two S7-200 SMART programmable logic controllers (PLCs; Siemens AG, Munich, Germany), which achieve the logical control of the test bench actuators and acquire sensor signals. The IPC connected directly to a high-speed camera to capture video of knotting actions, and the IPC was also in charge of sending the instructions and storing and displaying the data. The S7-200 SMART and the IPC used the Open User Communication protocol, which is a bare TCP Socket communication. The S7-200 SMART was directly connected to the IPC through a standard Ethernet direct line, and the PLC served as the TCP server, listening on port 2000. The IPC actively connected as a client. The PLC used timed interrupts (with a cycle of 1 ms) to collect sensor signals. Each time, 21 bytes were collected, and they were stored in a temporary buffer. A collection counter was set, and each collection was incremented by 1. When the counter reached 10, the 10 pieces of data (210 bytes) were copied as a whole to the sending buffer, and the frame number and cyclic redundancy check (CRC) were updated. Each frame was fixed at 216 bytes: frame header 0xAA (1 byte), frame number (1 byte), data length 0x00D2 (2 bytes), 210 bytes of valid data (10 sets of data packets × 21 bytes), and CRC16 checksum (2 bytes), and TCP_SND command was called to send. After sending, the counter was reset to zero and the next round of collection continued. After receiving the data, the IPC unpacked them according to the frame header and length, verified the CRC, and parsed 10 sets of data. The frame number was used for frame loss detection. Each sensor signal was sent to the PLC through a transmitter, which came with an adjustable digital filter. The pressure sensors were used to measure the contact force exerted on the twine by the twine-pushing block. The encoder and torque sensor were used to measure the main-shaft rotation angle and the torque of the tested knotter, respectively. A direct-acting limit switch was used to determine whether the internal-gear tripod had returned to its initial position after each knotting cycle, whereas a swing-arm limit switch was used to count the knotting cycles. The control flow of the test bench was developed [26,27,28,29,30,31] and is shown in Figure 6.

The control program of the IPC (as the upper computer) was written in Qt software, and the PLC program (as the lower computer) was written in STEP 7-Micro/Win SMART software V2.8.2.1. Once the system was powered on, the main-shaft rotation speed and preset number of test cycles were set through the interactive interface of the IPC control program. When the operator selected the command button on the interface, the knotting command was sent to the PLC, and the PLC started the main-shaft servo motor according to the set speed through the servo drive. The PLC first checked whether the sliding table and internal gear tripod had returned to their initial positions. If the transverse or longitudinal electric cylinders had not fully retracted, the corresponding retraction commands were executed. If the direct-acting travel switch did not provide an initial-position signal, the metering-wheel motor rotated in reverse until the signal was detected. Once all initial position conditions were satisfied, the PLC made the transverse and longitudinal electric cylinders extend outward simultaneously, and the metering-wheel motor rotated to trigger a clutch engagement. Using the initial installation angle of the main shaft as the zero position, the encoder signal was continuously read. If the difference between the main-shaft angle currently indicated by the encoder and the initial installation angle was greater than or equal to 0.1 degrees, it was determined that the clutch was engaged properly, and the first position of main-shaft rotation was marked. Otherwise, the encoder signal was repeatedly read. After the clutch engagement was detected, the PLC saved the main-shaft rotation angle $\theta$, main-shaft torque $T\left(\theta \right)$, left-side twine tension force $P_L\left(\theta \right),$ and right-side twine tension force $P_R\left(\theta \right)$ on the MicroSD card of the PLC. Next, if the marked position was the first position of main-shaft rotation, the PLC immediately triggered the high-speed camera to start capturing videos. Otherwise, the PLC checked whether the marker position had reached 3599. If it had not reached this value, the PLC repeatedly read and saved the $\theta$, $T\left(\theta \right)$, $P_L\left(\theta \right)$ and $P_R\left(\theta \right)$ on the MicroSD card of the PLC. Once the marked position was the 3599th position of the main-shaft rotation, the PLC triggered the high-speed camera to end the video acquisition and the IPC saved it on the hard disk of the IPC. Next, the PLC made the longitudinal electric cylinders retract to their initial position after the transverse electric cylinders retracted to their initial position. At the same time, the PLC sent all sensor data to the IPC, and the IPC program saved, processed and displayed them as the curves on the interface.

Before deciding whether to proceed with the next knotting cycle, it was necessary to determine whether there had been a twine breakage fault or knot-releasing failure in this knotting test. The criteria for determining a twine breakage fault or knot-releasing failure is described in detail in Section 3.4. In the control process, if the twine tension forces decreased to zero within a main-shaft rotation angle range of 175° to 296°, a twine breakage fault was identified. If the twine tension forces were greater than zero within a main-shaft rotation angle range of 175° to 296° but decreased to zero within the range of 296.5° to 360°, a knot-releasing failure was identified. If a twine breakage fault or knot-releasing failure occurred, the main-shaft motor was directly stopped and an alarm was raised to wait for manual intervention. If no fault occurred, but the set number of knotting cycles had been reached, the knotting test was terminated. Otherwise, the next knotting test continued to be executed.

3. Results and Discussion

3.1. Verification Test of the Working Principle of the Test Bench

Action recognition and state detection are widely used [32,33], providing a foundation for visually backtracking knotting actions. To assess the operational stability of the continuous knotting of the test bench and capture image evidence of key action phases, a high-speed camera was used. The camera can simultaneously record actions such as twine feeding by the needle, twine clamping by the twine-clamping disc, twine winding by the knotter jaw, twine hooking by the hook jaw, and twine cutting and knot releasing by the knife arm. Combining high-speed video acquisition with the angle signals of the encoder allows us to verify the actual mechanical state of the knotter when an anomaly occurs in the sensing signals. The test bench was manufactured in 2026. Figure 7 illustrates the working processes of the test bench.

As illustrated in Figure 7a, when the test began, the twine-pushing blocks moved transversely outward simultaneously, according to the direction indicated by the red arrows in the diagram. At the same time, the longitudinal motor propelled the sliding table forward to apply tension to the twine according to the direction indicated by the red arrow in the diagram, forming a simulated bale boundary under the path constraint of the twine, as shown in Figure 7b. After clutch engagement (see Figure 7c), the needle introduced the twine into the twine-clamping disc of the knotter and one end of the twine was clamped, as shown in Figure 7d. The twine-clamping disc, knotter jaw, hook jaw and knife arm sequentially performed the twine-clamping, twine-winding, twine-hooking, twine-cutting, and knot-releasing operations, and a twine loop was ultimately formed within one revolution of the main shaft, as illustrated in Figure 7e–k. The twine loop fell to the ground, and each component of the test bench returned to its initial position, as shown in Figure 7l. Due to the different rotation speeds of the main shaft, the time required for the test bench to complete a single test knot was between 10 s and 12 s. The knotting tests confirmed that the test bench can fulfill the specified functional requirements.

3.2. Visual Test and Analysis of Motion Timing Sequences for a Single-Knot Knotter

Motion timing sequences of the knotter are extremely important for the reverse design or optimization design of the knotter. This section presents the motion timing sequences of the knotter from the synchronized videos and encoder signals. To eliminate the effect of time-scale differences caused by the different rotation speeds of the main-shaft on the comparison of feature positions, the main-shaft rotation angle θ was adopted as a unified phase coordinate. In the same knotting cycle, the signals of the torque, left and right twine tension forces, and video frames can be obtained corresponding to the main-shaft rotation angle θ. This treatment is consistent with the angle-domain analysis of rotating machinery, in which synchronized signals are aligned according to the shaft’s angular position [14].

The test subject was a type of commercial single-knot knotter that is widely used in square balers; it was supplied by Heilongjiang Dewo Technology Development Co., Ltd., China. Polypropylene twine with a diameter of 3 mm was used and its breaking force of 1076 Newtons provided by the twine bale manufacturer. The elongation at break of the twine was 12% to 18%, and the testing method for the twine generally complied with GB/T 8834 [34] “Fiber ropes—Determination of certain physical and mechanical properties”. Prior to testing, zero calibration and synchronization checks were conducted on the encoder and cameras. Initially, the clutch was disengaged, the knotter was set to its initial phase, and the sliding table was retracted to its initial position. The twine was tensioned using a pretension adjustment device. To obtain the motion videos of each executing part of the knotter, the positions between the camera and knotter were placed according to different perspectives; the acquired frame images are shown in Figure 8. These configurations ensured consistent and observable video frames, providing stable conditions for extracting the action sequence based on the main-shaft rotation angle.

Because the shooting of the high-speed cameras was triggered by the high-speed pulse signal of the encoder, the camera captured an image for each signal emitted by the encoder. The main-shaft rotation speed was set to 10 revolutions per second in order for the camera to obtain clear image at the set frame rate every time the encoder triggered the camera. Twine pre-tension remained constant at 50 Newtons. Through 3 knotting tests, the camera captured the motion videos of the knotter from the back of the knotter, the front of the knotter and the side of the knotter.

Since the phase of the knotting motion was determined by the main-shaft rotation angle, this study adopts $\theta$ as a unified reference to obtain the video frame $I\left(\theta \right)$. By correlating the knotting actions and image states, the relationship within one cycle may be processed according to Expression (1):

$${\theta }_i\to I\left({\theta }_i\right)$$

(1)

where i = 1, 2…, 3599.

The motion timing diagram of the tested knotter was obtained, as illustrated in Figure 9. To accurately describe the continuous and overlapping motions of the knotter, the main-shaft rotation angle $\theta$ as the common reference is used in Figure 9.

The concentric tracks represent the needle, twine-clamping disc, knotter jaw, hook jaw, and knife arm. The blue sectors indicate the active angular intervals of the corresponding components, whereas the white sectors represented their stationary states. In the following description, the action timings of knotter components such as the rope needle, twine-clamping disc, knotter jaw, and knife arm are relative to the rotation angle of the main shaft. The needle performed twine feeding from 0° to 175° and subsequently reset from 175° to 360°. The twine-clamping disc operated from 140° to 215.5°. Loop winding, performed by the knotter jaw, occurred from 151.7° to 207.2°, while the hook jaw performed twine hooking from 178.9° to 208°. The knife arm performed twine cutting from 203.3° to 230°, knot releasing from 230° to 253.4°, and the return motion from 253.4° to 296.5°. Due to the limitations of encoder resolution, high-speed camera frame rate and signal-acquisition frequency, the measurement accuracy of knotter action timing was 0.1 degrees.

3.3. Continuous Knotting Tests for Determining the Success Rate of Knotting

The success rate of knotting is an important indicator for evaluating the performance of knotters. It can be calculated using Equation (2):

$$\eta =N\mathrm{s}/N\times 100\%$$

(2)

where $N\mathrm{s}$ denotes the number of successful knots formed, $N$ represents the total number of each group knotting test, and $\eta$ is the success rate of knotting.

By combining the rotation speeds of 30 r/min, 60 r/min and 90 r/min of the main shaft with twine pretension values of 50 N and 80 N, six groups of working conditions were created. Each group had 500 test cycles. For each group of knotting tests, the IPC recorded Ns and N; the success rate of knotting is shown in

Table 2. Knotting success rate statistics under various working conditions.

Main-Shaft Rotation Speed (r/min)	Twine Pretension (N)	Success Rate of Knotting (%)
30	50	99.8
30	80	100
60	50	99.4
60	80	99.6
90	50	99.2
90	80	99.4

.

Table 2 shows that the average knotting success rate of the tested knotter was 99.57%. In the experiment, two types of faults occurred: one was knot-releasing failure in the knotter, and the other was a twine breakage fault. Since the twine breakage fault stemmed from uncontrollable friction during twine feeding rather than a defect of the knotter itself, it should not be counted towards the knotting success rate. The knot-releasing failure of the knotter mainly depended on the stability of the knot-releasing mechanism in the knotter when the twine maintained appropriate tension. A higher pretension in the twine was beneficial for the release of knots, but excessive pretension in the twine may result in overload during twine feeding and the breakage of the twine. In addition, the torque response of the tested knotter can provide measured data for subsequent dynamic, strength and wear analysis of the knotter.

3.4. Knotting Fault Discrimination in Continuous Knotting Mode Based on Applying Pressure Monitoring to Simulated Bales

Due to the knot-releasing failure and twine breakage fault that occurred in the process of the knotting tests, the test bench should be able to automatically identify faults and shut down in order to meet the requirements of unattended and safe operation for long-term continuous knotting tests. Therefore, online discrimination of knotting faults needs to be added into the control process of the test bench. To determine online whether the knotting is normal, it is essential to identify the characteristic differences between normal knotting, knot-releasing failure, and twine-breakage faults.

By analyzing the pressure changes applied to the simulated bales in the knotting fault samples, we found that the twine tension force characteristics can thoroughly distinguish the three situations of normal knotting, knot-releasing failure, and twine-breakage fault, as shown in Figure 10. The pressure sensors measured the horizontal component P of the force exerted on the twine, which indicates the mechanical representation in Figure 4b. By observing the pressure indicators of the sensors at each stage of knotting, an online judgment may be obtained.

As shown in Figure 10, under normal knotting conditions, the horizontal component of the twine tension force continued to increase within the range of 140° to 175°, and it exhibited an abrupt decrease before it reached its main peak at approximately 217°. After successfully releasing the knot, the horizontal component of the twine pulling force rapidly returned to zero by approximately 296.5°. The twine breakage was caused by the twine pulling force exceeding the twine breaking force when the main shaft’s rotation angle was within the range of 60° to 175°. This interval corresponded to the overlap between twine feeding and the initial twine-clamping action, with the beginning of loop winding at 151.7°. Therefore, the twine breakage may be identified by a no-load response on the pressure sensors when the main-shaft rotation angle is within the range of 175° to 296°. For knot-releasing failure, the pressure indicators before the main peak were generally similar to those observed under a normal knot-releasing state. Although the knife arm returned from 253.4° to 296.5°, the knot remained on the knotter jaw, which resulted in a residual pressure after 296.5°. Therefore, this residual pressure was used to identify the knot-releasing failures.

Based on the above results, the criteria for fault identification were added to the control process of the test bench shown in Figure 6. It meets the functional requirements of unmanned operations for continuous knotting.

4. Conclusions

In this study, a novel test bench motion scheme was proposed to simulate the bundling process of a bale. Then, a performance test bench for a single-knot knotter was developed to meet the requirements of unattended and safe operations for long-term continuous knotting tests under controllable indoor conditions. The bench can perform single-cycle or continuous rapid knotting tests to verify the working principles of different knotters and assess the knotting success rate.

By establishing the corresponding relationships between knotting actions and image states, a test and analysis method for detecting the motion timing sequences of the knotter was developed to obtain a motion timing diagram of the knotter, which can provide references for the reverse design or performance evaluation of the knotter. Continuous knotting tests results shown that the test bench can meet the performance testing requirements of knotters.