Design and Experimental Evaluation of a Form Trimming Machine for Horticultural Plants

: Form trimming is an important practice in horticulture. Currently, handheld trimming tools are the most commonly used in China, which presents certain disadvantages including high human labor input, low productivity and inconsistent performance. In this work, a wheeled form trimming machine was designed for shrub plants with the aim of reducing labor input, increasing efﬁciency and improving trimming performance. The machine was mainly composed of three parts: a supporting frame, a rotary base and a knife system. The design and construction of the key components of the machine were introduced. The knife system was a combination of multiple cutter units with reciprocating motions. The number of units and their connecting angles could be adjusted to realize different trimming shapes. The knife system was carried by the rotary base and could realize 360 ◦ rotations to cut the plants into a desired form. Experiments were performed to determine the optimal working parameters (cutting frequency of the cutter unit and rotating speed of the rotary base). The similarity between the plant proﬁle after trimming and the proﬁle of the knife system and the consumed time in each operation were chosen as two evaluation indexes. Results showed that when the cutting frequency was 16.7 Hz and the rotating speed of the rotary base was 13.5 r/min, the trimming operation could be completed by two circles, and the time consumption was 8.89 s. Furthermore, to test the adaptability of the machine, ﬁve different shrub plants were chosen and trimmed by the machine, and results showed that the overall similarity was above 93%. Therefore, the form trimming machine developed could meet the requirements of shrub trimming in horticulture with desirable precision and adaptability.


Introduction
The green area per capita is an important index of the living environment and quality of life in a city. From 2000 to 2017, the area of vegetation coverage increased by 5% in the world and one quarter of that amount was contributed by China [1]. According to the report "Global Urban Land Composite and Expansion" from the National Remote Sensing Center of China, the green area per capita in cities increased from 23.14 m 2 in 2000 to 40.47 m 2 in 2020 on a global scale [2]. A rapid increase in green spaces (parks, gardens) in the cities means a booming horticulture industry and an increasing demand for supporting technologies and facilities. Pruning and trimming are common practices in horticulture. The superfluous or dead/diseased branches, twigs, leaves, and buds of horticultural plants need to be cut off on a regular basis. This practice is important for keeping the beauty of the landscape, promoting a healthy growth and adjusting the blossoms of plants [3][4][5].
Shrubs are a common type of horticultural plants used for landscape. They require regular trimming to keep desired forms and sizes. Although the horticulture industry is developing rapidly, the mechanization and automation level of some common practices is relatively low, especially in China [6]. Currently, form trimming of shrubs is mainly performed by human operators using handheld shears and trimmers. Obvious disadvantages include high human labor input, low productivity and inconsistent performance. Therefore, to provide a time-and-energy efficient solution for shrub trimming is of great value in promoting horticultural mechanization.
Through a comprehensive literature survey, there exists a significant body of work on trimming machines for various plants, such as grass [7,8], coconut [9], spinach [10,11], and hedge [12]. However, there are very few studies on form trimming of shrubs or bushes. Since each plant has its particular physical and mechanical properties, and the trimming requirements for different plants vary, the structural designs of those trimming machines cannot be directly applied in form trimming for shrubs. The most recent work on shrub trimming is reported in [13], in which a robotic arm with a vision system and a motion planning algorithm was developed. A circular double-blade cutter was mounted onto the robotic arm as the end-effector and was guided by the motion planning algorithm to cut a shrub plant into a designed shape. The accuracy of the system was tested to be lower than required, and further improvement on the vision system was suggested. Some machinery companies in Europe, such as Orlandi Group of Italy and Lanz Technik of Germany, have produced ground support trimming machines for form trimming of plants to reduce human labor demand. However, the design principles of those machines were not clear and no performance evaluation were performed.
Cutters and blades are important working parts in trimming machines for agriculture and horticulture. A series of work were conducted on designing suitable cutting elements and determining their optimal working parameters for trimming crops [14][15][16], fruit trees [17][18][19] and landscaping plants [20] etc. Such as in [21], the optimal combination of cutting speed, cutting gap and blade bevel angle were experimentally determined for the moving blade of a trimming tool for longan branches, and in [22], the optimal fruit feeding rate, rotating speed of the trimmed fruit and the knife height were experimentally determined for an automatic trimming machine for young coconut fruit. Overall, there are two main types of cutters used for plant cutting and trimming: disc type and blade type. Disc cutters are commonly used in cutting crops, vegetables, grasses and tree twigs through continuous rotations. The blade type can have various mechanical configurations, such as single moving blade, double moving blades, double blades with one blade moving and one fixed, multiple blades, and various motion types: circular, angular, and linear. Currently, the blade type cutters with linear reciprocating motion are mainly used in agricultural harvesters for grain crops. Double blade cutters, with one set of linearly moving blades or two sets of oppositely moving blades, are commonly seen on trimmers of tree branches and hedges. These machines are usually handheld, and form trimming cannot be realized without human operators' maneuvers.
Overall, with the rapid development of horticulture industry, the demand for more advanced horticultural machinery is increasing. The aim of this work is to design and test a form trimming machine for shrubs so that this labor-intensive operation can be completed by a more efficient and automatic solution. The design method and experimental evaluation process reported herein can be applied in developing form trimming machines for horticultural plants to promote the mechanization and automation level of trimming operation.
The remainder of the paper is organized as follows. The design of the form trimming machine and its key components are introduced in Section 2. Then, experiments for determining the optimal parameters and capacities of the machine and for testing the adaptability of the machine are detailed in Section 3. Results and discussions are presented in Section 4. The conclusions are drawn in Section 5.

Oveall Structure and Working Principle
The two main forms of shrub plants are spherical and conical, i.e., balls and cones, both of which can be shaped by revolving motions of the cutter. The general working principle of the designed form trimming machine was to rotate a flexible cutter by 360 • to complete trimming of the plant. The contour of the cutter could be adjusted to form different shapes. Figure 1 shows the overall structure and a prototype of the form trimming machine, which consisted of three main parts: a supporting frame with wheels, a rotary base and a knife system. The form trimming operation was completed with a combination of two motions: rotation of the rotary base and reciprocating motion of the cutter units of the knife system. The supporting frame was made of hollow square steel tubes to make the structure sturdy and light weight. The machine used two groups of wheels for supporting and transporting, two standard wheels and two caster wheels with locks, so that the machine can be easily moved and fixed. For easy maintenance and control, the machine used lithium batteries as the power source and electric motors for driving the working components. The rotary base was mounted on the frame through a pair of steel bars. The height of the rotary base could be adjusted to fit different heights of the plants. The knife system was carried by the rotary based and could perform 360 • rotations under its driving. The details of the parameters of the machine are shown in Table 1.  The rotary base and the knife system were the two key working components of the machine. Their design and construction details are given as follows.

Rotary Base
The rotary base provides the rotation motion required for the trimming operation. As shown in Figure 2, its main components included an upper cover, a driving motor, a slip ring, a motor coupling, a small driving gear, an internal driven gear, a slewing bearing and a lower cover. A key working parameter of the trimming machine is the rotating speed of the rotary base. A value too large will result in misses of plant branches and poor trimming performance, while a value too small means low working efficiency. Since the rotation motion of the knife system is analogous to the forward motion of the cutter bar on combine harvesters of corn, rapeseed etc. in the field, we referred to the working speed of combine harvesters and determined an initial speed range for the rotary base. The following equation was used for calculation: where n is the rotating speed of the rotary base (r/min), v is the maximum linear speed along the contour of the knife system (m/s), r is the maximum rotating radius of the knife system (mm). Generally, the field speeds of combine harvesters for crops are between 0.5 m/s and 1.0 m/s [23][24][25], i.e., v in Equation (1) was 0.5~1.0 m/s. The maximum rotating radius of the knife system was about 0.5 m, i.e., r = 500. Then, n was calculated to be 9.5~19 r/min. This range was set as an initial guideline for choosing the rotating speed for the rotary base. The optimal speed was determined through experiments after the cutter blades were chosen and the knife system was installed. The motor chosen for the rotary base was a 24V DC motor (model XD-37GB555, weight 310 g) with a rated speed of 93 r/min and rated torque of 8 kg·cm. Through the internal gear mesh, which had a reduction of 4:1, the torque transferred to the knife system was about 32 kg·cm, and the maximum rotating speed available for the rotary base was about 23.3 r/min.

Knife System
In order to fit different forms and sizes of shrub plants, the knife system needs to be flexible and can adjust shapes. In the designed machine, the knife system was a combination of multiple cutter units. As shown in Figure 3, the cutter units were connected in a sequence. Each unit was composed of a cutter holder, a driving motor, a driving gear, an internal gear with an eccentric wheel, a moving blade, and a fixed blade. The blades were made of 65 Mn steel with 0.62~0.7% carbon and 0.9~1.2% manganese. The hardness of the blades was 62HRC after a quenching heat treatment. The surface of the blades was coated with a layer of rust-resistant material so that the blades can have a long operating lifetime. To keep the whole knife system lightweight while ensuring enough cutting power, each cutter unit was driven by a small DC motor. The continuous rotations of the motor were converted into reciprocating swings of the moving blades through the eccentric gear mesh. The connecting angles of the cutter units could be adjusted to form different shapes for the knife system. The two commonly used shapes were a straight-line shape and an arc shape, respectively for cone form trimming and ball form trimming, as shown in Figure 4.

Principle of the Cutting Motion of Blades
Each cutter unit contains one fixed blade and one moving blade. The moving blade swings back and forth to make a cutting function. Figure 5 shows the principle of the generation of reciprocating motion of the moving blade. The driving motor drives the inner gear with the eccentric wheel to rotate around the center (O 1 ) of the inner gear, and then, drives the moving blade to rotate around the pin (O). When the eccentric wheel rotates for one circle, the moving blade swings back and forth once. According to the kinematic relationship, the coordinates of the eccentric wheel's center (O 2 ) is given by where L OO 2 is the distance between O and O 2 (mm), h is the length between point O 1 and point O (mm), e is the eccentricity of the eccentric wheel (mm), θ is the swing angle of the moving blade (rad), ω 0 is the angular speed of the driving motor (rad/s). The swing angle is an important parameter for the cutting blades. According to Equation (2), the swing angle meets the following Equation: tan θ = e sin ω 0 t h − e cos ω 0 t Then, θ can be obtained by The swing angular speed of the moving blade ω 1 (rad/s) is determined by The swing limit angle of the moving blade can be derived as According to the parameters of the chosen blades, e = 6 mm, h = 35 mm, the maximum swing angle of the moving blade can be calculated as 19.74 • . This swing angle could permit that the moving blade sweeps the full cutting gap with respect to the fixed blade in one reciprocating cycle.

Determination of the Number of Cutter Units
The knife system could adjust to different shapes and sizes of plants. By increasing or decreasing the number of cutter units, the contour of the knife system could be changed. A common shrub shape is a ball-like sphere and the knife system needs to form an arc, as shown in Figure 6.

The relation between the linear length and arc length is shown below
where L 1 is the arc length of the cutter unit (mm). If L is the total arc length determined by the shape of the plant, the number of cutter units required n can be calculated by

Determination of the Driving Motor Speed
The cutting ability of the blades is highly related to the frequency of the cutting motion, which is equal to the rotating speed of the driving motor. To keep the knife system lightweight while ensuring enough cutting power, a small DC motor (model JGB37-545, weight 260 g) with a rated speed of 1000 r/min and rated torque of 0.8 kg·cm was chosen for each cutter unit. After the cutter units and the knife system were assembled, the driving motor was set to run at different speeds to cut branches of different diameters, and the optimal driving motor speed was determined through experiments.

Experiments
This section introduces the experimental setup for determining the optimal working parameters, i.e., the cutting frequency of the cutter unit and the rotating speed of the rotary base, of the designed form trimming machine, and for evaluating the performance of the trimming operation.

Determination of the Cutting Frequency of the Cutter Unit
The driving motor of the cutter unit has a maximum rotating speed of about 1000 r/min, i.e., the maximum cutting frequency of the cutting blades is around 16.7 Hz. To test the cutting performance of the cutter unit under different cutting frequencies, the driving motor was controlled to run at four different speeds: 450, 650, 850, and 1000 r/min, to trim the branches of Aglaia odorata, a widely planted shrub in China. The diameters of the branches on the plant ranged from 1 to 5 mm. The branches, both cut off and un-cut off, were collected and their diameters were measured.

Determination of the Rotating Speed of the Rotary Base
The rotating speed of the rotary base is another key working parameter of the trimming machine. To determine the optimal rotating speed, the rotary base was controlled to run at five different speeds: 7.5, 10.5, 13.5, 16.5 and 19.5 r/min. The tested shrub plant was Fagraea ceilanica, with its size before trimming being around 500 mm and the average moisture of its branches being 36.2%.
To evaluate the performance of the trimming operation, the contour of the knife system and the contour of the plant after trimming were compared. To make a quantitative comparison, the trimming performance was recorded by a photography system (German PCO, series dimax HD+, 1920 × 1440 resolution). The experimental setup is shown in Figure 7. A grid paper was put in the background, and a white paper with angle marks were stuck on the ground. The photography system was placed in front of the trimming machine. After the trimming was completed, the contours of the knife system and the trimmed plants were captured. To record a full view of the plant contours, the plant was put in 12 different angle positions, and the angle marks on the ground were used for alignment. Pictures of contours taken by the photography system are shown in Figure 8.  After trimming, the pictures were then uploaded to a PC with a MATLAB program for processing and analysis. The profiles of the knife system and the plant after image processing are shown in Figure 9. Feature points (x i , y i ) were extracted from the profile of the knife system, and (a i , b i ) were extracted from the profile of the plant. The overall relative deviation of the plant profiles from the profile of the knife system can be calculated by the following equation where E S is the overall relative deviation, m is the number of pictures taken for each trimmed plant and m = 12, k is the number of feature points extracted in each picture, (a i , b i ) are the coordinates of feature points of the trimmed plant (mm), (x i , y i ) are the coordinates of feature points of the knife system (mm). The similarity S between profile of the trimmed plant and the profile of the knife system can be calculated by The second index for evaluating the performance of the machine was the time consumed for each complete trimming. During the experiments, the knife system was rotated for at least two circles. If the performance was not desirable, more rotations were carried out.

Adaptability Test
It is desirable that the designed trimming machine can work on different shrub plants. After the cutting frequency and the rotating speed of the rotary base were determined, the adaptability of the machine was tested. A total of five common shrub plants, Aglaia odorata, Murraya paniculate, Camellia oleifera, Osmanthus fragrans, Radermachera sinica, were chosen to be trimmed by the machine. The average diameters and moisture contents of the branches were measured before the test and are listed in the Table 2.  Table 3 shows the performance of the cutter unit under different driving motor speeds/cutting frequencies. The cutting capability of the cutter unit generally improved with an increase in the driving motor speed. Under the lowest speed 450 r/min, the cutter unit could only cut off the thinnest branches. Under the highest speed 1000 r/min, the cutter unit could cut off 4.5 mm branches in one run, while branches thicker than 5 mm were too strong for the cutter unit. As reported in [26], the cutting energy required for cutting Miscanthus x giganteus stems was proportional to the stem diameter and the cutting energy from the blade was directly proportional to the cutting speed. For the cutter unit in this study, a higher cutting frequency meant a faster cutting speed of the moving blade, and thus provided more cutting energy. Generally, the branch diameters of small shrub plants are smaller than 5 mm, and the designed cutter unit will be capable of trimming them. A shrub plant with branches thicker than 5 mm would be a plant too large for the trimming machine designed in this work. Since the cutter unit performed best under the highest driving motor speed, 1000 r/min, or 16.7 Hz cutting frequency was determined to be the optimal working parameter for the cutter unit.

Determination of the Rotating Speed of the Rotary Base
The trimming operation was performed when the rotary base was commanded to turn at different speeds. Taking the consumed time and similarity as the evaluation indexes, the results of the experiments were calculated and are listed in Table 4 as follows. From Table 4, the trimming operation under all rotating speeds could be completed with desirable performance, similarities all above 96%. However, during the experiments, it was noticed that when the rotating speed of the rotary base was too fast, larger than 13.5 r/min, the branches missed by the cutter units increased and more circles were required to complete a desirable trimming. A speed lower than 13.5 r/min could also result in a good trimming effect, but the time consumed increased. Considering both efficiency and performance, the rotating speed of the rotary base was determined at 13.5 r/min, and the trimming operation was to be completed in two circles with a corresponding time consumption of 8.89 s.

Adaptability Test
In the adaptability test, the cutting frequency of the cutter unit was fixed at 16.7 Hz and the rotating speed of the rotary base was fixed at 13.5 r/min. Each plant was trimmed by two rotations of the knife system. The results of the test are shown in Table 5. It can be seen from Table 5 that all five plants were successfully trimmed by the designed form trimming machine, and the profile of the knife system and the profile of the trimmed plants matched well. As the calculations show, the overall similarity was above 93%. This confirms that the designed form trimming machine could adapt to different shrub plants.
However, this similarity level was lower than the anticipated performance. As a preliminary analysis, the reason would be the differences in the branch densities and strengths. The plants Aglaia odorata, Murraya paniculate, and Camellia oleifera, which resulted in the smallest similarities in the test, had thinner, softer branches and the trimming process was not as smooth as what was recorded in trimming the other plants. A different rotating speed of the rotary base would be more appropriate for those plants. The correlation between trimming performance and the branch density and strength can be taken as future studies to further improve the performance of the trimming machine.

Conclusions
A form trimming machine for shrub plants was designed and tested in this study. The machine was mainly composed of three parts: a supporting frame, a rotary base and a knife system. Through the design, construction, and experimentation of the form trimming machine, the following conclusions are drawn: (1) The two key working parameters, the cutting frequency of the cutter unit and the rotating speed of the rotary base, were determined to be 16.7 Hz and 13.5 r/min respectively. The trimming operation could be completed by two rotations of the knife system with a time consumption of 8.89 s. (2) Through a performance evaluation using a photography system and an imaging processing method, the similarities between the profile of the knife system and the profile of the trimmed plants were calculated. An overall similarity of 96% was obtained when the machine was tested to trim Fagraea ceilanica plants. (3) Through an adaptability test using five different species of shrub plants, the overall similarity was above 93%, which showed that the designed machine could meet the requirement of trimming various shrub plants with stable performance.