*4.2.1 Structures of wheel-legged robots*

The structure of a wheel-legged mobile platform depends on (i) the number of legs, (ii) the leg type, and (iii) the leg arrangement. The feet consist of 2-DOF steerable powered wheels as illustrated in **Figure 5**.

*Number of legs*: The minimum number of legs required for statically stable walking is four-three legs providing support in the form of a stable tripod while the other leg performs the transference phase [38]. Combining sequences of leg


#### **Table 3.** *Robots designed specifically for agriculture.*

#### **Figure 5.**

*Wheel-legged structures. (a) 4-DOF articulated leg; (b) 3-DOF SCARA leg; (c) 2-DOF SCARA leg; (d) 1-DOF leg.*

transferences with stable tripods produce a walking motion. A wheel-legged robot requires only three legs for translational motion, which provides additional terrain adaptation.

*Leg type*: Legs are based on the typical configurations of manipulators; thus, articulated, cylindrical, Cartesian, and pantographic configurations are the types used most often.

*Leg arrangement*: The normal arrangement for a 2*n*-legged robot is to distribute *n* legs uniformly on the longitudinal sides. Four-legged structures present some advantages regarding terrain adaptability, ground clearance, and track width control (crop adaptability) but also have some drawbacks, such as additional mechanical complexity (complex joints designs, including actuators and brakes) and control of redundant actuated systems, which exhibit complex interactions with the environment and make motion control more difficult than that of conventional wheeled platforms. **Table 4** illustrates different theoretical wheel-legged structures.

#### *4.2.2 Examples of wheel-legged robots*

**Figure 6a** illustrates the structure scheme of a wheel-legged robot based on the 3-DOF SCARA leg (See **Figure 5b**) with full terrain adaptability, ground clearance control, crop adaptability, and capability of walking, and **Figure 6b** shows the structure of a wheel-legged robot exhibiting full terrain adaptability, ground clearance control, and crop adaptability; however, it cannot walk under static stability.

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*Unmanned Ground Vehicles for Smart Farms DOI: http://dx.doi.org/10.5772/intechopen.90683*

**Structure Characteristics**

Advantages:

• Crop control.

Disadvantages:

Advantages:

• Crop adaptability.

Disadvantages:

Advantages:

• Crop adaptability. Disadvantages:

• Limitations for walking. Use in smart farms:

used in the short term.

Use in smart farms:

• Full capability for walking.

• A large number of actuators (20).

Use in smart farms:

• Full capability for walking.

A 4-DOF articulated leg with a 2-DOF wheeled foot (**Figure 5a**)

A 3-DOF motiondecoupled leg\* with a 2-DOF wheeled foot (**Figure 5b**)

A 2-DOF motiondecoupled leg\* with a 2-DOF wheeled foot (**Figure 5c**)

Another interesting example is the structure of BoniRob [39], a real wheel-legged platform for multipurpose agriculture applications, which consists of four independently steerable powered wheeled legs with the structure illustrated in **Figure 5d** (1-DOF legs with a 2-DOF wheeled foot). This robot can adjust the distance between its wheel sets, making it adaptable to many agricultural scenarios. The platform can be equipped with common sensorial systems used in robotic agricultural applications, such as LIDAR, inertial sensors, wheel odometry, and GPS. Moreover,

• Full terrain adaptability and ground clearance control.

• Full terrain adaptability and ground clearance control.

promising for use in smart farms in the long term.

• Full terrain adaptability and ground clearance control.

• A medium number of actuators (16).

• A huge number of actuators (24) that jeopardize the robot's reliability.

• This structure is the most complex structure that exhibits complete wheel positioning and orientation in its working volume. However, the orientation of the wheel does not provide additional characteristics regarding stability or traction. Thus, this structure provides the same advantages as other structures (see **Figure 5c**) but with extra complexity, which will jeopardize its application in smart farms. This structure is presented here as the most complex platform.

• This structure provides full positioning of the wheel in its working volume and can control the robot's body leveling, which allows for the wheel plane to be aligned with gravity, which provides an excellent robot's stability using fewer motors than the structure illustrated in **Figure 5a**. In addition, this structure can walk under static stability, an interesting feature when the robot works in very irregular, soft, or muddy terrain. Its terrain adaptability, ground clearance control, and crop adaptability, along with its medium complexity, make this structure the most

• This structure can control the ground clearance, leveling, and distance between wheels; the latter determines the adaptation to different crops (distance between crop rows). Nevertheless, the wheel moves on a vertical-cylindrical surface rather than in a working volume. This fact impedes the robot from walking and, thus, exhibits worse characteristics than the structure illustrated in **Figure 5b**. In any case, it can be a proper structure to introduce wheel-legged vehicles and could be

*Agronomy - Climate Change and Food Security*

transferences with stable tripods produce a walking motion. A wheel-legged robot requires only three legs for translational motion, which provides additional terrain

*Wheel-legged structures. (a) 4-DOF articulated leg; (b) 3-DOF SCARA leg; (c) 2-DOF SCARA leg;* 

*Leg type*: Legs are based on the typical configurations of manipulators; thus, articulated, cylindrical, Cartesian, and pantographic configurations are the types

*Leg arrangement*: The normal arrangement for a 2*n*-legged robot is to distribute *n* legs uniformly on the longitudinal sides. Four-legged structures present some advantages regarding terrain adaptability, ground clearance, and track width control (crop adaptability) but also have some drawbacks, such as additional mechanical complexity (complex joints designs, including actuators and brakes) and control of redundant actuated systems, which exhibit complex interactions with the environment and make motion control more difficult than that of conventional wheeled platforms. **Table 4** illustrates different theoretical wheel-legged

**Figure 6a** illustrates the structure scheme of a wheel-legged robot based on the 3-DOF SCARA leg (See **Figure 5b**) with full terrain adaptability, ground clearance control, crop adaptability, and capability of walking, and **Figure 6b** shows the structure of a wheel-legged robot exhibiting full terrain adaptability, ground clearance control, and crop adaptability; however, it cannot walk under static

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stability.

adaptation.

*(d) 1-DOF leg.*

**Figure 5.**

structures.

*4.2.2 Examples of wheel-legged robots*

used most often.

Another interesting example is the structure of BoniRob [39], a real wheel-legged platform for multipurpose agriculture applications, which consists of four independently steerable powered wheeled legs with the structure illustrated in **Figure 5d** (1-DOF legs with a 2-DOF wheeled foot). This robot can adjust the distance between its wheel sets, making it adaptable to many agricultural scenarios. The platform can be equipped with common sensorial systems used in robotic agricultural applications, such as LIDAR, inertial sensors, wheel odometry, and GPS. Moreover,



#### **Table 4.**

*Wheel-legged structures.*

#### **Figure 6.**

*Model of wheel-legs: (a) full terrain-crop adaptability, (b) full terrain and partial crop adaptability.*

the robotic platform can be retrofitted and upgraded with swappable application modules or tools for crop and weed identification, plant breeding applications, and weed control. This robotic platform is completely powered by electricity, which is more environmentally friendly but reduces its operational working time compared to conventional combustion-engine systems. Nevertheless, this robot configuration requires custom-built implements, which prevent the reuse of existing implements and, thus, jeopardize the introduction of this robot to the agricultural market.
