**4. Specifically designed mobile platforms**

The second approach to the configuration of mobile robots for agriculture is the development of autonomous ground vehicles with specific morphologies, where researchers develop ground mobile platforms inspired more by robotic principles than by tractor technologies. These platforms can be classified based on their locomotion system. Ground robots can be based on wheels, tracks, or legs. Although legged robots have high ground adaptability (that enables the vehicles to work on irregular and sloped terrain) and intrinsic omnidirectionality (which minimizes the headlands and, thus, maximizes croplands) and offer soil protection (discrete points in contact with the ground that minimize ground damage and ground compaction, an important issue in agriculture), they are uncommon in agriculture; however, legged robots provide extraordinary features when combined with wheels that can configure a disruptive locomotion system for smart farms. Such a structure (which consists of legs with wheels as feet) is known as a wheel-legged robot. The following sections present the characteristics, advantages, and disadvantages of these specifically designed types of robots.

#### **4.1 Wheeled mobile robots**

#### *4.1.1 Structures of wheeled robots*

The structure of a wheeled mobile platform depends on the following features: *Number of wheels*: Three nonaligned wheels are the minimum to ensure platform static stability. However, most field robots are based on four wheels, an approach that increases the static and dynamic stability margins [28].

*Wheel orientation type*: An ordinary wheel can be installed on a platform in different ways that strongly determine the platform characteristics. Several wheel types can be considered:

a.**Fixed wheel**: This wheel is connected to the platform in such a way that the plane of the wheel is perpendicular to the platform and its angle (orientation) cannot change.

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**Figure 3.**

*(d) independent steering and traction system.*

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

its orientation can change freely.

orientation actuator.

classified as follows:

power.

b.**Orienting wheel**: The wheel plane can change its orientation angle using an

c.**Castor wheel**: The wheel can rotate freely around an offset steering joint. Thus,

*Wheel power type*: Depending on whether wheels are powered, they can also be

a.Passive wheel: The wheel rotates freely around its shaft and does not provide

*Wheel arrangement:* Different combinations of wheel types produce mobile platforms with substantially different steering schemes and characteristics.

a.Coordinated steering scheme: Two fixed active wheels at the rear of the platform coupled with two passive orienting wheels at the front of the platform are the most common wheel arrangement for vehicles. To maintain all wheels in a pure rolling condition during a turn, the wheels need to follow curved paths with different radii originating from a common center [29]. A special steering mechanism, the Ackermann steering system, which consists of a 4-bar trapezoidal mechanism (**Figure 3a**), can mechanically manage the angles of the two steering wheels. This system is used in all the vehicles presented in **Table 2**. It features medium mechanical complexity and medium control complexity. One advantage of this system is that a single actuator can steer both wheels. However, independent steering requires at least three actuators for steering and power (**Figure 3b**).

b.Skid steering scheme: Perhaps the simplest structure for a mobile robot consists of four fixed, active wheels, one on each corner of the mobile platform. Skid steering is accomplished by producing a differential thrust between the left

*Steering driving systems: (a) Ackermann steering system; (b) independent steering; (c) skid steering system and* 

b.Active wheel: An actuator rotates the wheel to provide power.

*Agronomy - Climate Change and Food Security*

Nevertheless, UGVs suitable for agriculture remain far from commercialization, although many intermediate results have been incorporated into agricultural equipment—from harvesting to precise herbicide application. Essentially, these systems are installed on tractors owned by farmers and generally consist of a computer (the controller), a device for steering control, a localization system (mostly based on RTK-GPS), and a safety system (mostly based on LIDAR). Many of these systems are compatible only with advanced tractors that feature ISOBUS control technology [25], through which controllers connected to the ISOBUS can access other subsystems of the tractor (throttle, brakes, auxiliary valves, power takeoff, linkage, lights, etc.). Examples of these commercial systems are AutoDrive [26] and X-PERT [27]. An important shortcoming of these solutions is their lack of intelligence in solving problems, especially when obstacles are detected because they are not equipped with technology suitable for characterizing and identifying the obstacle type. This information is essential when defining any behavior other than simply stopping and waiting for the situation to be resolved. Another limitation of this approach is that the conventional configuration of a standard tractor driven by an operator is designed to maximize the productivity per hour; thus, the general architecture of

The second approach to the configuration of mobile robots for agriculture is the development of autonomous ground vehicles with specific morphologies, where researchers develop ground mobile platforms inspired more by robotic principles than by tractor technologies. These platforms can be classified based on their locomotion system. Ground robots can be based on wheels, tracks, or legs. Although legged robots have high ground adaptability (that enables the vehicles to work on irregular and sloped terrain) and intrinsic omnidirectionality (which minimizes the headlands and, thus, maximizes croplands) and offer soil protection (discrete points in contact with the ground that minimize ground damage and ground compaction, an important issue in agriculture), they are uncommon in agriculture; however, legged robots provide extraordinary features when combined with wheels that can configure a disruptive locomotion system for smart farms. Such a structure (which consists of legs with wheels as feet) is known as a wheel-legged robot. The following sections present the characteristics, advantages, and disadvantages of

The structure of a wheeled mobile platform depends on the following features: *Number of wheels*: Three nonaligned wheels are the minimum to ensure platform static stability. However, most field robots are based on four wheels, an approach

*Wheel orientation type*: An ordinary wheel can be installed on a platform in different ways that strongly determine the platform characteristics. Several wheel

a.**Fixed wheel**: This wheel is connected to the platform in such a way that the plane of the wheel is perpendicular to the platform and its angle (orientation)

the system (tractor plus equipment) is only roughly optimized.

**4. Specifically designed mobile platforms**

these specifically designed types of robots.

that increases the static and dynamic stability margins [28].

**4.1 Wheeled mobile robots**

types can be considered:

cannot change.

*4.1.1 Structures of wheeled robots*

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*Wheel power type*: Depending on whether wheels are powered, they can also be classified as follows:

a.Passive wheel: The wheel rotates freely around its shaft and does not provide power.

b.Active wheel: An actuator rotates the wheel to provide power.

*Wheel arrangement:* Different combinations of wheel types produce mobile platforms with substantially different steering schemes and characteristics.


#### **Figure 3.**

*Steering driving systems: (a) Ackermann steering system; (b) independent steering; (c) skid steering system and (d) independent steering and traction system.*

and right sides of the vehicle, causing a heading change (**Figure 3c**). The two wheels on one side can be powered independently or by a single actuator. Thus, the motion of the wheels in the same direction produces backward/forward platform motion; and the motion of the wheels on one side in the opposite direction to the motion of wheels on the other side produces platform rotation.

c.Independent steering scheme: An independent steering scheme controls each wheel, moving it to the desired orientation angle and rotation speed (**Figure 3d**). This steering scheme makes wheel coordination and wheel


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electric energy [35].

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

algorithms, etc.).

*4.1.2 Examples of wheeled robots*

at the rear (**Figure 4c**), etc.

robot can work for 2–4 hours at a 2–5 km h<sup>−</sup><sup>1</sup>

requires eight actuators for a four-wheel vehicle.

steerable wheels and two rear fixed and active wheels.

position accuracy more complex but provides some advantages in maneuverability. In addition, this scheme provides crab steering (sideways motion at any angle α; 0 ≤ α ≤ 2π) by aligning all wheels at an angle α with respect to the longitudinal axis of the mobile platform. Finally, the coordination of driving and steering results in more efficient maneuverability and reduces internal power losses caused by actuator fighting. The independent steering scheme

**Table 2** summarizes the advantages and drawbacks of these schemes. Note that the number of actuators increases the total mass of a robot as well as its mechanical and control complexity (more motors, more drivers, more elaborate coordinating

Some examples of wheeled mobile platforms for agriculture are the conventional tractor using the Ackermann steering system (**Figure 2**) with two front passive and

Skid steering platforms can be found in many versions. For example,

• Two fixed tracks, each one placed longitudinally at each side of the robot,

• Two fixed wheels placed at the front of the robot and two castor wheels placed

An example of a mobile platform under development that focuses on performing precision agricultural tasks is AgBot II (**Figure 4c**). This is a platform that follows the skid steering scheme with two front fixed wheels (working in skid or differential mode) and two rear caster wheels. It is intended to work autonomously on both large-scale and horticultural crops, applying fertilizer, detecting and classifying weeds, and killing weeds either mechanically or chemically [31, 32]. Another robot is Robot for Intelligent Perception and Precision Application (RIPPA), which is a light, rugged, and easy-to-operate prototype for the vegetable growing industry. It is used for autonomous high-speed, spot spraying of weeds using a directed micro-dose of liquid when equipped with a variable injection intelligent precision applicator [33]. Another example is Ladybird (**Figure 4b**), an omnidirectional robot powered with batteries and solar panels that follows the independent steering scheme. The robot includes many sensors (i.e., hyperspectral cameras, thermal and infrared detecting systems, panoramic and stereovision cameras, LIDAR, and GPS) that enable assessing crop properties [34]. One more prototype, very close to commercialization, is Kongskilde Vibro Crop Robotti, which is a self-contained track-based platform that uses the skid steering scheme. It can be equipped with implements for precision seeding and mechanical row crop cleaning units. This

rate and is supplied by captured

Regarding the independent steering scheme, the robot developed by Bak and Jakobsen [30] is one of the first representative examples (**Figure 4a**). This platform was designed specifically for agricultural tasks in wide-row crops and featured good ground clearance (approximately 0.5 m) and 1-m wheel separation. The platform is based on four-identical wheel modules. Each one includes a brushless electric motor that provides direct-drive power, and steering is achieved by a separate motor.

• Four fixed wheels placed in pairs on both sides of the robot

*Agronomy - Climate Change and Food Security*

**Characteristics**

• Simplicity.

Disadvantages: • Large turning radii.

Use in smart farms:

• Few actuators (2). Disadvantages:

Use in smart farms:

Disadvantages:

Use in smart farms:

wheels steered independently.

• Compact size, robustness, few parts.

• Full mobility (including crab motion).

• Complex control algorithms.

intensively used in smart farms.

**Steering scheme**

Coordinated Advantages:

Skid Advantages:

Independent Advantages:

*Characteristics of wheeled structures.*

and right sides of the vehicle, causing a heading change (**Figure 3c**). The two wheels on one side can be powered independently or by a single actuator. Thus, the motion of the wheels in the same direction produces backward/forward platform motion; and the motion of the wheels on one side in the opposite direction to the motion of wheels on the other side produces platform rotation.

c.Independent steering scheme: An independent steering scheme controls each wheel, moving it to the desired orientation angle and rotation speed (**Figure 3d**). This steering scheme makes wheel coordination and wheel

• Few actuators (2) if based on the Ackermann device.

• Good turning accuracy if the front wheels are steered independently.

• Steering control on loose grounds, e.g., after plowing, is difficult.

• Agility (motion with heading control and zero-radius turns).

• The maximum forward thrust is not maintained during turns.

• Vehicle rotations erode the ground and wore the tires.

• Many actuators and parts (eight for a four-wheel robot).

Hence, such steering control is not expected to be used in smart farms.

• Terrain irregularities and tire-soil effects demand unpredictable power supply.

• This steering scheme is simple and robust, but not very precise in loose terrain; hence, it could be used in smart farms, e.g., for indoor tasks, but not for infield tasks.

• This steering scheme is the more versatile of the schemes, but it is also more complex and expensive. However, most of the engineering systems evolve by increasing their sophistication and robustness while decreasing their cost; hence, this scheme will be

• Ideal rotation in only three steering angles if based on the Ackermann device. • Requires three actuators and more complex control algorithms if based on front

• New mobile robotic designs are abandoning this scheme, which only offers simplicity.

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**Table 2.**

position accuracy more complex but provides some advantages in maneuverability. In addition, this scheme provides crab steering (sideways motion at any angle α; 0 ≤ α ≤ 2π) by aligning all wheels at an angle α with respect to the longitudinal axis of the mobile platform. Finally, the coordination of driving and steering results in more efficient maneuverability and reduces internal power losses caused by actuator fighting. The independent steering scheme requires eight actuators for a four-wheel vehicle.

**Table 2** summarizes the advantages and drawbacks of these schemes. Note that the number of actuators increases the total mass of a robot as well as its mechanical and control complexity (more motors, more drivers, more elaborate coordinating algorithms, etc.).
