YANMAR Technical Review

Abstract

In agricultural work, mowing the banks of fields requires a lot of labor, and with the aging of agricultural workers, the demand for labor-saving bank management is increasing year by year.
We closely observed the actual work of our customers in mountainous areas and developed a radio-controlled grass mower for steep slopes that reduces the labor required for mowing.
In addition to being compact yet having excellent stability on steep slopes, the adoption of Yanmar’s patented control technology allows it to cut grass at the same height as a large machine, and the adoption of a series hybrid system allows it to be transported into and out of the barn in electric mode, and it also meets customers' requests for low noise.
In this article, we will introduce these technologies that are installed in the radio-controlled grass mower “YW500RC” released in 2023.

1. Introduction

While rice cultivation machines are available to fill almost every need, one labor-intensive task that remains for rice farmers is that of mowing the banks or causeways between rice paddies. Driven by factors such as aging farmer demographics, the demand for labor-saving measures in causeway management is growing year by year. Moreover, in addition to the work itself, the manual mowing of sloped banks is also a dangerous activity, with the risk of falls or getting caught up in the mower.
Yanmar has conducted observational studies of how customers go about this work in intermediate and mountainous terrain and has applied the quality function deployment (QFD) method to develop a labor-saving radio-controlled grass mower that can operate on sloped land. In doing so, it has produced a compact mower with excellent slope stability for mowing the steep slopes and narrow working spaces that tend to be found on such terrain and the ability to cope with patches of tall grass, also utilizing Yanmar’s own variable speed control technique to equip the mower with the performance to mow the same grass heights as a large mower.
The mower is driven by an electric motor, using a series hybrid configuration in tandem with the engine for patches of grass that require a lot of power to mow while also being able to travel to and from the barn quietly without using the engine.

2. Product Overview

The YW500RC (Fig. 1) is a radio-controlled grass mower that can efficiently mow grass and weeds growing on the banks or causeways built between paddy fields, including around steps in the banks. It can operate on a 45° slope (Fig. 2) and is operated by radio control, with the operator using a transmitter to mow along contour lines. As there is no need for the operator to set foot on sloping ground, this mode of operation reduces the risk of injuries due to falls or getting caught in the rotary cutter. Quality function deployment (QFD) is a design approach for ensuring that products properly fulfill user needs. Having used QFD to identify the required quality characteristics (Table 1), Yanmar sought to create a mower that would excel on slopes in intermediate and mountainous terrain while also being compact and capable of mowing tall grass. At only 870 mm wide, the mower is compact enough to work in confined spaces and its low height of 630 mm makes it suitable for use in orchards. In addition, the variable speed control enables the mower to cut even tall grass that would be beyond the capabilities of competing small mowers. The YW500RC also offers excellent range (operating time) compared to other radio-controlled mowers in its class and can travel to and from the barn or shed where it is stored without engaging the engine.

Table 1 Quality Function Deployment (QFD) Example

3. Product Technologies and Features

3.1. Stability on steep slopes

Maintaining stability when operating on steep slopes is vital. While a wider wheelbase provides more stability, larger mowers are more difficult to navigate around the narrow slopes and pathways found in intermediate and mountainous terrain. The increase in machine weight will also have an impact on operating performance. This makes it important for the mower to be compact with a low center of gravity to provide stability. Moreover, as mowing a slope mainly involves alternately moving forwards and backwards along the contour of the land, the mower should drive just as easily in both directions to avoid putting the operator under excessive stress.
To achieve this, Yanmar optimized the positioning of mower components to combine both lateral balance for slope stability and longitudinal balance to improve handling when cornering. The engine is one of the heaviest parts of the mower. Accordingly, the mower was designed to keep its center of gravity as centered as possible, selecting a vertical two-stroke engine together with a simple design for the power transmission mechanism to achieve a 48:52 longitudinal balance and a 51:49 lateral balance. Furthermore, to improve durability, parts that can have their product life shortened by heat, such as the battery, were positioned away from the heat source of the engine (Fig. 3).
Fig. 4 shows the internal layout of the mower that achieves both lateral and longitudinal balance and features a low center of gravity. User requirements were satisfied by adopting a 50° target for roll-over during mowing to ensure that the mower could operate reliably on 45° slopes during on-farm use, and this was achieved without making the machine any wider. The result was a compact 870-mm wide mower. In addition, by placing equipment that is infrequently maintained in multiple layers and placing other major equipment on a flat surface, it is possible to easily access equipment that requires maintenance by opening the guard pipe integrated into the bonnet, ensuring high maintainability while being compact.

3.2. Grass height cutting performance comparable to large mowers

Engine stalling due to overloading or running out of fuel can make mowing more troublesome and less efficient. This is even more of a problem when the engine stalls on sloped ground and the operator has to climb down to clear the cause of the overload and restart the engine. This not only interrupts mowing, but it also poses a hazard. Even if the engine does not stall, the slowing of the rotary cutter when the engine is heavily loaded can make mowing more difficult, such as leaving grass uncut or narrowing the cutting area. To make the mowing of patches of tall grass 1 m or more in height easier and more efficient, Yanmar equipped the mower with variable speed control (Fig. 5). This feature helps avoid engine stalling by reducing the mower’s travel speed. The rotary cutter is connected to the engine directly via a centrifugal clutch and any increase in drive torque can be inferred by detecting the consequent drop in rotor speed. If this speed drop exceeds a threshold, the controller identifies it as an overload and automatically slows the travel speed to reduce the load on the engine and allow the rotor speed to recover. Use of this control technique minimizes engine stalls and uncut grass due to overloading. If slowing the mower does not reduce the load quickly enough, the engine is idled and the centrifugal clutch disengaged to avoid an engine stall.

3.3. Running on battery

The YW500RC has a series hybrid configuration (Fig. 6) in which the rotary cutter is driven by the engine directly while mower travel is driven by an electric motor powered by a lead-acid battery charged from an engine-driven generator. This means the mower can move with the engine turned off using energy stored in the battery.
The battery capacity was determined based on simulations of lead-acid battery discharge characteristics and degradation. This design provides the mower with 10 minutes of battery-powered travel on flat terrain —approximately 10 times the performance of competing mowers in the same class. This operating time is sufficient for the mower to travel to and from its storage location, such as a barn or shed, without using the engine. It also enables the mower to return to higher ground if it runs out of fuel while operating on a sloped bank.

4. Conclusions

Resolving the challenges that customers face and delivering better products are always front-of-mind considerations in Yanmar’s product development. While this latest development posed many new challenges, including ensuring stability when operating on sloped ground and implementing remote control, the production, sales, service, and development teams all worked as one to overcome the obstacles. In the future, Yanmar will continue contributing to the safety and efficiency of agricultural work by supplying better products that reflect customer feedback and requirements, incorporate automation and other new technologies, and deliver greater labor-saving and safety.