Lawnmower specification and sizing
The mower specification are the following:
- Weight: 25Kg (+- 20%).
- Delimitation of the surface to be treated by GPS RTK (Real Time Kinematic), to avoid the installation of a wire.
- Working speed: 1,8 km/h, that is 0,5 m/s. This seems to be a reasonable speed for a small vehicle on an irregular surface.
- Travel speed: 1,8 km/h (see comment above).
- Ground clearance (chassis to ground distance) min 10cm, to allow movement on an uneven surface.
The forward speed resistance is one of the parameters required to calculate the propulsion power of a wheeled vehicle. The two main parameters that influence the resistance to advancement are:
- The wheel forward resistance.
- The slope to be crossed.
According to wikipedia, rolling resistance calculations can be simplified if the vehicle does not move fast, which is our case. Not having found the rolling resistance coefficient for a tire on grass, we take the sand coefficient, 0.3, which is probably higher than that of grass. However, since the robot will move on uneven ground, which is not very similar to grass, his hypothesis leaves us a little margin.
The relation between the rotational speed and wheel radius is given by:
where:
V: speed in m/s
r: wheel radius in m
: rotational speed in rad/s
The rotational speed can be calculated from the linear speed as follows:
or, in our case: