Power Products Technology
Honda’s Robotic Lawn Mower
Leave it to Miimo
1. About Miimo
Honda’s first lawn mower, the HR21 walk-behind lawn mower, was released in 1978. Since then and after numerous enhancements, Honda’s lawn mowers gained popularity around the world, especially in North America and Europe. Keeping lawns in prime condition requires periodical mowing, which can be very laborious. For a long time Honda has been engaged in automating, and minimizing labor required for lawn mowing, and with interest growing in the European market and others towards automated robotic lawn mowers, Honda developed the Miimo Robotic Lawn Mower to be dependable, easy to use, and effortless.
Since its introduction to Europe in 2012, Miimo has been, and is still being improved to meet customers’ lifestyles.
Miimo is battery powered, so it can operate quietly at night, and the tiny grass pieces Miimo creates fall through the lawn, without need for collection. Miimo, programmed to operate according to the customer’s requirements, reduces the customer’s lawn mowing labor.
HR21 walk-behind lawn mower
Inside Miimo, there are two wheel motors (one for each rear wheel), the lithium-ion battery, a blade motor, and the ECU which controls the entire unit. Miimo also has a control panel to display the unit’s status and settings, and obstruction sensors. Underneath is the blade disc, which attaches to three blades.
The docking station charges Miimo via a transformer connected to household power, and also sends signals to Miimo to define the mowing area and docking station location.
1. Area sensor
Receives signals from the boundary wire and docking station
2. Obstruction sensor
Detects collision with obstructions
3. Lift sensor
Detects when Miimo is lifted
4. ECU (with various sensors)
Tilt sensor (G sensor)/ Rollover sensor / Yaw sensor (turning speed)
5. Blade motor
Max output: 28.8W
6. Wheel motors
Maximum output: 10.8W each (2 total)
7. Lithium-ion battery
High performance 22.2V / 3.6Ah lithium-ion battery
8. Control panel
For setting Miimo, etc.
9. Blade disc
Rotates, cutting grass with its 3 blades. The blades can swivel 360 degrees, prolonging blade lifespan.
2. Technologies for Automated Operation
Accurate mowing area detection
Robotic lawn mowers operate automatically, making mowing area detection and access to power vital. Miimo receives boundary signals from the boundary wire laid along the perimeter of the mowing area, and operates inside the boundary wire.
Signals sent to Miimo use Honda-unique codes. Signals are determined by code concordance, so Miimo can determine over a wide range its position in or out of the mowing area, and can continue operation accurately even with surrounding ambient noise.
Once Miimo’s battery is depleted requiring recharging, it returns along the boundary wire to the docking station. Once close to the docking station, Miimo receives a “station signal,” and accurately docks with the docking station, and begins recharging.
Autonomous Slope Control System
Miimo requires strong straight-line driving performance for stable, continuous operation on mowing areas with slopes and uneven surfaces. Miimo is equipped with an autonomous slope control system to realize this performance. When moving across a slope, the vehicle’s weight pulls it down the slope, preventing straight line movement. This minute change is detected by the yaw sensor, and the ECU compensates by separately controlling the motor speeds of each wheel motor. When moving diagonally up a slope, the difference in wheel speeds compensate and ensure a high degree of straight line accuracy.
Miimo’s low center of gravity, coupled with a well-balanced unit design, high-traction rear tires and high-output motors, allow it to climb slopes of up to 25 degrees.
Lawn mowing performance
In order to maintain an immaculate lawn, then entire lawn needs to be mowed uniformly. Manual lawn mowing can be efficient, as the person can decide which areas to mow first. By simply automating operations and setting ordered movement patterns, some areas may be mowed more often than others.
Miimo has 3 mowing patterns (Random / Directional / Mixed), and mowing / movement modes to suit the lawn’s condition, allowing the entire lawn to be mowed uniformly.
Miimo detects the extra load on the motor when it encounters lawn that is not completely mown, or the growth speed of the lawn is fast, and concentrates on these areas.
Miimo mows along the boundary wire, tidying the edge of the lawn.
When Miimo reaches the boundary wire, it turns without backing up, increasing efficiency in mowing narrow passages.
If the return route to the docking station is always the same, Miimo’s weight will gradually cause wheel tracks. To avoid this, Miimo can be set up to automatically select and follow one of multiple return routes.
Lawn mowing performance - Blades
On Miimo’s blade disc, three blades that rotate 360 degrees are attached. The blade disc protects the blades from obstacles, and is designed to efficiently cut grass without pushing lawn into the ground.
As Miimo’s blade disc is designed to rotate in both directions, blades do not wear unevenly, ensuring sharp cutting edges for a longer time, increasing lawn mowing efficiency.
Miimo’s blades were newly designed for durability and sharpness.
Blade edges are better protected from stones and other obstacles, by rotating 360 degrees to deflect collision shocks.
3. Functions for Using Miimo Safely
Automatic Load Control System
When the load on Miimo’s blade motor increases, Miimo slows down and maintains a constant blade speed through its Automatic Load Control System. When load on the blade motor increases, the ECU detects the increase in electric current from the battery to blade motor, and adjusts the wheel motor output to slow Miimo down. Under light load, Miimo moves at full speed, maintaining the blade motor at 2500rpm. When the load is high, (such as in long grass or dense lawns) Miimo reduces its speed and mowing load, and by maintaining blade speed, ensures lawn is efficiently mowed.
To operate a robotic lawn mower, the mowing area and operations need to be set up first. Effective operation schedules require skill and knowledge, but by using the setup wizard, the operator can smoothly set up automated operation suitable for the mowing area.
With Miimo’s setup wizard, the user enters settings as they appear on the control panel screen, which allow Miimo to automatically configure an operating program including complex settings such as operating time, and return routes to narrow passages.
Miimo’s setup wizard allows automatic operations matching the mowing area to be set up smoothly.
Various sensors and automatic stop
Robotic lawn mowers operate unattended, so safety is a concern when other people approach Miimo inadvertently.
Miimo is designed so that it is difficult to come in contact with its blades, but blade rotation and movement can be quickly stopped by pressing the manual STOP button.
By lifting Miimo up, the lift sensor activates the emergency blade stop function.
On collision, Miimo’s bumper is designed to move in any horizontal direction relative to the frame, and up. In conjunction with the obstruction sensor, Miimo can detect collision from any direction. This allows movement to be immediately stopped upon collision, and operation to resume after Miimo moves around the obstacle.
Miimo detects it has been lifted when the metal plate fixed to the cover is separated from the lift sensor.
Collision is detected through cover movement, when the mount shaft, magnetized by magnets fixed to the cover, moves across the obstruction sensor.