Grass Bag Fill Sensor for Robotic Mowers

A lawn mower is one of the most effective equipment for keeping grassy lawns trimmed and beautiful. Some mowers come with an alert to let the operator know when the grass bag is full, thus preventing a trail of mulch on the lawn that would require manual raking. While using a traditional mower without a bag level alert system, a mulch trail will appear once the bag is full which allows the operator to pause and empty the bag.

New, robotic lawn mowers deliver self-propelled, battery-powered operation that requires minimal user input or oversight. The absence of the operator necessitates a means to sense when the grass bag is full. When the grass bag is full, it prevents newly cut grass from entering and increases the pressure inside the bag resulting in lower efficiency and decreased battery life.

A cost-effective means to sense grass fill level in a robotic lawn mower is to use a MEMS gauge pressure sensor to measure the pressure inside the bag. As grass piles into the bag, air flow through the bag is reduced causing pressure to build up as the motor continues to transfer grass and air into the bag.

Mitsumi MMR920 is an ideal MEMS gauge pressure sensor for a grass bag fill sensor. Its ultra-small footprint (7mm x 7mm) takes up minimal space on the motor control circuit board and its high sensitivity (>1 Pa) allows progressive monitoring of the bag status. These features permit the robot mower to plan the most efficiency cutting path and bag emptying schedule. Its simple structure provides a significant cost advantage for consumer products.  Additionally, the robust design prevents grass and debris contamination to the sensor since neither the electronics or sensor element is exposed to the pressure medium.