Smart Lawn Mowers: Everything You Need to Know

According to Pro Landscaper Magazine, people spend almost 2 months of their lives mowing their lawn.   In the past, the only alternative was to hire a professional landscaper.  Now, technology has come to the rescue in the form of robotic lawn mowers.

There are a wide variety of robot lawn mowers available from well known manufacturers of lawn care equipment including two manufacturers with reputations for high quality products; Husqvarna and Honda.

As you would expect, robot mowers, that operate unattended, have a wealth of safety features.  First all mowers include collision sensors.  If they bump into an object they will reverse, turn, and proceed in another direction.  Second, unlike a standard mower, the blades of a robot mower are very small and designed to only cut grass.  Further, the blades will automatically stop if the mower is lifted or tilted.  The blades are also recessed, away from the edges of the mower, making it much more difficult to be cut by a blade if someone accidentally sticks their foot underneath the mower while it is running.

The small, razor sharp blades also contribute to longer battery life; as does the design of the mowing schedule.  A typical homeowner will cut their lawn once a week.  Robot mowers are designed to cut much more frequently.  Cutting more frequently means that each cut has to remove less grass growth.  Anyone who has tried to cut a lawn after missing a week while on vacation has found that the extra long grass requires much more effort to cut.  So, cutting more frequently saves battery life and allows the robot mower to cut more of the lawn between charging cycles

A regular mower, with its large blades, mulches the grass by cutting it multiple times. The large blade first cuts the grass at the selected height.  The shape of the blade creates an air  current that keeps the clipping from immediately falling to the ground and allows each clipping to be cut multiple times.  On the ground these small pieces of grass decompose, and provide nutrients for the lawn.

Instead of cutting each grass clipping multiple times to create the small pieces of grass that can easily decompose into mulch, robot mowers mulch the grass by cutting the lawn more frequently and only cutting small pieces from the top of the grass.

Robotic mowers also have features to prevent theft.  These range from requiring a pin code to operate the mower to a built in GPS that allows the location of the mower to be tracked.

Unlike a robot vacuum that can map your room by bumping into or scanning your walls, a robot lawn mower requires a boundary wire to be run around the perimeter of the lawn, flower beds, trees, and any other obstructions.  The mower senses an electronic signal that is sent along the boundary wire and stays within its border.

While it may seem obvious, it is important to note that if a yard is divided into several areas or levels, possibly with a fence or flower bed dividing them, a robot mower will not be able to automatically mow the entire lawn.  However, the software in the mowers allows them to be moved to a secondary area and a manual mowing session started.  But, if the area is too large for the mower to complete before the battery is exhausted, the user will have to carry the mower back to the charging station and then return it to finish mowing after the battery is recharged.  While this does make sense it makes a robot lawn mower less of a time saver for some people.

Husqvarna Lawn Mower

Husqvarna manufactures a total of eight different robot lawn mowers.  These can be split into two main categories:

• Mowers with Automower® Connect technology include a cellular radio, GPS navigation, theft tracking, and integration with Amazon Alexa, Google Home, and IFTTT
•  Mowers with Automower® Connect@Home can be controlled through Bluetooth using a smart phone app

For the purpose of this article I’m going to focus on the five mowers that include Automower® Connect as they provide the opportunity for integration in the smart home.  The primary differences between the Automower® 315X, 430X, 430XH, 450X, and the 450XH are the sizes of the lawn that can be cut by each mower.  The 315X is designed for a lawn of up to .4 acres.  Both the 430X and the 430XH are designed for lawns of .8 acres and, the 430XH allows for higher cutting heights.  Finally, the 450X and 450XH are designed for a lawn of 1.24 acres with the 450XH, again, allowing for higher cutting heights.

For safety, when any of the Automowers® strike an object they will backup, turn, and proceed in another direction.  In addition, the 450X and XH models include ultrasonic sensors and will slow down as they approach an object that blocks the path of the mower.

All of these Husqvarna mowers have a 19 button keypad and LCD display in addition to their ability to be controlled through a smart phone app.  All have an alarm, require a pin code, and include lift/tilt sensors.  All mowers have 18V lithium ion batteries.  But the battery capacity varies by model.  The 315X has a 2.0Ah battery with a 60 minute charging time and a 70 minute run time.  The 430X and XH models have 5.0Ah batteries with a 50 minute charging time and a 145 minute run time.  Finally, the 450X and XH models have 10.0Ah batteries with a 60 minute charge time and a 270 minute run time.

Like other robotic mowers the Husqvarna Automowers® are designed to cut the lawn frequently, removing only a small amount of growth.  The cutting width of the 315X is 8.7” while the other models all have a cutting width of 9.45”.  The 315X can climb an incline of 22 degrees while the other models can climb an incline of 24 degrees.  All the Husqvarna mowers are designed to operate in the rain and in temperatures up to 113 degrees Fahrenheit.  Husqvarna recommends that the mower be taken inside during extreme weather conditions.

All the Automower® models use three, small, razor sharp blades for cutting the lawn.  The blades will pivot aside if they encounter a hard object.  Blade life is estimated to be around two months for a quarter acre lawn and when dull, the blades can be replaced by the homeowner with just a screwdriver in only a few minutes.

All the Automower® models are quiet.  The sound levels of the various models are as follows

• 315X – 60 dB
• 430X – 58 dB
• 430XH – 59 dB
• 450X – 59 dB
• 450XH – 63dB

In comparison, a typical gas powered lawn mower generates 95 dB and even an electric push mower generates 75 dB.

Each mower comes with an operating manual, quick start guide, charging station, power supply, low voltage cord for connecting the charging station to the power supply, plastic screws for securing the charging station to the ground so it can’t shift, three spare sets of mower blades and blade securing screws, wire labels, and a ruler used during the installation of the boundary wire.

Unlike other the mowers from Honda, a separate installation kit that includes boundary wire, staples, and wire connectors must be purchased.  This adds between $100 and $250 to the price of the mower depending on the size of the yard.  The cost of installation must also be considered.  The Husqvarna dealer I spoke to said that professional installation typically costs between $200 and $500; depending on the size and complexity of the yard.  The dealer I spoke to said that while some homeowners do the installation themselves that most opt for having the dealer do the installation for them.

In addition to installing the boundary wire around the lawn, a piece of wire from the reel of boundary wire is run from a connection on the charging station to the boundary wire on the far side of the lawn.  This “guide wire” is used to help the mower efficiently return to the charging base and to guide the mower to areas of the lawn that it otherwise might have difficulty locating as it randomly cuts the lawn.  The 315X’s design allows for a single guide wire, the 430X and 430XH’s design allows for dual guide wires, and the 450X and 450XH’s design allows for three guide wires.  The larger number of guide wires allows for the mower to more efficiently operate in a more complex yard.

In conjunction with the guide wire the 315X can be programmed for three starting points while the 430 and 450, X and XH models, can be programmed for five.  Starting points allow the mower to more evenly cut the various sections of a yard.  For example, separate starting points can be set for the back yard, front yard, and side yard.  The mower would then rotate between these starting points making sure that each area was completely mowed before moving on to other parts of the yard and possibly needing to recharge.

The Husqvarna dealer I spoke to told me that some customers were very creative in how they ran the guide wires.  Some examples are:

• Though pet doors installed in fences allowing the mower to travel between areas it otherwise wouldn’t be able to reach
• Over a bridge the homeowner built across a small stream
• Between neighbors yards that shared in purchasing the mower

In addition, the Husqvarna mowers make use of their built in GPS to assist with navigation.  This feature primarily adds value in larger yards where the mower gains a better understanding of where it needs to go and where it has already mowed.

Similar to the Honda mowers, the Automower® models can optionally change to a spiral mowing pattern if the mower encounters an area with thicker grass.  The mower can also be manually moved to a specific location and placed in a spot cutting mode where it also uses this spiral pattern to cut an area that requires special attention.

The Automowers® include a basic weather timer that allows the homeowner to adjust the mowing schedule based on the weather.  There are five different options, from Low- to High+.  During the times where the grass grows rapidly, typically during the spring and fall, the cutting frequency can be raised by setting the weather timer to High, or High+.  During mid-summer, when grass typically doesn’t grow as quickly, the weather time can be changed to Low, or even Low-.

To prevent theft, the mower includes an alarm that sounds if the mower is lifted.  As the mower requires a pin code to operate, even if the mower was taken it would be of no use to the thief.  Further, the mowers are serialized and if a stolen reporting the mower to Husqvarna allows them blacklist the mower so it can’t be paired with the smart phone app for operation.

The on-board GPS is used for added security in several ways.  First, a geofence around a home can be setup and a notification will be sent to the homeowner’s smart phone in the event that the mower is moved beyond the geofence.  Second, the on board GPS can be used to locate the mower if it is stolen.

Honda Robot Mower

Honda manufactures two robotic lawn mowers.  The Miimo HRM310 is for lawns up to .37 acres and the HRM520 is for lawns up to .75 acres.

Like other robotic mowers, the Miimos rely on a boundary wire to keep them within the confines of the homeowner’s lawn and to avoid obstacles; such as trees.  Unlike the Husqvarna mowers, it doesn’t use a guide wire to help the mower return to the base station for recharging or to locate difficult to reach sections of a yard.

A homeowner can even view how a Honda Miimo mower would work in their yard using a simulator on the Honda Miimo web site. Using the simulator a homeowner can zoom in on a satellite image of their home, lay out the boundary wire around their property, and watch how the Miimo mower would cut the lawn.

Unlike the Husqvarna models with Automower® Connect, or Automower® Connect@Home, technology, there is no ability to control the mower with a smart phone app.  Both the HRM310 and HRM520 are controlled through an integrated 18 button keypad and an LCD display.  Both have an identical cutting width; 8.7 inches.  Both have an anti –theft alarm, require a pin code, and include lift/tilt sensors.

The HRM310 operates on a 22.2V, 1.8Ah lithium ion battery and the HRM520 operates on a 22.2V, 3.6Ah battery.  The larger battery allows the HRM520 to operate for 60 minutes vs. only 30 minutes for the HRM310.  The HRM310 can be recharged in only 30 minutes while it takes a full 60 minute to charge the HRM520.

The mowers come with a transformer, docking station, special screws that secure the docking station to the ground so it won’t shift, a reel of boundary wire that attaches to the docking station, pegs to secure the boundary wire, and a spare set of blades.

The Miimos software includes some more advanced mowing options than the Husqvarna mowers.  Homeowners can choose from three different lawn cutting patterns:

• Random – for cutting large open areas
• Directional –  more back and forth pattern (typically used by a human cutting a lawn) for obstacle free areas
• Mixed – for large complex yards

The Miimos seasonal timer offers more ability to adjust cutting to the season that the Husqvarna weather timer.  Through the keypad percentages can be entered for each month.  For example, in May and June a percentage of 100% can be entered and the mowers will cut at their maximum frequency.  In July and August, when temperatures are much hotter and grass doesn’t grow as rapidly, a percentage of 70% could be entered to reduce the cutting frequency.  During the fall, the monthly percentages can again be raised so the mower will again cut at their maximum frequency.

In addition, the Miimos can detect thicker grass areas and they will change to a spiral cutting pattern to cut these areas of the lawn.  They also include a special edge cutting mode for the area bordered by the boundary wire.

Like other robot mowers, the Miimos have three, small, pivoting, razor sharp, cutting blades that require replacement every one to three months.  These blades allow the mowers to operate very quietly.  In normal operation, the mowers only produce 58 db of noise.  In quiet mode, which can be used for operating the mower at night or when homeowners want to enjoy their yard while the mower is operating, the Miimo mowers only produce 55 db.   My experience with the Honda Miimo is that it could easily be scheduled to cut the grass at night and not disrupt anyone’s sleep; even with the windows open.

The Miimo HRM310 can be programmed for 3 starting points while the HRM520 can be programmed for 5 starting points.  Multiple starting points assist the mower in efficiently mowing the yard.  For example, if the yard consists of a front yard, back yard, and 2 side yards, starting points could be assigned to each area.  By then starting the mower in each area it helps assure that each is mowed evenly.  The mower will, eventually find its way to all areas but using multiple starting points makes the process go more efficiently.

It should be noted that Honda requires that the Miimo be professionally installed by a dealer.  At the time of this writing, Honda was offering a $500 credit towards installation with the purchase of a Miimo mower.  When I spoke to a local dealer, their business model was to simply install the mower at no charge to the homeowner and then to apply to Honda for the credit.

Integration

All the Husqvarna models that include Automower® Connect have a built in GPRS cellular radio.  Included with the mower is a 10 year mobile data contract.  This provides connectivity for the smart phone app, Alexa voice control, Google Home voice control, and IFTTT.

Voice commands provide the ability to start the mower, pause the mower, and tell the mower to resume mowing.  In addition you can tell the mower to park (return to its charging base), park until further notice (return to the charging base and ignore scheduled mowing until a command is issued), and request status of the mower.

The IFTTT service is even more full featured.  Similar to voice commands there are actions to start the mower, pause the mower, resume the mower, park the mower until the next scheduled start, and park the mower until further notice.  In addition, there are IFTTT triggers for when the mower starts charging, when the mower has an error, when the mower starts mowing, when the mower has paused, when the mower has stopped, when the mower has parked, when the mower is leaving the charging station, when the mower status has changed, and when the mower is going home.

Unfortunately, the Honda mowers do not offer any integration capabilities.  There is no ability to program, or control, the mower with a smart phone app or through a web service, such as IFTTT.

Grass Growth Model and Scheduling Software

The one common issue I see with all these mowers is with their scheduling software.  None of them is capable of truly taking environmental conditions into account.  The Honda Miimo mowers include a feature that allows you to adjust the schedule by month; taking into account that grass typically grows less in late summer than it does in the spring.  The Husqvarna models include a very basic weather timer that allows a homeowner to manually adjust the schedule themselves as the weather changes.  However, any abnormal weather conditions can cause the mower to cut the grass too often, wasting energy and causing undo wear on the mower; or not mow often enough.

With that in mind I wrote a scheduling system for a Crestron automation processor that estimates grass growth based on both historical norms and environmental conditions.  The software for this can be found on my GitHub.

The growth model took several months of research to develop with the help of people in turf grass research from across the country.  A detailed explanation of how the model works, for those interested, is provided below.

The example program I wrote measures how much water has been applied to a lawn through irrigation and rain. It then passes this data to the mower scheduler software that includes the grass growth model.  This is accomplished through integration with a Rachio smart irrigation system controller and through integration with a Weatherflow smart weather station.  The example program also gathers temperature data from the Weatherflow and passes it to the grass growth model.  The software could easily be changed to collect this data from a different smart irrigation controller.  Similarly, the weather data could be collected from a different smart weather station or the Internet.

The software uses this data, along with information on the type of lawn grass, to estimate growth and schedule the mower appropriately.

The only mowers that the software will currently work with are the Husqvarna mowers with Automower® Connect technology because of their integration capabilities through IFTTT.  However, as new mowers come available, that include either an IFTTT interface or an open API, there is no reason the software couldn’t be modified to work with them.

Parting Thoughts

Compared to a common gas or electric push mower, Robot mowers are fairly expensive.  Below is a chart that includes the list prices of each mower and the yard size the mowers are designed for

*This is just the price for the mower itself.  In the case of Husqvarna a separate installation kit must also be purchased.  The price also doesn’t include installation.  At the time of this writing Honda is including a $500 credit for professional installation; which is a requirement.  Husqvarna doesn’t require that their mowers be professionally installed but it is highly recommended by the dealer.

All the above mowers, when paired with the lawn size they are designed for, should provide relief to the homeowner from the time required to mow a lawn every week during the summer.  It is important to note that there is still a long list of tasks required to take care of a lawn.  For example, even though a robotic mower, with a properly installed boundary wire, can get closer to obstructions than most riding mowers, for a clean, manicured, lawn a homeowner will still need to use a trimmer to clean up around trees, shrubs, next to fences, etc.  However, the time to perform that task is still significantly less than it would take someone to mow their entire lawn.

It is also important for potential buyers to recognize that fences and gates will provide additional challenges to using a robotic mower.  All of the above mowers can find their way through gates; but only if they are left open.  Dog owners that allow their pet to roam freely in a fenced back yard will need to make adjustments for a robot mower that needs open access to all parts of a yard.  The scheduling software I wrote includes a trigger to open gates if a motorized system is installed.  However, this adds an additional cost to an already fairly expensive product.   Alternatively, a cutout that allows the mower access through the fence could be made but this may, or may not, work for every pet owner.

Both mowers are capable of operating in the rain.  However, as Honda warns on their web site, “mowing in the rain is not recommended for optimal cut performance”.  Neither the Mimmos nor the Automowers® have a rain sensor to avoid this situation.  Integrating an Automower® with a smart home processor, or hub, through IFTTT could resolve this by triggering the Automower® to return to its charging bay when either rain was detected by a smart weather station or through weather data obtained through the Internet.

Similarly, there are potential issues if one of these mowers is operating at the same time an irrigation system is active.  Again, the mower may not cut the lawn as well as it would were the grass dry.  Further, if the mower strikes a pop-up spray head it could break it, leading to a significant water leak.

Finally, even with the security features built into the robotic mowers, they will probably present an inviting target to thieves that don’t recognize that they can’t be used by anyone other than the original owner.  For that reason it is fortunate that the mowers are quiet enough to be run at night.  Scheduling the mower to operate late at night can provide an added degree of security.

For those left wondering how well these mowers work, all the dealers I spoke to discussed their adoption by professional landscaping companies.  After installing a robotic mower at a client’s home, the landscaping company would only have to dispatch a single person to visit the customer each week instead of a team of workers as the work to trim the edges of the yard, clean up flower beds, etc. could easily be handled by just one person.  This is a significant savings in labor and money.  In fact, one of the dealers talked about a pending order for almost 200 robotic mowers from one landscaping company.

Thanks for reading.

Growth Estimation Model

For those interested in digging into the details, I’m including this section on the model I developed for estimating the growth of turf grass.  This is an integral part of the software I wrote for automating the operation of an Internet connected robot mower using a Crestron automation processor, which can be downloaded from my GitHub.  I’ve included this explanation of the model so anyone with a different automation processor can port this to their programming environment

First I would like to thank Dr. Larry Stowell at PACE Turf, Dr. Paul Brown at University of Arizona Cooperative Extension, and Dr. Thomas Samples at the University Of Tennessee Institute Of Agriculture for their patience and assistance in helping.  The model estimates grass growth based on temperature and water (irrigation + rain).  While fertilizer is a very important factor in estimating grass growth I didn’t see a practical way of asking a homeowner to accurately report the amount of nitrogen, and other nutrients, that were applied to their lawn.  To work around this limitation I simply added an adjustment to the model that allows the homeowner to “tweak” it based on how well it represents the growth rate of their lawn.  For example, if their lawn is well fertilized then they can increase the calculated growth rate for their lawn by increasing the adjustment factor (AF).  Similarly, if they haven’t fertilized their lawn they can simply decrease AF.

The basic equation for grass growth is:

Grass Growth Rate = OSGR * GP * ETP * AF

Where

OSGR = Optimal Species Growth Rate – The growth rate for a given species of grass under optimal conditions

GP = Growth Potential

ETP = Evapotranspiration Potential

AF = Adjustment Factor where the value is greater than 0 and less than or equal to 2.  For example, an AF of 2 will double the rate growth that the model predicts.

The “Optimal Species Growth Rates” for popular species of lawn grasses are as follows:

Growth Potential is defined as follows

GP = e^{(-½ (((average temperature – optimum growth temperature) / variance) ²))}

Where

Optimum Growth Temperature = 67.5 degrees Fahrenheit for cool season grasses and 87.5 degrees Fahrenheit for warm season grasses

Variance = 10 for cool season grasses and 12 for warm season grasses

Before diving into the equation for ETP it is important to understand what evapotranspiration is.  Google’s online dictionary defines evapotranspiration as “the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants”.  It is also important to note that the amount of water required for plant growth, beyond what is defined by evapotranspiration, is negligible and can be ignored.

In addition, water isn’t applied to a lawn in an orderly manner.  It doesn’t rain every day and local ordinances may specify that irrigation can only be applied to a lawn on a non-daily schedule.  The water a lawn receives through irrigation and rain needs to be measured, accumulated, and removed through evapotranspiration.

The equations to precisely calculate evapotranspiration are quite complex.  Rather than trying to perform these calculations I chose a simpler route and leveraged the power of the Internet.  An online search for evapotranspiration by your location should provide you with historical ET values by month.  These can simply be entered into the software.

The ET value obtained from the Internet must be multiplied by a crop coefficient (Kc).  For warm season grasses, Kc varies from .6 to .8.  Below .6 the grass is in stress and above .8 there is no increase in benefit to the lawn and you are wasting water.  For cool season grasses the Kc ranges from .8 to .93.  ET * Kc defines the amount of water (in inches) required for grass growth where varying the value Kc defines how lush the lawn will be.

Each day the water in the soil (SW) equals the previous day’s water in the soil plus any rain and irrigation applied during the day.

If SW is greater than or equal to ET * Kc2 then water removed from the soil (WR) = ET * Kc2 and SW = SW – WR.  The assumption here is that the evapotranspiration will remove as much water as available up to Kc2 at which point water beyond that will simply remain in the soil.

If SW is less than ET * Kc2 then WR = SW and SW = 0.

ETP = ((WR / ET) – Kc1) / (Kc2 – Kc1)

Where for warm season grasses Kc1 = .6 and Kc2 = .8.  For cool season grasses Kc1 = .8 and Kc2 = .93.

If ETP is greater than one then ETP is equal to 1.

If ETP is less than zero then ETP is equal to 0.

References

Growth Potential Values for cool season and warm season turf – Paceturf.org –

https://www.paceturf.org/PTRI/Documents/0401ref01.pdf

Converting Reference Evapotranspiration into Turf Water Use – The University of Arizona College of Agriculture and Life Sciences, Cooperative Extension – https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1195.pdf