Blog #4 - Solar Powered Remote Controlled Lawn Mower

             Team 19 has completed the assembly of the solar powered RC lawn mower prototype (Figure 1). The prototype is fully functional, with all subsystems working properly. The design is fully self contained, requiring no charging or user input besides using the RC remote to control the mower.

                    Figure 1: Overall view of Team 19’s solar powered remote controlled lawn mower

We split our design into 4 main subsystems: the chassis, solar/charging, drive/RC, and cutting subsystems (See Figure 2 below for a labeled diagram of main components). The solar charging system consists of the 50 Watt solar panel fixed on top of the device, which feeds a solar charge controller that then safely and effectively powers the 20 Ah lithium iron phosphate battery. The drive system uses two planetary gear motors fixed to two 6 inch tires at the rear of the device, which are controlled using two brushed motor electronic speed controllers. The two front wheels are simple caster wheels, and act just as extra points of contact for stability. The caster wheels are not used for steering or power. These ESC’s receive user input from the RC receiver (mounted on-board the mower), which is controlled using the RC remote. The cutting system uses two high RPM DC cutting motors with plastic blades attached, and these motor speeds are controlled with the pulse width modulator speed controller. The cutting motors are turned on and off using an RC relay, which is mainly for safety purposes to allow the operator to have more control over the spinning cutting blades. The chassis is a simple ¼ inch ABS plastic sheet, with all main components bolted into place. On each corner of the ABS sheet is a custom made mount to secure the solar panel to the device, and there are two reinforcing metal tubes to add rigidity to the design while minimizing weight.

Figure 2: Component break down of final design

            Currently the team is working on validating our prototype to determine how well our design meets our initial goals. The first test was for device speed, and we had an initial specification goal of ~3 miles per hour. Testing showed that our device is capable of traveling at ~2.92 miles per hour (see Table 1 for data). 

                                                     Table 1: Speed test data

            Testing the range of our device yielded an operable distance of 245 feet, which is more than sufficient for our intended applications (yard size less than .2 acres). Battery consumption testing has given us an approximate run time of 3.3 hours, and our specified goal was one hour (Table 2).

                                                    Table 2: Capacity tests to approximate runtime

            Cutting time testing has also been performed to judge the device's efficiency to cut a specified area of grass (10ft by 10ft square). After performing three trials of the test, it takes an average of about 2.99 minutes to mow 100 square feet (Table 3). This rate results in about 4.35 hours to mow the average American yard of .20 Acres or 8712 square feet, which is much slower than our initial goal of 1 hour. In addressing this, the team considers the initial specification of 1 hour to be unrealistic, and that our final design is better suited for small yard applications.

                                                    Table 3: 100 sq. ft. timed cutting tests

            The most recent test the group has performed is the cutting height tests, which yields an average final cut height of about 2.4 inches. This is a bit under our goal of 2.5 inches, but the team considers this specification successful since the difference is very minor (.1 inches).

Overall the team is very satisfied with our final design solution, as we feel it meets the goals we have tested quite well and is expected to perform just as well in our remaining validation tests. The addition of the second cutting motor and PWM controller doubled the cutting width to 10 inches and made the device much quieter, and the aluminum frame supports seem to have greatly added to the longevity of our mower. The device has proven to be tricky to learn how to control due to the dual-motor tank-like steering, but once the user is acquainted the device is very easy to maneuver and use. As mentioned earlier the design doesn’t meet our goal of 1 hour to mow .2 acres, but this could be fixed by switching to a large metal blade and/or using higher RPM drive motors if we were to redesign to prioritize this specification. Team 19 feels our final design so far sufficiently meets our main goals sufficiently in the scope of a senior Mechanical Engineering Capstone project. 

All that is left for the project is to continue our validation testing: perform more trials where needed, noise level testing, traversable slope testing. Since the prototype is fully assembled and stress-tested quite well, we expect testing to go smoothly with minimal issues. Depending on how some of the testing goes, the methodology of some validation testing may be altered to prevent excessive wear on parts and produce more realistic data.