I'm designing a rover to use as a harvest aid on our vegetable farm. It will be GPS guided using an RTK base station to straddle our parallel fixed-width vegetable beds. (66" wide with 48" of growing area, the rest is 18" of path for feet and tires) The human harvester will be able to control the rover's speed along a fixed path so it can function as a harvest bin holder and carrier. Picture a rolling table 66" tire center to tire center, 48" or so long and 36" high. The axles are upside down U shaped so that there is clearance for the plants underneath. It will creep along at very slow speeds in front of the person harvesting and then return to our pack-shed at faster speeds.
It will have 4 wheels. The front axle will pivot on its center in relation to the rest of rover and have an Ackermann steering design, such that one motor, centrally positioned, will turn the front tires via tie-rods connected to steering arms. This is the first motor I need to select and size. Its shaft will move less than 180 degrees but needs to be precisely controlled.
The rear axle will have 2 drive wheels, each with its own motor, and will be fixed to the main body of the rover. These are the second motors I need to select and size.
My initial design will be for 500lb Max Gross Weight, but if it works well, it would be nice to build one that could carry a ton. I've listed some calculations below that relate the max speed, which is only 5-7mph, and the tire diameter to figure the max RPMs of the drive wheels. It also looks at the weight, rolling resistance and gradient climbing resistance to figure the power and torque requirements.
Could someone with experience in this check out my numbers, verify I'm on the right track with my calculations and make some recommendations for motors? In-wheel motors look really interesting for the drive wheels, especially with this design, which needs lots of open space in the center, making a differential difficult. I was thinking I could program differential speed via the 2 motors based on the position of the steering motor.
As for the steering motor, I'm not sure if a servo would be required here of if a stepper would work? Any suggestions on motor type and size would be greatly appreciated.
max speed 7 miles/hour, 10.27 ft/s, 3.13 meters/sec
tire diameter 22 inches, 1.83 ft, 0.56 meters
tire circumference 5.76 ft
angular velocity 11.20 radians/sec
RPM 106.95 RPM
total vehicle weight 500 lbs
number of drive motors 2 motors
motor efficiency 0.85
weight per drive wheel 113.398 kg resistance by weight 25% gradient percentage 15% rolling force 28.3495 N gradient force 17.0097 N
motor power 286 Watts motor torque 14.91 Nm wheel traction force 45.36 N