Radio-Controlled Lawn Mowers

The 22", 5 hp, pull-start R/C mower. 
From another angle. 
The throttle and shift servos.
Underneath the mower.
The 30", 8 hp, electric-start, 3-speed R/C mower.
Front view.
High view.
David Moyer's R/C mower.
The Evatech mower, a good design. 
The best steering servo ($300).
                     THE FLORIDA CRACKERS
   All lawn mowers have one basic flaw.  Get rid of it and it's much
lighter, goes faster, goes under low branches and between bushes, turns
on a dime, saves gas, mows in the rain and lets you have fun to as well.
That flaw is the rider.  So next we made two radio-control mowers.
   Earlier we had assembled a model R/C car with a small Sony TV camera
and a wireless video transmitter. It was an RPV, or remotely piloted
vehicle.  This seemed like a great way to mow.
  We assembled the back half of the mower first; the engine, transmission,
differential, axle bearing blocks and wheels.  Then we clamped a steering
mock-up in place.  It showed that we needed at least 18 in.-lbs. of torque
to steer in dry grass with no motion, much less if you're moving.  We
already had an electric screwdriver, so we modified it with servo electronics
and MOSFETs. It worked and was very strong.
  A key part is the Comet 203312A centrifugal clutch.  Many engines will
work, but one lever must operate the throttle-choke-kill functions.  Many
transmissions will work, but it should shift as easily as possible (remove
all detents).  Ours is a used Foote 1407 F-N-R model. Many differentials
will work (Peerless 108 shortened to 22").  The throttle and shift servos
are Futaba S34 (7 in-lb torque for big planes). The German Simprop steering
servo takes 7.2 volts from a standard nicad battery and delivers 33 in-lb of
torque.  A BEC drops the 7.2 volts for the receiver and smaller servos.  We
charge the 7.2 V with a peak-detect charger.
  The steering wheels are paired 6" wheels on a long bolt, giving a wider
footprint and easy turning. The front rocking axle has the steering servo.
The geometry of the steering servo arm gives a tight turning circle. The
rear wheels are 10" pneumatics on the 3/4" differential shaft.
  The centrifugal clutch is on the 1" portion of the blade/crank shaft. The
blade hub is on the 7/8" portion.  Power goes to the transmission only after
the engine RPM exceeds 1750 RPM.  So it won't move until then, regardless of
the shift setting. (But the blade is always spinning!)  The 10" rear tires
rotate at just over one rotation/second at that RPM, moving it at about 3
feet/second when first moving.  Braking is by drag.  We discovered that it
stops quickly in grass when put in neutral.  And reverse can be used for
quicker stops.
  The mower was designed to go under fruit trees and between shrubs that
are awkward or impossible with a rider.  And to go much faster than an
operator, walking or riding, would care to go.  For mowing large areas
you would need one with a wider cutting swath.
    We chose 5 HP somewhat arbitrarily.  It turns out the choice of
transmission pulley (5", for 5/8" shaft; 34" size A belt) and differential
sprocket (36T, 26" of #41 chain) was fortuitous and we left things as they
were.  It could also get by with a 4 HP engine for the 22" swath.  For a
larger swath (up to 30"), go to 8 HP.  The dimensions for bolt holes on the
deck will depend on the engine, transmission and swath chosen.  We used all
nylon-locking nuts and high-strength bolts.  The mounting plate for the
throttle servo mounts onto the engine and will have to be determined by your
engine layout. The same for the shift-servo plate that mounts on the
transmission.  The transmission has a lever-and-fork that slides the selector
dog gear with little friction after removing the detent plate.
  It mows equally well forward or backward.  Grass clippings fly out
the right going forward and out the left in reverse, leaving no unsightly
ridge of clippings behind as with standard mowers.
  Later we built a 30", 8 hp, electric-start version with 3 forward speeds.
The transmission is a modified Foote 2010-80 made to shift using two servos.
This 3-speed design was unecessarily complex and was modifed to F-N-R using
one shift servo instead of two.  Electric-start is very handy.
  Of course, any combination of features can be used for your design. Both
models mow right up to the edge of the deck on both sides for trimming and
this feature was designed in.  As was reverse mowing, without having the
grass clippings hit the pan and drop down, leaving a ridge.
CHECK LIST (every time mower is operated):
1.  charge steering and transmitter batteries
2.  check charge on all batteries
3.  check rear tire pressure
4.  check oil level
5.  check gas level
6.  turn on the transmitter first and then the receiver
7.  check servo throw directions and for any RF interference (twitching)
8.  move shift stick to NEUTRAL (center)
9.  manually pull the mower to the starting location in the yard
10. move throttle to the CHOKE position
11. place a foot against the rear wheel and pull the rope until it starts
12. set the RPM to the point of just engaging the clutch
13. shift transmission to FORWARD
14. set RPM to good mowing speed
15. to kill the engine at any time, move throttle to STOP (short ignition)
16. never turn the transmitter off before turning off the receiver, unless
    the engine is dead
18. stop immediately if any behavior becomes erratic, which could indicate
    a weak battery or interference


In the search for strong R/C servos for our radio-conrol gasoline mowers, our high-school engineering club found several approaches that all will work. Each has its good and bad points to consider before making a choice for your application. There was a German Simprop servo which was sold by Hobby Lobby having an output torque of 33 in-lb. It used a 7.2 volt battery pack plus the usual servo input wires from the receiver. We used these for steering both of our radio-control gasoline lawn mowers. But we can no longer get this servo, and all Internet references to it are in German. There are Tonegawa-Seiko servos with 50+ in-lb of torque and more. But one has a plastic output arm which is small and looks flimsy, plus they're expensive. The simplest approach for a stronger servo is to increase the voltage to 6 volts instead of the usual 4.8 volts if your receiver permits it. Most receivers will. But this does not increase the torque very much. Mechanical reduction can be added to any servo. The feedback pot must be moved to the new output shaft. Gears reverse direction, but toothed belts do not. The outer feedback pot terminals must be tested. The use of any mechanical reduction requires metal-working capability. We have a modest home machine shop. Servos have plastic output arms on splined plastic shafts. It is not a simple matter to attach a metal gear or toothed-belt sprocket onto these plastic parts solidly while staying centered and aligned true. Geared servo using an Expert SL-800 servo. If you prefer to buy add-on geared output heads made for standard servos: Servo City A very strong, commonly-available servo is the Hitec HS-805BB or HS-815BB, with a measured 14 in-lb of torque using 4.8 volts. These servos appear to be the same as the Hobbico CS-802BB. With 6 volts from an outboard MOSFET driver, they develop 16 in-lb. With 7.2 volts it develops 19 in-lb and with 8.4 volts it develops 22 in-lb. This is a linear correlation. Hitec HS-815BB with MOSFET driver on perfboard. The Hitec servo itself must be modified very little: 1. Remove the six screws and hold the servo upper part together. 2. Drop the bottom plate while holding the gearhead to the body. 3. Secure the gearhead to the body with a rubber band. 4. Drill 3/16" thru the bottom plate at the two existing depressions. 5. Cut the copper traces leading to the motor pins as shown in the photo using a hobby knife or a Dremel tool with an abrasive disk. (This voids the warranty, of course.) Foil cuts on servo PC board. 6. Solder two color-coded or marked wires to the servo circuit traces that had previously gone to the stock servo motor before being cut. 7. Solder a ground wire to the original servo board. 8. Solder two color-coded or marked wires to the servo motor pads. Wire connections to servo PC board. 9. Feed the 5 wires out through the 2 holes you drilled in the base plate. 10. Replace the base plate with the O-ring still in place on the plate. Insert the screws and secure them (but do not overtighten them). 11. Seal the wire-exit holes with caulking. NOTE: To convert the servo back to it's original condition, just remove the five wires and bridge the foil cuts. After determining where the MOSFET board will be located, cut, strip and tin the wires. If an enclosure is used, feed the wires thru the enclosure's hole drilled for the purpose. Solder the heavier motor wires to the drains traces on the MOSFET board. Solder the battery wires to the MOSFET board, black to ground and red to positive. The control wires coming from the motor control traces inside the servo will go to the MOSFET gates input. These wires will be soldered to the opposite sides of the H-bridge that their respective motor wires go to. Solder the ground wire to the MOSFET-board ground. Converted Hitec servo w/ steel extension arm. Making the printed circuit board for the MOSFETs is easy if you are familiar with the procedure. The circuit itself is very simple: Etched H-bridge printed circuit board, foil side. Parts side of board. You can also use perf board as shown below. Add extra solder to the perf board foil traces to allow them to carry more current. MOSFET H-bridge on perf board, minus one MOSFET. Two MOSFETs are N-channel and two are P-channel, matched to work together in this configuration. Each can dissipate up to 75 watts and can carry up to 8 amps. MOSFETs can also be paralleled to carry more current. Parts: (2) NTE2382 N-channel MOSFETs (or (2) NTE2395s, 50 amps) (2) NTE2383 P-channel MOSFETs (or (4) NTE 2371s, 19 amps each) .1 or .2 mfd capacitor, 15V to 25V 38-amp H-bridge with (4) 2371s and (2) 2395s. Gearmotor servo with 38-amp H-bridge driver. The above gearmotor has the feedback pot linked by cord wrapped around the gearmotor arm hub and around a knob on the feedback pot. There is a spring to hold tension on the cord for friction. This is similar to a toothed-belt or chain drive linkage. Any gearmotor can supply the torque to the arm of a servo. The outboard MOSFET H-bridge can supply the extra power from a strong battery to the new gearmotor. The feedback pot must be linked to the output arm shaft directly or by drive parts that cannot slip. The outside wires from the feedback pot to the servo electronics PC board will need to be determined by trial and error, depending on the motor rotation. Or the motor wires can be reversed. Servo guts removed from housing. Internal feedback pot cut off. Motor removed and MOSFET H-bridge input leads added. MOSFET H-bridge servo-driver schematic. This uses four NTE2371s for the P-channel MOSFETs and two NTE2395s for the N-channel MOSFETs. This has given us good service with the three gearmotors shown below. No external diodes are necessary because they are on the chip. The foil is easy to etch using nail polish because the gates, sources and drains all line up. Leave room for the heat sinks. The capacitor value is not critical. Be certain that your orignal servo works properly before removing the electronics for use with the H-bridge driver. Three good gearmotors for steering servos. The smallest one draws 100 ma, the medium-sized one draws 200 ma and the biggest one draws either 750 or 500 ma, depending on which hot wire is used. All gearmotors have speeds and torques suitable for steering servos. Caps from the motor leads to ground were .003 mFd. .......................................................................... There need only be three wires coming out of the stock-servo electronics. The two from the motor solder pads and the ground. Connect the external gearmotor's power terminals to the MOSFET board motor foil traces. Cut the three stock servo's feedback pot's leads. You will have to run three wires from the external feedback pot's terminals to these stock servo PC leads. Of course, ground goes to ground. The two other leads must be determined by trial and error. Use jumpers to connect them temporarily. Connect the receiver, turn on the transmitter very briefly, and observe the centering action. If the servo does not center, switch the jumpers. Solder the wires permanently. Inside the enclosure for a 38-amp H-bridge. The MOSFETS are on a heavy brass heat sink which is tight against the sides of the aluminum box. The top left connector is to the gearmotor. The top right connector is to the 7.2-volt battery. The servo electronics will be located near the gearmotor. The long wire from the servo electronics goes to the receiver. The feedback pot must be connected to the gearmotor output shaft. The MOSFET tabs are insulated from the brass heat sink with nylon grommets in the tab, which is also the drain, and a mica strip under the tab. The printed circuit board layout. All P-channel sources are (+). All N-channel sources are (-). Each motor lead connects the drains of one side of the bridge. The gates of one side of the bridge are connected to an input from the servo electronics. Ground goes to ground. The cap goes from (+) to (-). A very basic circuit. ---------------------------------------------------------------------------- Four solid-state relays were tested in an H-bridge in place of discrete MOSFETs. The caps from the motor leads to ground were .006 mFd. Sketch of the layout using four SS relays. The four SS relays are on a steel heat sink . Gate-closer gearmotor driven by SS relay H-bridge. (The feedback pot has not been connected for this initial smoke-test.) Good electronic components company.