I am a bit bored while the printer is working all day long. So I have continue with the design of the new drive system for the robot. I ordered new motors the other day and while they not yet have arrived I have the ability to compete the first concept of the drive-system before they are arrive, so I can test them as soon as they arrive!
As I wrote earlier I want be able to change pulleys and the center to center distance easy. This allows me to change gear ratio with out hassle and without the need to re-design the drive-system.
So lets discuss the basic before moving on.
A lawnmower in not in need of any fast speeds. A slow speed with smooth movement is the goal. The BLDC motors tend have quite high RPM possibilities. The BLDC Kv is often taken to be the RPM/Volt of an unloaded motor. In other words, if you apply 1 Volt to an unloaded motor, the Kv constant tells you how fast the motor will rotate. Then if you apply 2 Volts, the motor will rotate twice the value of Kv. My new motors have 190Kv value meaning if I apply 22.2V I can have a possible 4218RPM at the motor shaft. In other words it spins fast, to fast for a lawnmower.
To increase or decrease the RPM on the motor shaft one can use pulleys to alter the gear ratio in transition between motor shaft and wheel axle. To decrease RPM on the wheel axle I will mount a small pulley on the motor shaft and a bigger pulley on the wheel axle, in other words I will gear it to to a lower RPM at the endpoint.
I have not yet decided on the actual pulleys that will be used in the end, but I have ordered some test pulleys that can lead me to the right way. The pulleys I will use is XL 12 Teeth and a 100 Teeth 8mm Pilot Bore Screwed Timing Pulley. The pulleys Center-Center distance Is not yet decided, But with this drive-system I can easily slide the motors to change the belt length. Also It will be easy to adjust the belt tension. I will also be able to change pulleys and and adjust the belt length to suit its needs.
So some test calculations
720RPM (UNLOADED) is still a quite high for a lawnmower. The transition from the Wheel axle to the wheels will also needed to be calculated. The Valify Robot Lawnmower will use 220mm wheels. But of course this values have been calculated on full throttle (22.2v) Lets calculated more about this (Editing).
Here is the BOM for the first test
- 2 of uxcell Aluminum XL 12 Teeth 10mm Bore Timing Belt Pulley
- 2 of uxcell Silver Tone Aluminum Alloy 100 Teeth 8mm Pilot Bore Screwed Timing Pulley
- 2 of Uxcell a16111400ux0248 120XL 60 Teeth Synchronous Closed Loop Rubber Timing Belt 10mm width 304mm Perimeter, Rubber
- 2 of Hardened shaft Ø08mm 240mm
- 10 of Shims 8X14X0,5
- 6 of bearings 608 ZZ 8 x 22 x 7
- 4 of Stop ring SR-08 – DIN705
- 2 of Radial seal 8 X 20 X 7 AS
The brackets will be CNC milled. But before making the trouble I will print them in my 3D Printer to confirm the design.
Note: The 5 parts will be ready in a couple of hours. This will mean that I will have all parts for the lower chassis for the robot lawnmower. GREAT! I can finally see the layout in real-life.
Another note: I hate messy cables and as it turns out the support beams have room for cables I can use 3/4 slots on each beam to lead cables through the robot. GREAT!