Building an autonomous truck from scratch: Part 2
In my previous post, I covered the process I went through in choosing and constructing the platform for my autonomous truck. Although I have not fully begun the software development, I have been able to complete the powertrain. Before I continue with the project, I wanted to ensure the power system was up to the job of hauling everything that I would need to have on board.
Luckily, I had a spare RC receiver from an old tricopter laying around which I was able to quickly hook up to the ESC (Electronic Speed Control) and steering servo. This enabled me to give the truck a manually-controlled performance test with the operational weight that would be required for autonomous driving.
With all of the electronics loaded in the back, the truck weighed 0.95kg.
The truck was able to crawl through thick grass without much issue. Since a varying throttle was needed to maintain a good velocity, I decided that the final software would need to have a rudimentary cruise controller in order for the truck to keep to a reasonable pace. This would also be useful for hills, where the motor would need to exert more power to keep a constant velocity.
In terms of suspension, the truck has ample ground clearance and suspension travel to deal with all but the most difficult obstacles I gave it. I also found the tyres to be excellent at dealing with all of the surfaces I encountered in the testing session. However, I did have an issue with the rear right tyre as it kept on popping off its wheel. This seems to be a combination of the fact the the wheel is slightly broken on part of its rim and because the battery sits on the right side of the bed. (The battery is the heaviest single part of the truck). When the truck was travelling across a slope from left to right, almost all of the frictional force keeping the truck from sliding down the hill had to come from the rear right wheel. I will try to use superglue to hold it in place, and failing that, the spare wheel that came with the truck.
With the full load of the electronics in the back, the truck had good power in the low end but topped out at ~1.6ms^-1 due to its high gear ratio. However, this suits the operational requirements I had of the platform, as it means I can easily stop the truck if it decides to go full throttle in an arbitrary direction. Furthermore, this gearing will enable the truck to traverse extremely steep hills where a faster vehicle might struggle to put down enough torque. Another positive effect of the truck's transfer case is its high friction when not powered by the motor. This enables the truck to come to a complete rest on a hill, acting as a handbrake and stopping it from rolling backwards.
The battery the truck is designed to work with is a 700mAh 4.8V NiMH unit. The battery I have selected is a 2200mAh 2S (7.4V) LiPo battery. The LiPo is lighter, higher capacity and higher voltage. The greater voltage aids the motor with its peak RPM, giving the vehicle a greater top speed. The capacity of the battery means that even after ~15 mins of hard driving, the truck had lost less than 10% of its capacity. While the load on the battery will increase with the addition of the Raspberry Pi, GPS module and WiFi adapter, it should still provide a good running time even when pushed to its limits.
The greatest issue I had with the truck was the steering radius. The servo that I had selected does not turn the wheels at a great enough angle. This is something that I will need to work on as it could severely impact the capabilities of the truck when driving autonomously.
Luckily, I had a spare RC receiver from an old tricopter laying around which I was able to quickly hook up to the ESC (Electronic Speed Control) and steering servo. This enabled me to give the truck a manually-controlled performance test with the operational weight that would be required for autonomous driving.
With all of the electronics loaded in the back, the truck weighed 0.95kg.
The truck was able to crawl through thick grass without much issue. Since a varying throttle was needed to maintain a good velocity, I decided that the final software would need to have a rudimentary cruise controller in order for the truck to keep to a reasonable pace. This would also be useful for hills, where the motor would need to exert more power to keep a constant velocity.
In terms of suspension, the truck has ample ground clearance and suspension travel to deal with all but the most difficult obstacles I gave it. I also found the tyres to be excellent at dealing with all of the surfaces I encountered in the testing session. However, I did have an issue with the rear right tyre as it kept on popping off its wheel. This seems to be a combination of the fact the the wheel is slightly broken on part of its rim and because the battery sits on the right side of the bed. (The battery is the heaviest single part of the truck). When the truck was travelling across a slope from left to right, almost all of the frictional force keeping the truck from sliding down the hill had to come from the rear right wheel. I will try to use superglue to hold it in place, and failing that, the spare wheel that came with the truck.
With the full load of the electronics in the back, the truck had good power in the low end but topped out at ~1.6ms^-1 due to its high gear ratio. However, this suits the operational requirements I had of the platform, as it means I can easily stop the truck if it decides to go full throttle in an arbitrary direction. Furthermore, this gearing will enable the truck to traverse extremely steep hills where a faster vehicle might struggle to put down enough torque. Another positive effect of the truck's transfer case is its high friction when not powered by the motor. This enables the truck to come to a complete rest on a hill, acting as a handbrake and stopping it from rolling backwards.
The battery the truck is designed to work with is a 700mAh 4.8V NiMH unit. The battery I have selected is a 2200mAh 2S (7.4V) LiPo battery. The LiPo is lighter, higher capacity and higher voltage. The greater voltage aids the motor with its peak RPM, giving the vehicle a greater top speed. The capacity of the battery means that even after ~15 mins of hard driving, the truck had lost less than 10% of its capacity. While the load on the battery will increase with the addition of the Raspberry Pi, GPS module and WiFi adapter, it should still provide a good running time even when pushed to its limits.
The greatest issue I had with the truck was the steering radius. The servo that I had selected does not turn the wheels at a great enough angle. This is something that I will need to work on as it could severely impact the capabilities of the truck when driving autonomously.
Overall, the truck performed very well, especially when you consider the price (£23 shipped w/o battery). I will need to make further adjustments to the steering arrangement before I can begin work on the hardest parts of the project -- the electronics and programming. Stay tuned!
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