jjrreett

So I am on my school's competitive science team and have an event. This event's goal is to design and build an electric vehicle that both as quickly as posable and as accurately as posable reaches a target distance between 9 and 12 meters. ( only the first 8 meters are timed)
Last year I got second but want to improve 10 fold this year.
I need the most help with the battery and motor system.
I am limited at 9 VOLTS
My original idea was to use a 2s lipo with a bldc motor but now find it favorable to run a brushed DC motor because of simplicity and the ability to short the motor to craft an active brake.
I am looking for a favorable battery motor combo
Any ideas

jester_s1

You realize that most brushless ESCs for surface vehicles have a brake feature, right?

After getting brushless power worked out, your real trick will be getting that power to the ground. That means working out your acceleration settings, clutch tension, and any suspension your are allowed to get maximum traction.

red head

Different props may be your answer.

jjrreett

Well the track is completely smooth so I don't have to worry about suspension. I didn't think about using a clutch because I will control the motor with an Arduino, so changing settings is simple enough.

So I did the math and calculated that my output power would have to be close to 25 Watts. This would mean about 300mNM torque at 1600 RPM. Obviously I can gear that to different ratios. The problem still lies with finding a motor that can work that at less than 9 volts. (preferably a 2s lipo)

red head

Sorry ~~ My bad, thought it was a plane . ENJOY !!! RED

jester_s1

If you actually want to do this project well, you need to get a basic understanding of performance electric power systems and model powertrains. Getting on the internet and asking hobbyists to do your homework for you isn't going to teach you anything and won't likely help you much in the contest. Read up on what slipper clutches do in RC cars, then read up on motors and what kind of performance can be expected from them for various weight cars. As it is, we can't help you. You don't have any specs for the project like vehicle size and weight, amp or watt limits, what kind of tires and gearbox you will use, or what materials and supplies you are allowed to use. Sit down and actually put some effort into the project, design your vehicle, and do your own research. Then not only will you have learned something, but you'll also done your own work and will be able to be proud of it.

jjrreett

So I have done the calculation. I want 27 Watts output I can change the gearing as nesisary for my needs. The ideal motor has 318mNM of torque at 7.4 volts at 1624 rpm. This motor is ideal because I would not lose efficientcy due to gears. With gears, any ratio of rpm to mNM (torque) that is close to 5 at 9 volts or less is perfect.
Now I don't want to use a giant ESC with a huge fan (this will only be used in 5 second bursts so a fan is not nesisary. I wanted to use a quad ESC but also want the brake. (Can I toggle that with an Arduino)

jester_s1

Those specs aren't anywhere close to reality for any kind of electric motor that I've heard of. So like I said above, do your own research about motors and what's available, compare them to what you used last year, and actually approach the project with some real knowledge. Just so you know, a motor putting out that kind of torque at that speed would be pulling roughly 53,690 watts, give or take a few. Not that that would even be possible at 7.4v, but if it was, you would need batteries and wiring that could handle over 7000 amps of current.

init4fun

To the original poster ,

Jester is spot on about you doing this yourself so that you learn something in the process . There are guys here at RCU that can bang out electric motor specs like a football fan knows his quarterbacks , but if one were to drop this right in your lap what will you have learned ? Sure , you can come back and say you learned to go with a pro's advice and in "real life" that would be great advice indeed , but here your instructors obviously believe there is some important lesson for you to learn from this , and I doubt that lesson is to defer to someone with more knowledge . I think you'd be best served to first exactly define the vehicle , determine how many watts it will take to get the desired acceleration result , and build from there . Now I know that for model airplanes there are pre existing charts that tell how many watts per pound will result in what type of flying the plane can do , everything from putting around the field in a biplane at 50 watts per pound to unlimited vertical in an extra with 400 watts per pound . It's those kinds of figures you'll need but specific to your particular application and figuring that out may be part of what they want you to learn .

jester_s1

I'm not even bothered by a guy showing up with specific questions about a project like this. Sometimes the hobby literature doesn't spell things out the way a beginner needs them to be. But I do expect a guy doing a school project to take the time to design his vehicle, know what the actual power requirements are, know what limits are being placed on the design, and go ahead and study up on the basics of the equipment. So if he were to ask about the suitability of a specific motor to turn a particular set of tires with a particular gear ratio within a given amp draw and voltage limit on a vehicle that weighs a certain amount, I'd help him if I could. But the above posts make it pretty clear that he's taken no time at all to learn about these power systems, so there's really not much for us to do for him. He's even talking about using an Arduino to control this thing, which makes no sense at all to me unless the project is specifically geared toward creating an autonomous vehicle.

jjrreett

Is there any way I can calculate torque per volt (like kV but torque), or calculate the speed torque curve at different voltages?
Yes this is autonomous and I am limited by a very specific rules set.
The standard specs seem very arbitrary, but now that I know my expectations are unrealistic I can look differently at these motors.
Are there fanless ESC's (like a quad but with brakes) I'm looking at 30 A.
Are there any resources on controlling ESC's with arduino. (Expecaly the brakes)

jester_s1

Voltage only determines RPM. Amps determine torque. Motor manufacturers will know the operating ranges of their motors and the amp limits they can handle. I can go ahead and tell you though that it's not just a quick plug in the numbers kind of thing. The way an electric motor works is that it tries to turn the RPM that the voltage you give it says that it should, and will draw as many amps as it has to to get there. But as amp draw increases efficiency goes down, heat production goes up, and voltage is lost. That all results in the motor turning more slowly. As the load increases, eventually the motor won't be able to draw enough amps to go the speed that the voltage says it should. That's when things start melting and smoking. The weight of the vehicle, the terrain, the rolling resistance of the tires, and the type and efficiency of the motor you choose will all affect your equipment choices.

Why do you care if the ESC has a fan or not? Most surface brushless ESCs do because ESCs in trucks and cars don't have much airflow over them so they tend to run hot. Aircraft ESCs are not suitable for surface applications because the firmware is so different.

Arduino based autonomous vehicles have been done before. I doubt there is anywhere for you to download code that someone else has written, ie, you'll have to do your own homework. You can buy an APM 2.6 and use the Ardurover software that 3DR developed if you want a plug and play solution. But I imagine if you have a teacher who assigned an autonomous vehicle project you've also received some training on how to develop the software for it.