Quote:
Originally Posted by Nathan
Thanks for your replies, everyone, we really appreciate your help!
Here's a little more information to clarify a few things:
- We're wanting to use normal wheels (not a holonomic drive) and steering like on a car
- We had a mechanical engineer give a presentation to our club and he also recommended Ackermann steering, so we'll be trying to implement it somehow with a servo
- We'll be using a cRIO for the control system
- We estimated that the weight should be around 30lbs, but to be safe we should probably plan for twice that, or 60lbs
- If we end up using FIRST motors, then we'll probably get one of the standard batteries that FIRST robots use (12V motorcycle battery?), as long as that doesn't put us way over our weight limit
Wow, that looks like it might be really helpful. Thank you!
Thanks for your help! Here's some more information:
- The weight we're shooting for is 30lbs (not sure how realistic this is..), so let's plan for up to 60lbs
- The maximum speed needs to be at least 4 mph, but we'd like it to go up to 6 mph
- I'm not sure on acceleration, but it's not a huge issue since speed isn't our primary concern
- We need it to be battery operated. What type of battery we use mainly depends on how much power the motors draw, so we don't know what battery we'll be using yet
- We'd like an operating time of at least 20 minutes of running non-stop, which would hopefully translate to at least 40 minutes of on and off use. Obviously a longer running time is preferred, though!
- We'll be using a cRIO as the control system and will have a couple of low power sensors and some kind of camera (possibly an Xbox Kinect)
- We need at least 2" of ground clearance, so we were planning on using 6" wheels
So far we've had these motors recommended: - Banebot (possibly with a 4:1 or a 16:1 ratio)
- BAG motors with the versaPlanetary gearbox of 10:1 (but they might be bad, according to a few)
- miniCIM in a versa-planetary
Where do you all buy your parts from? Is there any particular place you recommend? |
*Standard disclaimer* The following is meant to be a guide, and by no means is an exhaustive list of what needs to be done.
I understand your objective weight is 30 lbs, but that's really hard to achieve; 60 lbs is more reasonable. I would suggest using this number, assuming no mechanisms or devices are mounted to your vehicle. 30lbs will likely be your drive chassis, wheels, and transmissions.
Those are good numbers on speed. You can take these (6 and 4 mph) to determine the velocity against the ground required at each wheel. This speed will be used to get a gear ratio with the motor.
The reason for acceleration is to find horsepower (think in terms of Work). For now, you may want to make an assumption - maybe 6 mph can be reached in 6 seconds (just make something up). The horsepower rating is required to size the motor needed to accelerate your vehicle in a given time. In conjunction with the scrub force on the wheels and your wheel size (width and diameter), calculate the torque required for each mode of operation (forward, reverse, turning). This will allow you to size the motor get torque.
From here, build a "mission profile", where you lay out how the vehicle is intended to be used: turn on with a systems check, 10 seconds idle, 15 seconds full power acceleration, 85% cruise for 10 minutes, deceleration to 0 mph in 10 seconds, accelerate to 3 mph for 4 seconds at 100% power, drive an 8% grade hill while maintaining 4.5 mph, accelerate to 6 mph at 85% power, etc. This is a simple bookkeeping method that will allow you to size your battery. Using the required current draw from this profile, you can verify or select a battery that will provide the proper current for the time you need. For this vehicle, you may be able to use the standard 18 Amp-Hour 12V battery used by FRC, but it has to be proven first.
Some prototyping of the drivetrain and measurement of each motor's current draw in a loaded system will help validate your initial assumptions or prove them to be incorrect. From here, you can size your battery appropriately.
Is there a reason for the cRIO? Speed of development (LabVIEW), sponsorship? Ruggedness? If weight is a concern, I would suggest driving this system using an Arduino and coding through LabVIEW or the Arduino IDE to save at least a pound and a half. Additionally, the
MyRIO should be entering the market soon (if not already). It is the platform that the 2015 FRC RoboRIO was leveraged off of (as well as the NI cRIO 9068). Either of these should handle the Kinect output in real-time, assuming there is a LabVIEW driver built.