Name: Michael Shimniok
Member since: 2007-12-23 16:33:37
Last Login: 2013-08-22 04:57:14
Author of Bot Thoughts blog, interested in robotics since '89. Since 2007, dove in full force, built Pokey the firefighter (failure = learning), and then Data Bus, 3rd place in 2012 AVC, my Rapsberry Pi tele-rover, a beam robot, and have tinkered with lots of other electronic thingies.
Recent blog entries by shimniok
Trash Robot Design, Part 2
Previously we looked at an option of lifting the trash can and driving it down the driveway to the curb. This time around let's look at the other option of grabbing the can's handle and wheeling it to its destination.
Grab the handle and wheel the cans to the curb
Probably a good idea to do some preliminary calculations on this tilty thing. The force required to tilt the 25kg can around its wheel axles measures 45-50N.
|About 45-50N is required to tilt the can at its handle.|
Minor problemWhile tilting the can, you need to counteract the force required to tilt it. If the chassis has trailing wheels as pictured above, they will tilt up if the force f2 exceeds the gravitational force at the trailing wheels. So to keep the robot from tilting, put a mass over the trailing wheels at least f2/9.81.
There's leverage at play here. If the length of the robot equals the height to the pivot point of the arm, then f2 = f. If the length is shorter as pictured, then f2 > f, by a factor equal to the ratio of these two distances, otherwise f2 < f, by the same factor.
A longer arm means less leverage on the trailing wheels by the arm pivot point. But the motor torque on the arm will have to increase to apply the same force. A bigger motor or lower gearing, meaning lower speed, will be required.
Another option, the robot could drive the motors backwards toward the can while tilting to counteract the force with the main drive motors' torque.
Still another option, put the upright and arm more toward the robot's middle, reducing leverage of the pivot point.
I'm sure you figured out five minutes ago that the simple solution is to put the trailing wheels on the other side, but then one side of your chassis will have to be open which means the chassis will be less rigid unless you add additional reinforcement. Seems like a reasonable solution.
How do you turn?One more thing to think about when wheeling instead of lifting. How do you turn the robot when you're clamped onto a trash can?
Let's assume the robot will have differential drive because that will provide the best maneuverability in tight quarters the robot is likely to see. To turn the robot, one wheel goes forward, the other in reverse. Each exert an equal force, fw. The total turning force at the robot's track width is 2fw.
The drive wheels will exert a force at the trash can wheel. That force will be perpendicular to a line drawn from the can's wheel to the robot's center. How much is the force?
Imagine a there's a virtual arm with a force at a distance r2 shown below. To find the force, take the torque and divide by the arm's distance.
So, for one thing, the force is smaller than it is at the drive wheels. More importantly, the force is trying to move the trash can's wheels mostly sideways. Yeah. Good luck with that.
So either we put our drive wheels next to the can's or we have to have a pivot point where the robot grabs the can.
Pivot PointWith a pivot point we're now able to exert a torque on the trash can. The force at the pivot is still 2fw. The torque exerted on the can is proportional to the radius of this pseudo-trailer's tonque, 2fwrt. The force exerted at the can's wheels is the torque divided by the wheel radius of the can, rc.
If we have a trailer, and we have trailing wheels behind the robot to deal with the lifting torque of tilting the can, that means we have to make the chassis wide enough to let the trash can turn. Something else to think about.
But hey, at least we have a few major design options to try and choose from:
- lift the can and drive,
- drag the can on a pivot, or
- drag the can with wheels coaxially located.
There's a lot more design to work out just on the physical side let alone software and electronics. We'll get to all that. But for now, I'd like to start prototyping the chassis. I like to build the electronics once I have some idea of what the chassis is going to look like.
Next: Prototyping the chassis
Trash Robot Design, Part 1
|To be taken out.|
- Convert the trash cans into robots
- Put the trash cans on trailer and tow them
- Lift the trash can and drive it to the curb
- Grab the handle and wheel the cans to the curb
After thinking about zany ideas it's time to be the dark-suit-wearing buzzkill. Frown and look stern. It helps get you in character.
The first two options are out. Why? I don't want to add extra weight to cans that the trash carriers must deal with and I only want one robot.
Let's talk about lifting the can and driving it to the curb first. Some potential difficulties may arise designing for the weight of the cans, their center of mass being so high, the inclined path, among other things.
Lift the trash can and drive it to the curbOnly one robot required. The chassis will have to be able to hold a full 50lbs (20-25kg). And something will have to lift that weight up a short distance off the ground. This might work, but we probably should do some quick calculations.
Drive Motor TorqueSo, first of all, how much torque is required to move a 25kg (+5kg robot?) on an inclined driveway? Same answer whether I'm lifting or rolling the can, by the way. And the answer depends on the wheels and gearing and on how much acceleration you want. Actobotics has a 6" heavy duty wheel and a variety of precision gear motors.
You may know that torque is equal to force at a specified distance (wheel radius). And force is equal to mass times acceleration.
I'm using SI units like all the cool kids. Mass is 30kg. Wheel diameter is 0.1524m (6") and the torque acts at the radius. Here's a diagram from Wikipedia showing the forces involved in a free body on an inclined plane.
Gravity, g, is 9.81 m/s2 and the angle is up to 8 degrees. If we were climbing the hill, the geartrain and motor need to generate a force equal to f to stay still (zero acceleration), greater than f to achieve any acceleration uphill, and less than f to accelerate downhill. So how big is f?
We're talking about 41 Newtons (kg-m/s2) to stay still. I have no concept of 41N because I live in America but no matter, let's just figure out motor torque. For a 0.1524m (6") diameter wheel, the torque generated by the motor and geartrain at the wheel radius must exceed 3.12N-m. (If the wheel radius was 1m then the torque required would be 41N-m). It's possible that if the motor is in braking mode, with the torque multiplication of the geartrain it'll be ok. I'll have to verify that later.
Lifting the canHow much force is required to lift the can vertically? The force would have to exceed the can's mass times the acceleration due to gravity, mg or 9.81m/s2 times 25kg which is about 245N. Let's suppose you use a motor or servo with an arm to somehow lift the can. Or a winch with a drum. If the arm/drum has a 0.01m (4") length/radius, the force at that distance is 245N so the motor driving it needs to supply at least 0.01 times that, or 2.45N-m of torque. Two motors halve that requirement. The shorter the arm, the less motor torque required but also the less movement possible.
ServoCity offers a selection of precision gear motors that look like possible candidates for driving such an arm. There are also hi-torque, geared servos available. The 45rpm gear motor supplies a torque of 1.963N-m. Two of them give 3.93N-m which should be adequate for a 0.01m arm. There's also a 20rpm model as well as lower and higher geared models.
ServoCity also offers some killer geared servos. The one pictured above puts out 1281 oz-in of torque when geared 7:1, or over 9N-m. Just one of these servos should be more than enough to lift 50lbs with a 0.01m arm.
|Vertical force from rotating arm|
Something else to consider is that the torque required is at a minimum when the force is perpendicular to the Earth's surface.
Using a rotating arm, the force will be applied at increasing angles to the force of gravity (see picture). The vertical component will diminish to zero as the arm reaches vertical.
Therefore, it'll be best to operate the arm in a narrow range of angles. Below is a plot of available torque for several motors, given a 0.01m arm, at different angles.
Here's a plot of 0.01m arm angle versus vertical force. The 60rpm motor can operate up to 30 degrees. Any of the lower-geared motors can run up to 45 degrees, no problem.
Ok, that design could work. I still want to double-check the lifting capacity of these motors and servos in the real world in case I goofed up something.
Next: Grab the handle and wheel the cans to the curb
Robot, take out the trash!
I hate wheeling the trash to the curb on trash day and sometimes I forget. Screw that noise, man. A robot wouldn't mind, wouldn't forget and wouldn't care if it's freezing outside. I'm building Take Out The Trash Robot. TOTT Bot, for short. So there.
|A few of the many Actobotics parts I'm evaluating|
What the Isaac Asimov is Actobotics, you ask? Lots of cool parts. Precise parts. With drill holes in compatible locations. So, it's easier to prototype. Between the ball bearings, precision shafts and tubing, lightweight aluminum channel, gears, sprockets, chains, belts, and pulleys, not to mention ServoCity motors and giant servos I'm seriously giggling with glee here.
As it turns turns out, I've been wanting to build a trash robot for a few years now. After wondering "what am I going to do with all these parts?" for awhile, I decided the trash robot is the perfect project to see what the Actobotics platform can do for us robot-builders. It's going to take several weeks to build and I'm capturing it all here. Hope you'll tune in while I prototype, build, figure, calculate, screw up, fix, and all that joy. On to the problem statement...
|Problem: This. To curb.|
|Almost 50 lbs. Pretty typical.|
Size? About 0.41m wide by 0.93m tall (look at me, ma, I'm using SI units). The handle is floppy and is hinged at a height of 0.84m. The cans have 0.15m diameter wheels with a track width of only 0.36m.
Tippy much? Uh yeah. Their center of gravity is always high but varies. If I had a nickle for every time these stupid things fell over, spewing trash all over, while I'm dragging them to the curb in my jammies... Well, that's some fun stuff right there.
|Floppy handle hinges at a height of 33"|
|Angle finder shows about 6 degrees, steeper elsewhere|
|My humble abode. Note trash cans and missing Jeep.|
When the trash is picked up the trash collector lifts up the can and dumps it into the truck, then runs the cans back up to the garage where you see the cans pictured above. (It's a neat little value-add)
I should mention, too, that I only want to build one robot to carry cans one at a time, not some trash carrying swarm.
Oh man, this is going to be great! No more trash carrying! Yes! I invite you to join in, follow along, subscribe, share, all that stuff.
Next Time: Evaluating Candidate Designs
Pololu Black Friday
Pololu is doing another Black Friday sale this year! Lots of great deals. (No affiliation, I just like Pololu). (Update: click here for deals from Sparkfun, GHI, Parallax and Bot Thoughts!)
Pololu Robotics and Electronics is having its biggest Black Friday sale yet, discounting hundreds of sensors, actuators, motor controllers, and other robot parts by 30% to 60% and offering an additional 11% to 15% off orders over $100! Buy one Zumo Robot and get one free, save on a 3pi Robot and get a free programmer, and take advantage of great deals on select Arduinos, Raspberry Pis, and mbeds. The first doorbuster deals go live Wednesday, November 27, and the sale runs through Cyber Monday (December 2). For details, visit www.pololu.com/blackfriday.
Cyber Monday, New Products!
Announcing Bot Thoughts Cyber Monday discounts! Details here.
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