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91
it sounds like you have already worked through many challenges by yourself.

- you mention a rotary encoder, are you using this for closed loop control or the operator interface?

- when the motor stalls does it always stall on the upswing? is the arm balanced?

- have you adjusted the current limit on your stepper motor driver, if you are limiting the current of your larger stepper motor to the same value it may be the reason you did not see any improvement

- steppers can face some issues with natural frequencies so it is possible this is part of the problem but it wouldn't be my first guess



92
Mechanics and Construction / Re: mistake in meachnics gear tutorial?
« Last post by bdeuell on March 15, 2016, 08:25:41 PM »
you are correct there are several errors, below are what the equations should be.


writing down the equations:
torque_motor / radius_gear1 = torque_gear2 / radius_gear2
torque_gear2  = torque_gear3
torque_gear3 / radius_gear3 = weight

simplifying, we get:
torque_motor / radius_gear1 = weight * radius_gear3 / radius_gear2

so therefore the minimum required motor torque is
torque_motor = weight * radius_gear3 / radius_gear2 * radius_gear1


as radius_gear3 = r and radius_gear2 = r you can further reduce the equation to
torque_motor = weight * radius_gear1

Also weight has the units of force not mass.
93
Hello Robot Experts!

This is a bit of a long story, and I'll try to keep the level of detail appropriate, but I can say that this is all embarrassingly well documented and I am more than happy to provide more pictures, part numbers, video, etc. I'm a mathematician working on a little robot project for my calculus class; a robot that throws a small ball from a rotating arm. I built a working prototype from VEX parts (you can see it in action here)

https://www.youtube.com/watch?v=qGkNjDZgBNQ

but VEX is quite expensive and a bit flaky mechanically, so I set out to build one from off the shelf parts with an Arduino for distribution to other schools.

I got a suggestion to use a stepper motor for the basic drive (this may be my mistake already), a rendering of the entire build is attached; basically the idea was to use the stepper to directly drive an arm about 35 cm long. I'd like to rotate the arm at about 150 rpm (max). I built a test rig to hold the motor and cut a test arm from MDF to evaluate various stepper motors.

I first tried use an Arduino Motor Shield and the Stepper library to drive the steppers. That didn't work at all; after some reading, I read the recommendation that a driver board is usually much better, and that plenty of people recommend the AccelStepper library instead of Stepper. (I'm running an Arduino MEGA b/c I needed extra pins to run a display, a rotary encoder, and a relay as well as the stepper.)

I was also very puzzled by why the coil voltages listed were so much lower than the voltage I needed to apply to actually get things to run, but I think I get it now that as long as current is limited, high supply voltage can charge the coils faster and help a stepper run quickly.

I bought a BigEasy driver board from SparkFun. I'm powering the BigEasy with a 24 vdc/1.33 amp power supply; I've also tried a 24 vdc/2.65 amp power supply. I'm running AccelStepper in DRIVER mode, and I've tried all the combinations of full stepping/half stepping/.../16x microstepping that I can think of. The steppers do run smoothly at low speeds, so I don't believe that I've simply wired them backwards (but could I be wrong?).

I've tried several different kinds of stepper motors now, and all of them start to get flaky (randomly stopping, shaking, and refusing to go faster) about 60-80 rpm (I'm measuring the speeds with a laser tach, so I'm pretty sure of them and can provide more detail about settings and speeds if it helps).

The first stepper I tried was a 125 oz/in 200 step/rev motor from SparkFun:

https://www.sparkfun.com/products/10847

at 16x microstepping, I got up to 74.69 rpm at 4120 steps per second. After which, things started to get weird.
Shaking, stopping and starting, failing when I put any weight on the arm.

After some reading around, I read that steppers lose torque at high speeds and may be unable to drive their own shaft if you rotate them too fast. So I figured that perhaps 125 oz/in just wasn't enough power, and tried out two gigantic stepper motors, both from Trinamic (the QMot series), with 238 oz/in and 435 oz/in.

Interestingly, the results were about the same. I can get each of them up to about 70 rpm, after which things get weird.

I was really surprised that the very different torque ratings of these motors didn't change anything; just to test on the other end, I tried an Applied Motion stepper motor (it's a 6-lead motor, but one I got on a special clearance sale for $10 each; I think it's even less powerful than the SparkFun motor, but I don't have any really obvious way to test it). The results were just about the same. Works ok at slow speeds, I could only keep it running stably at a max of 181 steps per second.

I have several working theories at this point, but maybe they are all implausible:

1) The BigEasy can't put out enough steps/second to run the motors fast. I'm willing to try the Trinamic StepRocker controller, which is advertised as "matched to the QMot series steppers", but it will be a pain to interface it with the Arduino. It has an internal driver, which presumably can put out many more steps/sec and also can take voltages up to 70v (the BigEasy seems to top out at 35).

2) I read that underloaded steppers can act strangely, and I've cut parts for damper to add mass to the rotating arm in the hopes that it steadies the motion.

3) Steppers just can't run this fast, and I need to gear up the motors to spin the arm faster. I have parts coming in to try this, but I am really confused because I see listed torque/speed curves for these particular stepper motors with speeds seemingly in hundreds of rpm.

I feel like I've put the time into trying to figure this out for myself, but there's clearly something that I'm just missing here. Can anyone help?
94
Mechanics and Construction / mistake in meachnics gear tutorial?
« Last post by cervantes on March 15, 2016, 01:12:17 PM »
Hi

I just started to read whole website.
On page: http://www.societyofrobots.com/mechanics_gears.shtml I think there is a mistake.
In section Compound Gears there is a formula for torque gear calculation:

------
In this example, what minimum torque does the motor need to pull the weight up?

writing down the equations:
 torque_motor * radius_gear1 = torque_gear2 * radius_gear2
 torque_gear2 * radius_gear2 = torque_gear3 * radius_gear3
 torque_gear3 * radius_gear3 = weight * radius_gear3 

simplifying, we get:
 torque_motor * radius_gear1 = weight * radius_gear3 

so therefore the minimum required motor torque is
 torque_motor = weight * radius_gear3 / radius_gear1
-----

BUT THE UNITS are not ok...? how can moment can be compare to load (mass)?....
can someone verify my opinion?
95
Mechanics and Construction / Re: DC motor torque control with PWM?
« Last post by mstacho on March 15, 2016, 07:07:48 AM »
Wow! This is an ancient thread, but I used force sensing resistors like these: http://www.robotshop.com/en/lynxmotion-fsr-01-force-sensing-resistor-kit.html although there are larger/smaller versions, they all worked well.
96
Mechanics and Construction / Re: DC motor torque control with PWM?
« Last post by bmagyar on March 15, 2016, 06:07:56 AM »
Hi Mike,

Hope you are still around :)
What sort of force sensor did you use? What would you use now?
97
Misc / Re: Best Programming Language Used in Robotics
« Last post by johnnywheels on March 13, 2016, 11:49:17 AM »
For simple, and lots of support, the Arduino platform, which uses the C++-like Processing language.
airwheel q1 unicycle
For more real-world stuff, C and C++, or with full computers on board, the Robot Operating System (ROS) is worth looking into.  ROS has APIs in both C++ and Python.

My roommate uses C++ in coding as well...haven't heard of ROS though, i'll give it a look!
98
Misc / Robotic blocks even 3 year olds can use?
« Last post by Browsingrobotics on March 11, 2016, 06:45:12 PM »
Hi there, I stumbled upon an article on Forbes about these robotic blocks to move and program old toys, gadgets, 3D prints and it seems a lot more. They are fully wireless and can be controlled with a smartphone or tablet. What got me excited is the smartphone controlled GoPro you can drive around, the fact you can bring 3D prints to life and that even young kids can play with it cause it doesn't require any screws, wires, soldering etc. I checked out their kickstarter page to find out more and I am actually thinking about it.

It seems pretty cool to me. I have seen other kits around but none that is so easy to use and that allows you to control multiple blocks from one single device. Let me know your thoughts before I get one. The article on forbes is http://www.forbes.com/sites/andyrobertson/2016/03/11/tio-brings-old-toys-to-life-with-robotics-and-iphones
99
Software / communication with PC via the com port (serial port)
« Last post by ansh on March 10, 2016, 05:39:56 PM »
Hello

I have computer that has internet connection and Microcontroller is connected to computer via usb to db9 cable. LED is connected to port pin of microcontroller

Computer -> web page (com port of PC)----- DB9 connector -> max Rs232 - > micro c -> LED

I want to control LED via web page I have created a web page , there are two buttons on web page , start and stop, if I press start button on web page , led should be “on’’ and If I press stop button on web page ,led should be turn “off’’
I am using Windows ,I want to send this information (LED ON/OFF) from internet to one of the serial ports of my computer? I am looking example code , can anyone tell me How does web page send data to port of computer ?
100
Robot Videos / MeccaNoid does- Mc Donald's Comedy Sketch
« Last post by MeccaNoid Madness on March 10, 2016, 12:41:19 PM »
https://www.youtube.com/watch?v=2k1H39xV-YU

I did this in LIM mode. I changed the voice to +30% pitch to sound more like MeccaNoid

Notice how he looks down to find that extra fry.

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