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21
Electronics / Re: Controls Braking vs Dynamic Braking for Motors
« Last post by Tranq97 on October 20, 2014, 03:35:51 PM »
Large motor, large motor... I'm not talking about one I possess, and I'm not sure how to judge it in kW. Let's just say the nominal dimensions are 150mm/150mm/250mm. That's probably as much as I can give you, sorry.

Now, this thing is going to go at high RPM and low torque, attached to a couple of gears to convert that to low RPM and high torque for the robot's arm. Before, I was trying to design a type of motor which could just run at high torque, low RPM. But, I found that that would hinder it's precision, accuracy and braking mechanisms. So, then I decided to use the gears with a high RPM motor. So, the braking on the arm will have to be virtually instant on the arm (big gear), so I don't know how quickly the motor (small gear) will have to stop to make that happen.

Basically, I'm building the motor and controller myself. The controller I have sorted. But electronic motors have always made me a little confused. With all this back EMF and whatnot. The things I need help with are as follows:
- Calculating motor requirements
- How to build a motor and account for those requirements
- What system can I use to change the acceleration rate?
- What system can I use to change the deceleration rate?
- What system can I use to hold a stationary motor in place?
- How to calculate gear sizes
- Are there any systems which can make the large gear more compact?
I'm 17, I've been interested in robotics for 4 years and it's all been textbooks, internet and college. I haven't had much practical experience at all, and about half of my education hasn't been learning, but rather developing ideas. As for the last point I made, basically I am worried that the large gear will be too big to fit in the robot's arm. So, are there any special gears which have the same effect as a big gear but a more compact? You seem like an expert on robotics, David, so I was wondering if you could give me a hand with these hurdles?
22
Mechanics and Construction / Re: Questions about robot actuators
« Last post by sdk32285 on October 19, 2014, 04:40:37 PM »
I am not positive why they are not used.

In most of my past projects we have used brushless DC motors with various types of gearing. I think one advantage to this approach is the ease of using your own gearing to modify speeds and torques.

I have used frameless motors before and they are very tricky to get right. You need to be very careful with the mechanical design and assembly to get the rated performance specs of the motor.
23
Electronics / Re: Controls Braking vs Dynamic Braking for Motors
« Last post by sdk32285 on October 19, 2014, 04:33:09 PM »

In many digital motor drives you can specify acceleration and deceleration rates.  After that a lot will depend on the type of gearing that you use and how much resistance that puts on your drive motor.

How much braking do you need? What do you consider a "large" motor?
24
Mechanics and Construction / how to make tiny pistons move
« Last post by amrosik on October 18, 2014, 12:37:18 PM »
Hello,

I want to know which of the following methods would be best, to make tiny pistons (8mm diameter of the cylinder, max. 6mm diameter of the piston) move. Of course the piston should have a recoil spring. It is important that the system has a quick response time, about 5 Hz should be no problem.

Indirect methods:

1. Using pneumatics and pushing solenoids.
2. Using a pressure reservoir and magnetic valves.

Direct method:

3. using a self made pushing solenoid in that size, with a neodymium magnetic core.




Concerning

1. A pushing solenoid is attached to a big cylinder with piston (isolated with a rolling membrane (diaphragm) or a bellow), and that is connected with a thin air hose to the tiny   
piston, again using rolling membranes or a bellow.
As the Solenoid is activated, the volume of the system is decreased, therefore the pressure increased, making the tiny piston push with a certain force. I need a force a 6 N.
I calculated that then the pressure should be about 22 Psi, and , given that the Solenoid has a stroke length of 1 centimeter and that the air hose has a inner diameter of 4mm and a length of 2 meters, we need a force of  about 300 Newton for the Solenoid, to create that pressure.
There is no Solenoid that could do that, in fact the best thing that I know creates 20 Newtons for that stroke length.
Can you do something about it, or is that method already checked off?

2. A pressure reservoir of big volume, holding air at about 22 psi is connected to a 3/2-way valve, which is connected with the tiny piston. Turing of the valve connects the piston with the reservoir, turning it of connects it with atmosphere.

Does that work? How to get those valves, working at these pressure levels? The should not be too expensive, because I need 40 of them.


3. I dont know how to wind the cupper wire with that tiny size restriction, to get 6 N out of it.


I will be grateful for any suggestion. Thank you in advance.
25
Software / Re: Axon II button not being recognized?
« Last post by Hero I on October 17, 2014, 02:18:51 PM »
The above solution worked.

26
Mechanics and Construction / Parallel Gripper Kit - $14.99
« Last post by ServoCity on October 17, 2014, 07:15:04 AM »

Get a grip! The first in our line of standard size gripper kits is here. The Parallel Gripper Kit A is a simple, durable, and versatile kit that is great for all kinds of robotic applications. The grippers move towards one another in a parallel motion, hence the name, and have a maximum width of 2.80. By incorporating the Actobotics 0.770 hub pattern into the back plate, and 1.0607 hole spacing on the grippers, you can easily attach nearly any Actobotics component to the assembled kit. Designed for use with any standard size Hitec or Futaba servo (sold separately). The kit is easy to assemble and requires only a Phillips Head Screwdriver. Watch our step by step assembly video for detailed instructions.

Actobotics Parallel Gripper Kit A (for standard size Hitec or Futaba Servos) - $14.99
http://www.servocity.com/html/parallel_gripper_kit_a__637092.html



Assembly video:
Standard Parallel Griper Kit Assembly (Full Instructions)


More gripper kits coming soon: http://www.servocity.com/html/robots___accessories.html
27
Mechanics and Construction / Questions about robot actuators
« Last post by ganvarel on October 16, 2014, 10:26:14 AM »
Hi to all, I'm new to this forum ( but not to robotics ). I'm no pro but just a hobbyst, however ( being a programmer ) I have a basic knowledge of physics and other stuff necessary to build robots.

I have built some simple robots ( some crawlers and a bioloid, plus some wheeled robots ) and I have always used dc motors and servomotors. But I'm trying to build something more interesting, a two legged humanoid the size of a child to experiment with AI and machine learning in robots.

I have made my homeworks and researched a bit the topic of actuators. What I have seen is that hobbysts use servos ( obviously ), but pros use custom built motors, linear actuators with dc motors, linear motors ( the ones that work like the maglev ), pneumatics, hydraulics, and so on.

For various reasons I don't think that hydraulic or pneumatic actuators are a good practical choice, so the only remaining realistic actuator type is the electric motor.

I have seen that the universities and labs around the world use frameless dc brushless motors ( torque motors ) + harmonic gears or custom built linear actuator with springs and other mechanisms to implement compliant joints.

But there is another type of electrical actuator, the so called linear motor ( also called direct drive linear motor ). I'm talking about this thing https://www.youtube.com/watch?v=SHTvv47Cs-E

This type of motor is fast, has great peak force, no bearings, pulleys, gears, etc... It seems a really good choice, but no humanoid or legged robot in existence uses them. Why? Is there a catch?
28
Misc / Fall Wire Sale - 50% OFF!
« Last post by ServoCity on October 16, 2014, 07:44:17 AM »
Don't miss our 50% off Wire Sale going on now thru October 19th!

Save big on select servo extensions!


http://www.servocity.com/html/servo_extensions___leads.html
29
Electronics / Protection circuits for GPIO pins of a Raspberry Pi bot
« Last post by RpiHacker on October 15, 2014, 07:39:23 PM »

Greetings,

I am making plans to build a GPIO breakout board for the Pi B+ and intend to use two (2) SN7407 ICs to make available 12 buffer circuits (3.3V to 5V level shifter) for connection with motor controllers, servo controllers, sensors, and other bells / whistles I will be cobbling in the future as the Pi bot gets built-out.

In addition to the level shifter for GPIO outputs, I intend to include a number of voltage dividers on the breakout board to protect the Pi GPIO inputs.

In looking at the value of R1 in the attached circuit I interpret the value of R1 as shown 4k7 to be 4.7 K ohms, on the other hand, I suppose it it could mean R1 can vary between 4k and 7k as the applied voltage varies between 5V and 30V.

I would appreciate it if someone would clarify the nomenclature 4k7 and any comments or advice you care to share on building the GPIO breakout board.

The above circuits, along with other Pi GPIO protection circuits, can be found at:
 < http://elinux.org/RPi_GPIO_Interface_Circuits >

Thank you for any thoughts and comments you may share.
30
Electronics / Controls Braking vs Dynamic Braking for Motors
« Last post by Tranq97 on October 15, 2014, 03:55:48 PM »
Okay, so for a very large brushless DC motor, I need to be able to stop and slow the motor at chosen rates. I've looked up a lot of different options:
Mechanical Braking wear would cause too much hassle.
'Crossing the Poles' wouldn't be powerful enough and I wouldn't be able to chose the deceleration rates.
Controls braking, where the H-Bridge is toggled to run back on the motor, giving an opposing force against the momentum for the motor.
And dynamic braking, mostly used for locomotives and gives the 'generator effect' a hard time by giving the motor a load of resistance.
These motors would go on the arms of the robot, not wheels.
I simply need to explanation as to which braking method I should use and why.
I found a lot of the information here: http://www.societyofrobots.com/schematics_dcmotorbraking.shtml.
Thank you in advance.
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