PDA

View Full Version : [Question(s)] simple roller

stothepizzle
04-22-2008, 03:50 PM
I'm looking to build a simple roller with some very simple specifications, i hammered out some basic details with somone in irc but I'm here to get some specific components.
Specification
1: Powers four motors by usb
2: can shift up to two kilograms of weight at walking speed
the motors are likely to be arranged in a diamond with each one facing outwards (ie top and bottom motors for forward and backward, left and right motors for moving to the left and right).
I've built some mechanum wheels in order to help achieve this.

Basic component list:
1X servo board with four connectors
4X bane bots dc motor controllers (connected to servo board)
4X motors (probably ghm-02)

04-22-2008, 04:50 PM
Hi, welcome to the forum!

You're not going to get enough power from USB to drive four motors. The power is going to need to be external. The USB specification limits the current to 500mA, and an inrush of 100mA typical. That's 0.5A and 0.1A respectively.

A typical motor to do what you're looking for is probably going to be looking for 9+ A with an inrush current near 30A. Or 20-300 times what you can get from a USB port respectively.

Now multiply that by 4 and we see that you're looking for up to 1200 TIMES what USB can provide.

Further limiting is that you can't get more than 5v from a USB port. You're going to be looking at 12v-24v motors in the power range we're talking about. Now you're multiplying the current requirement in a DC-DC conversion by a good 5 fold. So now you need at least 6000x what USB can offer.

It's just going to HAVE to be done with independant power.

I suspect your motor choice is also not going to do the job for you. You need to figure-out the drive wheel size before we can calculate "walking speed". 5mph is 2.25m/s, approximately. The motor you're looking will stall when it lifts 4KG for one inch, approximately. It is only turning at 120rpm, or about 2revolutions/sec UNDER NO LOAD. So figure half that, about 1 revolution/sec. So you would need to have a wheel big enough to shift your track 2m for every revolution... That's putting a lot of load on the motor as that's a very large drive wheel.

If you give us all some more specification of your desired application (wheel size, track length, loss in the system), we can all help you crunch some more numbers and figure out what you're actually looking at.

stothepizzle
04-23-2008, 06:51 AM
ok, so i've got a 12 volt battery which i'm probably going to use to power the motors, the wheels have a diameter of about 2.5 inches hope that helps

stothepizzle
04-23-2008, 02:00 PM
http://img187.imageshack.us/img187/63/planex4.jpg

basic plan view (robot is inside clear plastic sphere mechanum wheels face outwards

LinuxGuy
04-23-2008, 04:45 PM
I'm looking to build a simple roller with some very simple specifications, i hammered out some basic details with somone in irc but I'm here to get some specific components.
That was me. We met on the #robotics channel at irc.freenode.net the other day. :happy: I'm glad to took my suggestion and signed up over here. You won't be sorry! :veryhappy:

8-Dale