Hi and thanks for the advice. The motors have the following specs. Based on
these, would I need an H-bridge rated at 12 A or 65A+ ?
Since I would probably only have about 50lbs total weight on the bot, I
wonder if I could away with running it on 12vdc ?
a.. Peak Rated Power Consumption 560W
b.. Rated Voltage DC24 V
c.. Rated Current 11.5 A
d.. Rated Torque 16.5 Nm
e.. Rated Speed 166 RPM
f.. Efficiency at Rated Speed & Torque 52%
g.. Peak Current 65 A
h.. Peak Torque (at 70A) 55.4 Nm
i.. Gearbox Ratio 1/28
j.. Minimum Brake Torque 2 Nm
k.. Maximum Brake Release Current 0.4 A
l.. Mass 6.9 kg
m.. IP Rating IP50
n.. Noise 65 db(A)
I have a pair of wheelchair motors on my bot. They are easy to mount up,
have lots of torque and should work well. I have not finished the motor
controller that I am building, so it is not fully operating yet. I did
power it up on 12 volts and it was pretty slow, but everthing worked.
The wheelchair motors I have came with an electro-mechanical brake on
the back of the motor that you need to energise to be able to move. I
took them off of my motors and it proved to be an excellent place to
mount the quadrature encoders that I am using to close the loop on the
motors. The brake covers protect the encoders nicely.
My motors came off a chair that had been in a car accident, so I had the
original wheels. The wheels were a direct drive arrangement and they
were 13" in diameter. My bot has a 1:1 ratio chain drive down to some
13" wheels that I got from the Surplus Center (Good People), so I think
that it should have the torque and speed that I want. The powered chairs
can zip around pretty well, and seem to deal with a couple hundred
pounds of person and probably 150 pounds of wheelchair and batteries.
Your Drive H Bridge must survive the 70A and in my opinion should
survive 100A for short periods. It only needs to survive that load for
brief amounts of time while starting up or stalled, and does not need to
support that for normal operation. If you are not up to building a speed
control, there is a company called Roboteq that sells a dual channel
controller suited for this for something around $400 or $500 US.
The motor data listed 6.7kg per motor or about 15 pounds each. Two
motors, wheels suitable for that kind of torque and a frame large and
strong enough for the motors will not leave much out of your 50 lb
budget for batteries. You probably could run it on 12 volts, but your
top speed would be cut by 1/2 and your availible power will only be 1/4
of the 24V capability. If you are going to be operating on hard surfaces
and can keep the weight down, you might be able to use a 1.5:1 or 2:1
chain or gear drive (or larger wheel) with 12 volts, but I would design
for the 24V.
Good Luck, I think they are great motors.
Hey thanks for all of the great and well thought out advice! Wow - those
"store bought" motor controllers are expensive !!!
I am more than capable of building the controllers but these days I am
predisposed to a more modular approach to things, although budget
constraints seem to be pushing back in the old direction. :-)
50 lbs is not really a hard and fast budget, but rather a guestimation based
on nothing much more than gut feeling.
That is very valuable info on removing the brakes and mounting encoders !
Thanks again !
I'm glad to help.
The purchase cost on motor controllers is pretty high, but by the time
you have the code written and debugged for the controller and the boards
laid out, debugged and sorted out, they are a bargain! At $500, by the
time PWB's are done and the cost of the processor development tools, it
would leave about fifty cents an hour for your time.
Try Googling on "Open Source Motor Controller", these guys have done a
pretty decent power board and I think that they have firmware for
downloading for a couple of processors and such. The Yahoo group for it
has some very sharp motor control people that frequent it. I forget the
prices, but I think the bare power board can be had for under $50 US.
I like them, but they aren't strong enough for a really
heavy robot (40+ lb).
Groucho runs nicely, but it's only 40 pounds. The wheels have
a bit of slop in them that I don't like, but the motors are
D. Jay Newman
The limitation seems to be due to the motors. They are
fairly powerful motors for a 30 lb robot.
There is also a limitation due to the use of PVC for
the decks. Heavy batteries on the lower level cause a
bit of bending.
However, I'm pushing the limits with Groucho. The motors
work with a robot much heavier than the design specs
D. Jay Newman
The ABS base really is a limitation, although it's fine for decks. You can
brace the ABS base with a bit of aluminum channel stock, or you can simply
cut a plywood base and use the ABS stuff for decks.
If you don't want the plywood to look like plywood, polyurethane, sanding
and metallic paint goes a long way. I have gone both routes -- I kind of
like the plywood base, since I can also cut out wheel wells, and lower the
robot's profile a bit.
(Replies: cleanse my address of the Mark of the Beast!)
Well. I broke down and purchased 2 of the REXC Drive Unit w/ Optical Encoder
from Zagros, so the journey begins in earnest! This newsgroup and this
thread, especially, have been very helpful in me making my decision, so
thanks again to everyone !
I bought the max99 from the zagro site, and want to add three polaroid
sonars to it since I bought the interface from zagro as well (which lets
you connect three polaroids to them).
Has anyone experimented with the Max99 using the HC11, and the sonar
interface from Zagro?
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