Basic question about steering

Perhaps I misunderstood, but typically you won't need H-Bridges if you are
using mechanical switches. Typically, to turn on the spot you make one motor
go forward and the other go backwards.  This can be done by using servos to
activate switches.  Others have used this approach but the mechanics become
more involved.  Basically, to turn, you'd need to operate two switches with
one servo, via mechanical linkage of some sort.  Or, you could hook up two
servos to the same channel and reverse the wiring to one of the switches.

If you have an RC setup that can be programmed to do differential steering
as in a tank, use a separate servo for each switch and let the RC do the
figuring.

DPDT switches can reverse the motor if wired as shown on page 349 of RBB 3rd
Edition

HTH
-Will

Thank you for the reply. . I have toggle switches in the properly wired
 configuration to use in this application (maybe I should have stated
that in my op). The problem is that, 1. eventually I want to interface
this to a microcontroller, so I would like to make that transition as
simple as possible, ie, I can use the data output pins from the uc,
through the already installed 1K resistors to the H bridge to power the
motors. 2. I tried using the 2 switches on one servo and it was
becoming a nightmare because every time one switch  would work, the
other wouldn't. I had to keep readjusting the position of one of the
toggle switches in order to make it work but then the other would not
work. I also rebuilt the linkages I don't even know how many times.  It
was just not reliable enough. This is an old futaba system and I
suspect the servos are not as " torquey" as they used to be as I have
used it for different things over the years.  So I decided to use the H
bridges. So I need to find a way to either cut off power to one of the
wheels to make my turn, or just apply power to one of the wheels and
not the other without causing any other problems with the system.

Maybe better put, I tried using the 2 switch idea for over a week with
no results except frustration. In one night, just an hour or so, I was
able to breadboard the H bridge and test it (It also gave me a chance
to play a little, checking the speeds of the motors one at a time for
now to be sure they both turned at the same rpm. They do, within 1 rpm
or so).
My RC system is not that sophisticated. Two channel, 1 for forward and
back, and one for left right. It cannot be programmed.

Joe

Hello Will,

Today, I wired the both motors to the one switch, one servo, the
forward, back, one with reverse polarity as you suggested, and it
works, like the H bridge did, but I still have the same dilemma. How do
I stop one using the other servo, and keep the other moving. OR how do
I start one and keep the other one stopped so that I can turn? I still
have one servo and one toggle switch DPDT left, but not sure how to
accomplish this. Can you maybe expound on your suggestion a little bit
more? The forward and back motion is not the problem, the turning is.

Joe


Joe wrote:

The other answer which I don't think anyone suggested is to just wire the
servos and switches tank-style so that one servo/channel controls the right
wheel and the other controls the left wheel.

Then the trick becomes clever use of your transmitter.  Is it one joystick
with both controls?  Or two sticks with one going up and down the other
left and right?

If it's a single stick, you can just rotate your transmitter 45 degrees
when you hold it and everything will work correctly (assuming you got the
switches wired correctly).  If you have two sticks, you have to logically
do the same thing in your mind and it will be a confusing, but with a
little practice you should be able to make it work.  And if this is just
temporary before switching to computer control, it will allow you to get it
working without buying more parts.

--
Curt Welch                                            http://CurtWelch.Com/
curt@kcwc.com                                        http://NewsReader.Com/

No, it's a 2 channel futaba, so there's 2 sticks. I figured I wanted to
use both of them as I have gotten used to using them in the past. The
forward/back is up/down, and the left/right is, well, left/right. It's
a first project and I wanted to see what I could do with a couple of
decent motors and a light frame. Thanks for the suggestions

Joe

Looks like you are approaching the problem in a purely digital (on/off
manner). When you put the radio controls forward, you engage power to
both drive motors. When you put the radio controls backwards you engage
power backwards to both drive motors. So far so good right?

Have you heard of a momentary switch which is normally closed? Well,
imagine splitting the two wires from your forward backwards switch
connection, have two of these switches which will be controlled by the
left-right servo. When the servo is left, it opens the momentary switch
going to the left motor, it stops, and the robot turns left. When the
servo is right, it opens the momentary switch going to the right motor,
it stops, and the robot turns right. When in the middle, the two
momentary switches are in the normal state (closed) and the robot goes
straight.

Joe, what you are pondering as a general principle is called servo
mixing. It's a pretty long way from the digital bang/bang method you
are approaching it on, and has to do with PWMing the H-bridges. You
might google on the subject. Also you might want to do some reading on
differential steering.

http://rossum.sourceforge.net/papers/DiffSteer/


--
Randy M. Dumse
www.newmicros.com
Caution: Objects in mirror are more confused than they appear.

No I never knew there were normally closed dpdt toggle switches. I can
check with some of my suppliers. I am not using the H bridge now, just
the switches. I looked at the waveforms tonite on the scope from the
receiver that drives the servos, but it was very difficult to observe.
I was able to measure the frequency and duty cycle as I moved the
levers on the transmitter though. The duty cycle changed, which is
expected for pwm and the frequency, of course stayed the same. I was
exploring how it may be possible to get rid of the switches and
possibly use the pwm signal from the RC receiver to drive the motors
thru the H bridge. I guess that's what you meant about PWMing the H
Bridges. I will google and see what I can find. Oh, and thanks for the
link. It was interesting reading. Thank you again for the info.

Joe

Normally closed switches come in many varieties. You only need SPST.
You just need one of these in series with each motor. When the servo is
left it should open the one in line to the left motor, and to the right
open the one in line to the right motor. It will stop the motor on the
side of the turn. Works both for going forward and going back.

Also consider you could do the same thing for forward and back, if you
had a double pole version.

Have a look at this:
http://www.bu.edu/eng/design/rules/switch.shtml

And you can even get them at Radio Shack:
http://www.radioshack.com/sm-pushbutton-normally-closed-momentary-switch4-pk--pi-2062540.html

or digikey or mouser for example.

Really what you need is to have a 3 pos switch. That way you can choose
fwd - off - back with the vertical stick. This needs to be DP3T. In the
middle position both circuits need to be open, so you can stop.

For the left right stick you need turn-left, straight, turn-right for
the side to side. In the middle position both circuits need to be
closed. That's rather a complicated switch that would be a DP3P. That's
why I suggested the two simpler switches.


The signals from the radios to the servos are a kind of PWM, but it is
used for signalling, and its purpose is to have a very short on period
(from 1 - 2 ms) and a long off period (from 19-18ms) so you can overlap
several as you serialize them and send them through the radio. In any
case, they don't go through in a format suitable for application to
motors.

So you will have to convert these RC Servo pulses to a different
frequency and periodicity suitable for driving DC motors.

With RC Cars, this is often done with a ESC (Electronic Speed
Controller) which is somewhat like an PWM-converter and an H-bridge put
together.

So you could use prebuild ESC's and Servo Mixers, which are somewhat
expensive and a dead end if you want to do non-radio control, or you
can get a micro and convert the incoming signal from RC Servo to PWM
signals and apply those to the motors, again, somewhat expensive and
rather a complicated process requiring you to know lots about micros.

--
Randy M. Dumse
www.newmicros.com
Caution: Objects in mirror are more confused than they appear.

http://www.radioshack.com/sm-pushbutton-normally-closed-momentary-switch4-pk--pi-2062540.html

Yes, I picked up a couple of NC (or NO) slide switches today at the rat
shack. I don't know if that's the right name for them, but Gordon has
them in his book for making "whiskers" for robots. It has a long lever
on it, but is spdt, so you can wire it as either NO or NC. Thank you
for that idea. I am now in the process of wiring this thing up. All I
wanted to do for my first project was to build an ROV, for starters,
just to learn how all these things actually work in the real world.
Sometimes, I make things more complicated then they really are. I have
a good education, but as many people have told me, and I laugh about
it, I just lack common sense. I would never have thought of using a nc
type momentary switch. The link you provided me with last night was
interesting. That's basic physics and some elementary calculus. I have
no problem reading articles like that. It is all in how I was trained.
When I was undergrad, I would stop the professor and ask him how
something like this could be applied in the real world, and he would
look at me like I had 3 heads and say something to the effect of
"that's what engineers are for" , or some such nonsense. Now I am
studying applied physics with my first formal electronics course this
coming semester. So maybe I will develop some common sense in the next
few years. But back to that article, what struck me the most about it
was that he explained it so clearly and concisely that someone who
didn't understand the math, or even the basic physics behind it could
still understand it, and just use the result. Anyway, I will be on this
forum for awhile. I hope folks don't mind a really basic question once
in awhile. I hope I may be able to contribute something useful once in
awhile. So thanks again, and I will hopefully have this up and running
by New Year's Day.

Joe

The PWM from an RC receiver is different from th PWM used to drive a
motor.

For the PWM from an RC receiver, the duty cycle isn't so important, but
the width of the pulse is. It's between 1 msec and 2 msec which repeats
about every 15 msec to 30 msec.

For driving an H-Bridge, the PWM duty cycle is used to determine the
speed of the motor. So a 75% duty cycle runs the motor at 3/4 speed.

So they're both PWM, but the PWM is totally different. You can use
something like an ESC (Electronic Speed Control) to convert RC stylw
PWM pulses and directly drive a motor.

Dave Hylands

Ideally you would want your steering and speed to be proportional to
your stick input. This will give a better control of your "robot". i.e
true differential steering.
Obviously there are a number ways to do this using non-microcontroller
circuits, a 555 timer comes to mind. There are plenty of circuits for
PWM if you do a Google search.

If you want to get something up and running quickly and with minimum
problems, then you might want to take a look at the Oopic
Micro-Controller. It has predefined objects which are designed to do
what you require. The objects are oNavCon for steering left/right and
and oDcMotor which provide forward/reverse and the PWM to your H-Bridge.

With both the above suggestions you would link your servo to a
potentiometer to give a smooth control as opposed to the bang-bang
control you will achieve using switches.

Ian

I have heard of the oopic. As a first project, I wanted to see what I
could do with a couple of decent motors and a light frame. It's more of
a ROV. I need to look into different microcontrollers because I plan on
eventually putting one in either this vehicle or my next one. Not sure
yet. I have also been looking at the basic stamp because I know the
basic language, I have an extra serial port on my PC, and it seems to
have some advantages over some of the other uc's, like built in voltage
regulator, a whole lot of info out on the web. The only drawback is the
price. They're about $50 a pop, so I am still contemplating.

I actually have a schematic of a 555 being used as a PWM. I am just
learning, so, for this one, at least for now, bang bang will do.

Thank you for the info

Joe

I've seen this done with an "8-way" joystick, which is just a 4-way (X, Y)
with a hole where you can go inside the quadrants, i.e., you don't only
get (+X,0) (-X,0) (0,+Y) (0,-Y)
you get (+X,+Y) (+X,0) (+X,-Y) (0,+Y) (0,0) (0,-Y) (-X,+Y) (-X,0) (-X,-Y)

turned 45 degrees, i.e., X is upper right to lower left, and Y is upper
left to lower right. Then you just control one motor with X, the other
with Y, and when you push the stick straight forware, X and Y increase the
same; when you push the stick sideways, the motors are driven
differentially.

Good Luck!
Rich