555 Timer triggered fropm a differentiator.

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MrAl
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Post by MrAl » Sat Jun 28, 2008 10:14 am

Hi Frank,


Ok, all the values check out good, so it must be something more
basic like a bad connection, bad socket, bad pot, or noise on the power
supply line. Did you try adding another 0.1uf bypass cap
across the second 555 chip yet?
LEDs vs Bulbs, LEDs are winning.

Rodney
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Post by Rodney » Tue Jul 01, 2008 6:56 am

Frank_fg, in looking over your circuit, I am surprised to not see a jumper or connection between pins 6 and 7 on the last LM555. There should be one even if it doesn't show up on the schematic as I do not think the circuit will work without that connection.

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MrAl
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Post by MrAl » Tue Jul 01, 2008 7:36 am

Hi Rodney,

Good catch! (he he)

If this is true, it will never work. Pin 7 *must* be connected to
pin 6 in the monostable mode or else i think you only get a short
pulse out of the second 555 which will more or less follow the input
pulse width.

Hope this is it, because everything else looks just right and well
calculated out.
LEDs vs Bulbs, LEDs are winning.

frank_fg
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Post by frank_fg » Tue Jul 01, 2008 4:15 pm

:smile: Hi! Al


Good show, as they say, must have stared at the schematic several times
and not notice that. By the way pin 6 & 7 are jumpered under the pc board.

Al being we are back to this circuit, I'm trying to get the servo to run more
smoother and it won't cooperate with me.

I thought maybe I could reduce the gear ratio if I can find one, or I
could try to program some code into a PIC such as 16F84A, I'm not really a programmer but I try, and I have programed several chips from other sources and a few of my own, not much though. Do you know of some good code for servo motors?

Thanks

Frank G.

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MrAl
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Post by MrAl » Wed Jul 02, 2008 5:28 am

Hi again Frank,


Oh ok, well that's good to hear.

What is the servo doing now that you dont want it to do?
When you say 'smoother' what exactly do you mean? Is the
motion jerkie or does it turn funny, maybe you could try
to describe your experience a little more.

What is your gear ratio now?

You may wish to use a PIC or something like that?
From what i can see so far (i havent worked directly with
servos since the mid 1970's, more than 30 years ago)
your circuit requires that you generate a pulse of
predetermined width which must be constant unless you
decide to change it, and that the frequency may be
varied from 50 Hz to 250 Hz, right?
Since it's been so long since i worked with these,
perhaps you can tell me briefly how the servo reacts
to your pulses, ideally, as to what the shaft does
(rotate etc) when you change the pulse width output.
I assume the rotation speed changes when you change
the frequency?

The last time i used a servo was around 1975, and that was
to raise and lower the round, clear plastic bubble cover
on a precision weigh scale. The user would press a foot
switch and the scale would 'tare' out, then the cover would
raise up (servo), then they would place the body organ
or part on the scale platform, then release the
foot switch and the bubble top would close, the scale
would then measure the weight and display on a 7 segment
display. The cover was necessary because small drafts
in the laboratory would cause inaccurate readings on
the weigh scale, so the cover prevented the air drafts
from bothering the scale platform.

As far as using a microcontroller, you would have to try
to determine the required resolution for the application
you are working on and see if you could get this from
the uC with the frequency of operation. Sometimes the
resolution isnt that good, but with the max frequency
being 250Hz, i would think this is possible. If i rem right,
you said pulse width of 800us to 2.5ms, and even 800us
is 800 instructions at 4MHz (a typical PIC system clock).
That's a lot of time to do the pulse i think. The resolution
(i think without doing any calc's or code) would be 1us
i guess, so that's 0.800ms for one pulse, then the next
width available would be 0.801ms, under control of
some external signal (probably digital in nature).
Perhaps an interrupt to change pulse width/frequency.
I dont have any code on hand for this, but i bet there
is much on the web as this subject must have come up
many times already.
LEDs vs Bulbs, LEDs are winning.

frank_fg
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Post by frank_fg » Wed Jul 02, 2008 8:20 am

:smile: Hi! Al


I'v heard so much about servo's, I thought I would do some experiments and it was great, However the servo just went +/- 90 degrees.

I'm using it to steer a robot, this robot will carry an arm that I have which will pick up wooden blocks and who knows maybe bring my coffee.

At 50 Hz it works fast and will sometimes jitter, and at other times after it has stopped it will keep on crepping in the direction it was going.

At 250 Hz it works better, but also fast. I want it to turn say +/- 45 degrees
but at a rate that is resonable. Think of it, as a truck pulling a load and you need to turn carfully.

I have done some experiments with the servo to stop at various degrees but it has always done it fast, almost like a stepper motor. Now dont tell me to use a stepper motor, then again it is a thought.

As for using the scope, I only use it to check the timing or a problem if any.

It seems to me, that at 1.0 ms the servo will stop, there should be a time period were one could slow it down.

I'm still in the learning stage open to any suggestions.

I don't know the gear ratio, but I'll be sending an e-mail to Futaba

Thanks
Frank G.

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MrAl
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Post by MrAl » Wed Jul 02, 2008 10:46 am

Hi again Frank,


If i were you i would first check the pulse width to make sure the
circuit is working properly, even though the design seems reasonable.

Once done, you might try looking up the specs for the motor you
are dealing with. It could be that the thing does not take the
kind of commands you think it should. I have read a little now
on the web about various servo motors and have found that
they may differ greatly as to what pulse width does what.
Interesting, everything i found was talking about a running
frequency of 50 Hz and no higher.

As a last thought, the command line to the servo may have to be
AC coupled ie through a capacitor of reasonable size.
LEDs vs Bulbs, LEDs are winning.

Rodney
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Post by Rodney » Thu Jul 03, 2008 9:23 am

I use these RC servos all the time. All popular current designs (Futaba, Airtronics, JR, Tower etc.) all use 1.5 millisecond pulse as center with .5 milliseconds as one end and 3 milliseconds at the other end. Rep rate can vary from 15 to 50 milliseconds. The output to the servo is a DC positive going pulse with a peak value of 2.5 to 4.8 volts when using a 4.8 volt (4cell power supply). Most servos will work with a value of 4.2 to 7 volts with no problems. All have a rather limited range (+-30 degrees) except for retract servos which will have a 180 degree (usually not proportionsl) throw. Power varies greatly between brands and size, anywhere from 8-10 inch/oz to 120 in/oz or more.

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MrAl
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Post by MrAl » Thu Jul 03, 2008 9:58 am

Hello Rodney,


Thanks much for your input here. I am a little interested in
servos now too (he he) but have not used one in over 30 years
so i dont rem too much about them except the ones i used had
3 wires and i think they were power, ground, and signal.
From what you say the pulse width is centered around 1.5ms
and the frequency is around 20 to 67 Hz right?

I have seen some on the web for as little as 10 dollars
for one that controls over a pound (at one inch) which
isnt too bad. What i also saw that was a little confusing
however was that they also boast "compatible with all types
of radio control" which doesnt make sense if they only take
a pulse input (plus power). Any idea what they mean by
this? The radio receiver isnt built into it is it? I assumed
that would be more expensive then 10 dollars.

A note for others here too...
I have also read now that there are certain types of servos
that have 'smoother' operation than others, and these seem
to be called "coreless".
LEDs vs Bulbs, LEDs are winning.

Rodney
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Post by Rodney » Fri Jul 04, 2008 6:08 am

Mr. Al, what they mean when they say compatable to all radio types is that the signals from the receiver to the servos is pretty much standard. There are some variation is connectors but, again, most are interchangeable. On the Futaba's, you may have to shave off the little key tab to enable them to plug into other brands. As to types of servos, there are the older design styles using a conventional brushed motor, Coreless and the newest are the digital. They still all require the same signal from the reciever though (1.5 +-1.0 millisecond) positive going pulse every 20 milliseconds or so. The digital versions have additional circuitry internally that keep a higher holding power (accompanied by a higher current drain) between the pulses. The coreless motor ones are usually slightly faster in response but not as good on holding power as the digital ones are.

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MrAl
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Post by MrAl » Fri Jul 04, 2008 12:39 pm

Hi again,

Ok, that helps to explain a bit more then. Thanks.
LEDs vs Bulbs, LEDs are winning.

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Post by frank_fg » Mon Jul 07, 2008 12:16 pm

:smile:

Hi! Al

Well I got my project working up to the servo steering phase, would you believe it, it was the servo that was bad. The trailing edge measured 1.2 ms the stop position, and the +/- 90 degrees measured 1.22 ms which was totally not right. It was supper sensitive to the touch, so I replaced it with another servo I found and it worked out better, not what I would really like, but it will do for now. The circuit has a positive going glitch at the trailing edge, which I’ll get latter.

The servo that is bad is a Futaba S3003 Standard 4.8 to 6.0 V, 44 to 57 oz-in Torque, 0.23 to 0.19 sec/60 degrees speed. Do you know if the servo has any circuitry inside?

By the way what can you tell me about Transmitters and receivers in respect to servos?
They are so many that it gets confusing.


Thanks

Frank G.

P.S. Can you translate 0.23 sec/60 degrees for me.

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MrAl
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Post by MrAl » Tue Jul 08, 2008 2:41 am

Hi again Frank,

Oh ok well glad you got to the main problem and got it fixed
pretty much.
The servo is going to have a circuit inside yes, but i dont
have any particular circuit to look at in front of me.
Perhaps on the web we can find one?
I dont have a lot of experience with servos as i havent worked
with them since mid 70's, and then they were the kind that
are driven with the three wires, no receiver.
If you want to use stepper motors instead i would be able to
help more there. You might look into these too because they
allow electronic control of mechanical systems that are more
continuous than with servos as these motors rotate 360 degrees
and beyond (any number of revolutions with fairly precise
control).
LEDs vs Bulbs, LEDs are winning.

frank_fg
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Post by frank_fg » Wed Jul 09, 2008 9:45 am

:smile:

Hi! Al

I took the servo apart and yes it has circuitry inside. I like to do my own trouble shooting but in this case I might just send it out to get fix. But before I do I’ll see if I can get a schematic for it. It’s all SMT circuitry now days and that would give me some problems, both in vision and dexterity.

Guess what it cost $30.00 plus parts and shipping to get the servo fix, $15.00 minimum. They do not sell any schematics.

If it’s alright with you, every now and then I’ll sneak a question or two on stepper motors.

Let me ask you this, are you familiar with the microcontroller chips from Microchip say the PIC16F84A or PIC16F88, mind you as I said before I’m not a programmer, But I took a course on it, actually two or three of them, but far from being real good at it. Let’s say I’m still learning.

Here is the scoop! I have a library of programs that I would like to implement but before I bore you with that I need to know if you’re interested ok.

To give you an idea of what I’m trying to do I’ll say this, I have two programs that allow me to run via remote control a D.C. motor (s) and stepper motor (s), and that’s good. However I would like to modify them to suit my purpose, and that’s were the problem starts. I’ll try to explain.

As you know programming is very detailed and you could get lost real quick.

All the commands are via keypad, that go to a master module that in turn sends commands to the slave module via a USART . PIC to PIC commuications. I’ll stop here for now.


Thanks,

Frank G.

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Post by frank_fg » Wed Jul 09, 2008 1:31 pm

:smile:

Hi! Al

Its me again, I was wondering if it would be better to post my programming questions on the programming section next door?

Let me know ok.


Thanks

Frank G.

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