Using a pot and rctime
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I have project that involves the use of a 12V motor and a pot to return
positional data. The pot is a ten turn one and is mechanically coupled to
the output gear of the motor. I have found a nice ready built dual
H-Bridge and I will be getting the data back via Rctime.
First question:- Which combination of values of pot and Capacitor would give
me the best resolution over the range (the full 16 bit's would be the
perfect) BUT still keep the time as low as possible, because in one part of
the program I would have to poll three Pots and a light dependent diode (or
fourth pot) and then move the motors in real time. In another part of the
program I would have to measure the length of a frame pulse do a load of
maths then move the motor.
Second question:- How is such a setup affected by temperature? Most
studios are not heated and then temperature at 8am can in the winter be Zero
which then rises to 35-40 degrees (Celcius)by the afternoon. If we record a
setup first thing and then repeat it all day am I going to see a difference
?
Justin Pentecost
Focus Puller and Stamp Idiot
positional data. The pot is a ten turn one and is mechanically coupled to
the output gear of the motor. I have found a nice ready built dual
H-Bridge and I will be getting the data back via Rctime.
First question:- Which combination of values of pot and Capacitor would give
me the best resolution over the range (the full 16 bit's would be the
perfect) BUT still keep the time as low as possible, because in one part of
the program I would have to poll three Pots and a light dependent diode (or
fourth pot) and then move the motors in real time. In another part of the
program I would have to measure the length of a frame pulse do a load of
maths then move the motor.
Second question:- How is such a setup affected by temperature? Most
studios are not heated and then temperature at 8am can in the winter be Zero
which then rises to 35-40 degrees (Celcius)by the afternoon. If we record a
setup first thing and then repeat it all day am I going to see a difference
?
Justin Pentecost
Focus Puller and Stamp Idiot
Comments
>positional data. The pot is a ten turn one and is mechanically coupled to
>the output gear of the motor. I have found a nice ready built dual
>H-Bridge and I will be getting the data back via Rctime.
>
>First question:- Which combination of values of pot and Capacitor would give
>me the best resolution over the range (the full 16 bit's would be the
>perfect) BUT still keep the time as low as possible, because in one part of
>the program I would have to poll three Pots and a light dependent diode (or
>fourth pot) and then move the motors in real time. In another part of the
>program I would have to measure the length of a frame pulse do a load of
>maths then move the motor.
If RCtime returns maximum (65535) when the resistance of the pot is
maximum, that is also the most time, about 0.13 second. But due to
noise the RCtime command is really good for only about 10 bits of
resolution. So you wouldn't lose much in the way of accuracy by
shortening the time to, say, 0.02 seconds and a maximum count of
10000. Put a resistor in series with the pot if necessary to keep
the minimum RCtime above ~50 microseconds. What is the max
resistance value of your 10-turn pot? How long are the leads from it
to the Stamp.
>
>Second question:- How is such a setup affected by temperature? Most
>studios are not heated and then temperature at 8am can in the winter be Zero
>which then rises to 35-40 degrees (Celcius)by the afternoon. If we record a
>setup first thing and then repeat it all day am I going to see a difference
>?
Here is detailed information on temperature dependence etc.
http://www.emesystems.com/BS2rct.htm
>Justin Pentecost
>
>Focus Puller and Stamp Idiot
-- Tracy
>maximum, that is also the most time, about 0.13 second. But due to
>noise the RCtime command is really good for only about 10 bits of
>resolution. So you wouldn't lose much in the way of accuracy by
>shortening the time to, say, 0.02 seconds and a maximum count of
>10000. Put a resistor in series with the pot if necessary to keep
>the minimum RCtime above ~50 microseconds. What is the max
>resistance value of your 10-turn pot? How long are the leads from it
>to the Stamp.
The MAX resistance is up to me [noparse]:)[/noparse] What would you suggest ?
Justin
Focus puller
resolution that way.
--- In basicstamps@y..., "Justin Pentecost" <justin@k...> wrote:
> I have project that involves the use of a 12V motor and a pot to
return
> positional data. The pot is a ten turn one and is mechanically
coupled to
> the output gear of the motor. I have found a nice ready built
dual
> H-Bridge and I will be getting the data back via Rctime.
>
> First question:- Which combination of values of pot and Capacitor
would give
> me the best resolution over the range (the full 16 bit's would be
the
> perfect) BUT still keep the time as low as possible, because in one
part of
> the program I would have to poll three Pots and a light dependent
diode (or
> fourth pot) and then move the motors in real time. In another part
of the
> program I would have to measure the length of a frame pulse do a
load of
> maths then move the motor.
>
> Second question:- How is such a setup affected by temperature?
Most
> studios are not heated and then temperature at 8am can in the winter
be Zero
> which then rises to 35-40 degrees (Celcius)by the afternoon. If we
record a
> setup first thing and then repeat it all day am I going to see a
difference
> ?
>
> Justin Pentecost
>
> Focus Puller and Stamp Idiot
>resolution that way.
I thought about but the problem is my obsession with making one unit do so
many things. An unfortunately I don't think a stamp can read from two
encoders at one time and in fact this will probably be four encoders but the
advice to keep it down to 10000 was very good and makes allot sense to me.
Of course the other good thing about pots is that even if the unit is off
they know if motor (in this on an iris ring) has moved.
Justin Pentecost
> >maximum, that is also the most time, about 0.13 second. But due to
> >noise the RCtime command is really good for only about 10 bits of
> >resolution. So you wouldn't lose much in the way of accuracy by
> >shortening the time to, say, 0.02 seconds and a maximum count of
> >10000. Put a resistor in series with the pot if necessary to keep
> >the minimum RCtime above ~50 microseconds. What is the max
> >resistance value of your 10-turn pot? How long are the leads from it
> >to the Stamp.
>
>The MAX resistance is up to me [noparse]:)[/noparse] What would you suggest ?
>
>Justin
>
>Focus puller
Hi Justin,
The equation is,
t = 0.30111*R*C
t: time in microseconds
R: resistance in ohms
C: capacitance in microfarads
Okay, make it come out at about 20000 microseconds maximum, 0.02
second. That will be 10000 counts from the RCtime command. Choose
0.68uF to start. For best stability the capacitor should be a film
type. From there the equation comes up with a 100kohm pot, which is
readily available in 10-turn. Put a 1000 ohm resistor in series to
set a minimum value, a time of about 200 microseconds.
Vdd 5V
|
===== 0.68 film
|
|
Vdd -o--/\/\---/\/\---o--/\/\-- P0
`---^ 1000 220
100k
pot
tm var word
ohms var word
high 0
loop:
RCtime 0,1,tm
high 0
ohms=tm*10 ' approximately
debug ? ohms
goto loop
-- regards,
Tracy Allen
electronically monitored ecosystems
mailto:tracy@e...
http://www.emesystems.com
Oops, the Vss was incorrectly marked. Here is the corrected
schematic with the pot connected to Vss. The leads from the stamp
out to the pot should be as short as possible, or shielded if noise
pickup is a problem.
Vdd 5V
|
===== 0.68 film
|
|
Vss -o--/\/\---/\/\---o--/\/\-- P0
^ `---^ 1000 220
100k
pot