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Old 01-28-2012, 12:36 PM   #1
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Default Servo pot

If I solder the 3 wires on a servo pot together, will that make the motor stop spinning? Will the servo board still be usable?
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Old 01-28-2012, 03:51 PM   #2
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Default Re: Servo pot

Pretty sure it wouldn't make the motor stop spinning it would just cause a short on the board.
You'd be better off picking up a pair of resistors and soldering them in place of the pot. That should stop the motor spinning, but it still allows the board to be used as a mosfet switch/winch control.
The resistor value will vary depending what value pot is in the servo. But each resistor needs to be half the value of the pot for it to work properly.
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Old 01-28-2012, 04:27 PM   #3
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Default Re: Servo pot

The resistor values aren't really critical. Two 4K7 ohm resistors would be great. If not, I wouldn't go lower than 500 ohms or higher than 10K ohms. If you still have problems with winch creep, look me up!
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Old 01-28-2012, 09:48 PM   #4
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Default Re: Servo pot

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Originally Posted by heyok View Post
The resistor values aren't really critical. Two 4K7 ohm resistors would be great. If not, I wouldn't go lower than 500 ohms or higher than 10K ohms. If you still have problems with winch creep, look me up!
x2...
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Old 01-28-2012, 11:58 PM   #5
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Default Re: Servo pot

Learn something new everyday....I've always used the value of the pot divided by 2 to get my resistor value cause that's what i was told by a buddy who is a elec engineer, now i know i don't it makes it a bit simpler. Cheers Heyok.

Make me wonder where he got the info from though....
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Old 01-29-2012, 12:28 AM   #6
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Default Re: Servo pot

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Learn something new everyday....I've always used the value of the pot divided by 2 to get my resistor value cause that's what i was told by a buddy who is a elec engineer, now i know i don't it makes it a bit simpler. Cheers Heyok.

Make me wonder where he got the info from though....
well, actually with an analog servo you may have todo it that way because amount of resistance is important in analog stuff (i'm unsure whats analog in a servo though). So that may be why your buddy said that

But the servo's i've looked at they have a/d converters which is voltage based and the way pots work they just increase/decrease the voltage by changeing resistance values between the outer and inner lead (a pot is basically two variable resistors)...
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Old 01-29-2012, 06:55 AM   #7
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Default Re: Servo pot

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well, actually with an analog servo you may have to do it that way because amount of resistance is important in analog stuff (I'm unsure whats analog in a servo though). So that may be why your buddy said that

But the servo's I've looked at they have a/d converters which is voltage based and the way pots work they just increase/decrease the voltage by changing resistance values between the outer and inner lead (a pot is basically two variable resistors)...
I believe the "analog" part is that a varying voltage is what is being read by the controller vs. a pulse that comes from the A/D board.

You are correct, both have a pot for position, it's what happens with the pot voltage in terms of controlling the servo.

Hope that makes sense. If not, HeyOK or others may add explanation.
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Old 01-29-2012, 08:08 AM   #8
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Default Re: Servo pot

The pot forms what is called a Voltage Divider. One end of the pot is connect to ground, the other is connected to the positive voltage that is running your servo (often 6 volts). As the wiper of the pot is slid up and down, it basically takes a tap of the voltage. The closer it is to the positive end, the higher the voltage it will have on it. This voltage, which is an indicator of the rotational position of the servo output, is used by the servo circuit to feed a comparator. This comparator has two inputs. The other comparator input is fed from the servo signal that comes from the receiver. If the two inputs of the comparator don't equal zero, an error voltage is output from the comparator. This error voltage is amplified and fed to the servo motor which will then spin in the direction needed to try and make the error voltage become zero - this is the feedback loop.
Losikid indicated that some servos use and A/D converter to sample the signal from that pot. In that case, the error condition is determined using digital methods (binary numbers are subtracted from each other). Either way, that is the basic operation of a servo.

When we modify a servo for continuous rotation to make a winch or whatever, we are relying on the components to remain stable enough when we tweak them by hand. We decouple the servo pot from the servo output and modify the output gear so that there is no mechanical stop to limit it from turning all the way around. By leaving the pot adjusted to the center and probably tweaking the transmitter trim on the channel that is feeding it, we are able to make the feedback loop happy and output zero error voltage which stops the servo from rotating. Then as the rig gets used, parts warm up a bit or get jostled around. After a while the servo starts to creep as the error voltage starts to develop. Now, if you are lucky, you notice this happening before your winch line unspools and gets wrapped up in your wheels or, maybe you are unlucky and the winch was pulling in and rips itself apart.

People often feel that the pot has drifted around from the rig's bouncing and caused the winch to creep. Often a bit of glue is used on the pot shaft to prevent this - I believe that does stop the pot from moving - yet the winch still creeps. This creep happens, in my opinion, because of the very high gain of the comparator circuit sensing very small changes either its own circuitry or the transmitter's output, often due to heat making component values change slightly. The comparator's job is to notice any of the slightest change and then slam the power to the servo motor to bring that error back to zero. This why you always have to fight with your trim controls to keep the creep under control.

All this essay is meant to pass along is that I don't really think replacing the pot with a voltage divider made from discreet resistors will help much with creep. The pot is only one of a few factors that generates the creep.

Thanks for reading my book!
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Old 01-29-2012, 08:18 AM   #9
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Default Re: Servo pot

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The pot forms what is called a Voltage Divider. One end of the pot is connect to ground, the other is connected to the positive voltage that is running your servo (often 6 volts). As the wiper of the pot is slid up and down, it basically takes a tap of the voltage. The closer it is to the positive end, the higher the voltage it will have on it. This voltage, which is an indicator of the rotational position of the servo output, is used by the servo circuit to feed a comparator. This comparator has two inputs. The other comparator input is fed from the servo signal that comes from the receiver. If the two inputs of the comparator don't equal zero, an error voltage is output from the comparator. This error voltage is amplified and fed to the servo motor which will then spin in the direction needed to try and make the error voltage become zero - this is the feedback loop.
Losikid indicated that some servos use and A/D converter to sample the signal from that pot. In that case, the error condition is determined using digital methods (binary numbers are subtracted from each other). Either way, that is the basic operation of a servo.

When we modify a servo for continuous rotation to make a winch or whatever, we are relying on the components to remain stable enough when we tweak them by hand. We decouple the servo pot from the servo output and modify the output gear so that there is no mechanical stop to limit it from turning all the way around. By leaving the pot adjusted to the center and probably tweaking the transmitter trim on the channel that is feeding it, we are able to make the feedback loop happy and output zero error voltage which stops the servo from rotating. Then as the rig gets used, parts warm up a bit or get jostled around. After a while the servo starts to creep as the error voltage starts to develop. Now, if you are lucky, you notice this happening before your winch line unspools and gets wrapped up in your wheels or, maybe you are unlucky and the winch was pulling in and rips itself apart.

People often feel that the pot has drifted around from the rig's bouncing and caused the winch to creep. Often a bit of glue is used on the pot shaft to prevent this - I believe that does stop the pot from moving - yet the winch still creeps. This creep happens, in my opinion, because of the very high gain of the comparator circuit sensing very small changes either its own circuitry or the transmitter's output, often due to heat making component values change slightly. The comparator's job is to notice any of the slightest change and then slam the power to the servo motor to bring that error back to zero. This why you always have to fight with your trim controls to keep the creep under control.

All this essay is meant to pass along is that I don't really think replacing the pot with a voltage divider made from discreet resistors will help much with creep. The pot is only one of a few factors that generates the creep.

Thanks for reading my book!
Wow, I never thought that much stuff was going on when I turned my steering wheel and used the winch. I have heard nothing but good things about your(heyok) fix to make a servo a winch.
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Old 01-29-2012, 08:26 AM   #10
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Default Re: Servo pot

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Wow, I never thought that much stuff was going on when I turned my steering wheel and used the winch. I have heard nothing but good things about your(heyok) fix to make a servo a winch.
Thank you
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Old 01-29-2012, 08:29 AM   #11
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Default Re: Servo pot

Heyok, excellent post, I learned a bit from it.

I agree, temp change alters the resistance of parts, thus changing the signal and the beginning of "drift". I see the same changes in the equipment I use at work, but it does not try as hard to "stay in place" as a servo controller does.

Question, do you think the change to a higher wattage (5 watt if it would fit) higher tolerance part (like a 1% or tighter) would change the circuit and make it more stable?

I guess taking a warm servo and dunking it in water would drastically change the temp, thus aggravate the resistance change. This may be why people are chasing "0" all the time with a more basic controller.
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Old 01-29-2012, 08:55 AM   #12
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Default Re: Servo pot

Thank you, Charlie-III

I'm not sure if a tighter tolerance resistor would help. Probably worth a try, but I think it is only responsible for maybe 1/3 of the causes of the drift.
You still have the rest of the electronics inside the servo, and you have the transmitter think about too. So, it might help, but if the transmitter is wiggling around, you will still have some drift.

Last edited by heyok; 01-29-2012 at 08:58 AM.
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Old 01-29-2012, 10:10 AM   #13
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Default Re: Servo pot

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Originally Posted by heyok View Post
Thank you, Charlie-III

I'm not sure if a tighter tolerance resistor would help. Probably worth a try, but I think it is only responsible for maybe 1/3 of the causes of the drift.
You still have the rest of the electronics inside the servo, and you have the transmitter think about too. So, it might help, but if the transmitter is wiggling around, you will still have some drift.
Fair enough.
Also, the reason I stated a higher wattage was to see if it was more immune from the heat changes (I realize most of the wattage rating is how much power it can handle without going "poof"). Not sure if you could even put a higher wattage resistor in it due to size issues.

Hey, I know, why not just buy a Heyok winch controller!!
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Old 01-29-2012, 10:21 AM   #14
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Default Re: Servo pot

I didn't comment on the wattage vs tolerance because I'm too lazy to try and figure it out, but it sounds reasonable to me!

Thanks for the props on my controller. I was resisting the temptation to push my stuff (I know, that's rare)
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Old 01-29-2012, 02:14 PM   #15
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Thanks for reading my book!
so whats the difference between a analog servo and a digital servo?
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Old 01-29-2012, 04:19 PM   #16
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so whats the difference between a analog servo and a digital servo?
Here is Futaba's take on the subject. It is an interesting read without being too technical.
http://www.futaba-rc.com/servos/digitalservos.pdf
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Old 01-29-2012, 04:38 PM   #17
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so whats the difference between a analog servo and a digital servo?
Posts 7, 8 & 16???



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Old 01-29-2012, 04:41 PM   #18
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Here is Futaba's take on the subject. It is an interesting read without being too technical.
http://www.futaba-rc.com/servos/digitalservos.pdf
Nice link, thanks.

Now to book-mark it.
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Old 01-29-2012, 07:04 PM   #19
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Posts 7, 8 & 16???



7 & 8 don't really explain the difference, just what i already know...16 (posted after my post) helps abit but still unsure on the technical aspect
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