Analog servomotors

[JAY X]

Hi!

I would like to know what is the main drawback when using servos (360º) to move a rotary pot. Most datasheets only talk about stall current (up to 1A or more). But keeping a potentiometer in a position also draws current...or not much? Often this data is not shown in servos spec sheets, so maybe i would need a 1.5A TO220 regulator just for the servo?

Thanks for your advise!

Jay x

 

[abbey road d enfer]

Hi!

I would like to know what is the main drawback when using servos (360º) to move a rotary pot. Most datasheets only talk about stall current (up to 1A or more). But keeping a potentiometer in a position also draws current.

Due to solid friction, once the motor has reached its position, it does not take any current to maintain it in place.

..or not much? Often this data is not shown in servos spec sheets,

And for good reason, because it's always zero.

so maybe i would need a 1.5A TO220 regulator just for the servo?

That really depends on the servo's specs. Some run with less than 100mA.
https://www.dfrobot.com/product-2120.htmlMake sure you choose a 300° type.

 

[sahib]

I am assuming (from Abbey's post) you want to use those RC servos.

I do not think there is RC servo with 300 degrees travel. There are multi turn sail servos that you may limit the travel to 300 degrees.

You can modify a standard RC servo for 300 degrees (provided that the feedback potentiometer lets you), First you have to cut away the stopper lug on the output gear/spindle. Then apply wider range of pulse. Normally 1mS to 2mS will move the servo to 90 degrees. 0.5mS to 2.5mS will move it to 180 degrees and so on. In 8 bits you'll get about 1.7 degrees resolution for 300 degrees travel.

The servo will only start to draw current if a force is applied to it in one direction. It will not draw anything if it is holding a potentiometer unless you manually try to turn the potentiometer while the servo is holding it.

I have not played with RC servos for over 10 years, but now there are digital ones that are all reprogrammable. At some point I was planning to design a multi-axis controller. So, I still have quite a bit of M51660L analogue chip.

For 360 degrees travel you will require a special potentiometer to replace the standard feedback pot. I do not remember the part number but again I have some made by Murata that I purchased at that time. See picture. The PCBs were designed to modify Hitec HS805BB and 645MG servos.

 

[JohnRoberts]

The main drawback IMO is that a servo pot is overengineered for the task...

Servos come in many flavors and power levels. One well respected high power LF loudspeaker (Servodrive) used a servo motor driven woofer. That was also overkill, no longer in production.

JR

PS: Technically servos are not digital.... while stepper motors are.

 

[abbey road d enfer]

I am assuming (from Abbey's post) you want to use those RC servos.

I do not think there is RC servo with 300 degrees travel. There are multi turn sail servos that you may limit the travel to 300 degrees.

There are many 300) servos.
https://www.amazon.fr/Steering-0101-Robot-dedicated-300-Degrees-Prevention/dp/B01EHS3E1Chttps://french.alibaba.com/product-...bb-300-degree-rotation-angle-60007573771.html

 

[abbey road d enfer]

One well respected high power LF loudspeaker (Servodrive) used a servo motor driven woofer.

Actually, the motor in Servodrive was a DC motor with brushes. Distortion was awful, well-hidden by the 4th-order low-pass cabinet. In addition, the fan, powered by the rectified signal, continued to wail long after the last notes.
I tried vainly to distribute them way back.

 

[sahib]

Thank you for keeping me corrected Abbey. Also good to know. These must have come out in recent years.

 

[sahib]

PS: Technically servos are not digital.... while stepper motors are.

John, they call them digital because of the modern control electronics they contain, otherwise indeed the motors are not.

 

[JAY X]

Hi!

Thanks for the replies!. Solid Friction!! never thought about that!!. And me worrying about current draw!!

I plan to use analog servos controlled by PWM. With an arduino supposedly you can program the servo travel range (0 -180) or (0-300), but I never seen an example with a continuous rotation servo... Nice to have found 300º servos!! that will simplify things! Time to experiment...

Jay x

 

[abbey road d enfer]

I never seen an example with a continuous rotation servo...

Continuous rotation servos are not really servo mechanisms, in the sense that they don't have positive indexing.
The command just tells it at what speed and in what direction you want it to rotate. That means the NFB loop must be provided by an external sensor, which, in the case of rotating a potentiometer would be you deciding it's loud enough and telling it to stop or go backwards.
In any case, a CR servo cannot be recalled, unless you add EOT switches; then it's not a CR servo anymore.

 

[JohnRoberts]

Actually, the motor in Servodrive was a DC motor with brushes. Distortion was awful, well-hidden by the 4th-order low-pass cabinet. In addition, the fan, powered by the rectified signal, continued to wail long after the last notes.
I tried vainly to distribute them way back.

Yup the old belt driven servo drive was a weird beast back in the day, kind of a science fair project, "I wonder if this works."

Tom Danley the man behind that old beast was involved an several big/loud bass programs for military/government. His current business Danley Sound Labs Has developed some remarkable speaker systems for covering large venues with good sound integrity.

I just looked at his website and now he is pimping smaller speakers (probably because that is where the money is). I have been impressed with his big speaker sound quality (no not the servodrive) so his new small ones probably don't suck.

JR

 

[Matador]

Did you consider a micro stepper motor?

 

[JAY X]

Hi!

I found this 300º servo model:

https://www.dfrobot.com/product-2118.html
From the description:

"This servo features large torque, stable performance, accurate running angle and easy to install. The operating voltage is 4.8~6V. When the servo is powered at 6V, its stall torque will exceed 1.2Kg. There are no limited switches inside the servo so it can rotate smoothly in 360 degrees (manually only). Meanwhile, the servo could operate fluently with minimal power, and you can just supply power to it by normal battery or USB. In addition, users can directly control the servo using servo libraries in Arduino IDE and let it reach a maximum 300 degrees rotation angle within the pulse range of 500-2500μsec."

Seems interesting...or not?

Jay x

 

[abbey road d enfer]

"This servo features large torque, stable performance, accurate running angle and easy to install. The operating voltage is 4.8~6V. When the servo is powered at 6V, its stall torque will exceed 1.2Kg. There are no limited switches inside the servo so it can rotate smoothly in 360 degrees (manually only). Meanwhile, the servo could operate fluently with minimal power, and you can just supply power to it by normal battery or USB. In addition, users can directly control the servo using servo libraries in Arduino IDE and let it reach a maximum 300 degrees rotation angle within the pulse range of 500-2500μsec."

What exactly do you want to achieve with this servo?
What feedback? If you want to turn a pot to set something to your liking, like setting the volume of your stereo, you don't need a servo, because the FB mechanism is you.
You just need a up/down control, maybe with speed control.
If you want to remotely control something where you don't have direct FB, like setting the temperature of a thermostat in your datcha in Koursk from your ranch in Texas, you need a servo.
The problem with a real servo is that you have to have a MCU (or some kind of logic) to deliver the PWM command, which is not as simple as having a bi-directional switch to drive a DC motor.

 

[buckethead]

Watching this thread with great interest!

I've been working on a motorized potentiometer solution with huge oversized steppers for some time now. One gang of a multi gang pot is used for feedback (10bit AD into MCU).

What exactly do you want to achieve with this servo?

That would be interesting to know

Looking forward to where this thread is going.

 

[abbey road d enfer]

I've been working on a motorized potentiometer solution with huge oversized steppers for some time now. One gang of a multi gang pot is used for feedback (10bit AD into MCU).

In many cases you shouldn't need FB with steppers.
Many CNC machines use steppers without FB. The only issue is making sure the stepper doesn't loose a step. Making sure the max torque isn't exceeded is generally enough. Slouldn't be too hard with potentiometers.
Seems a tad overengineered to me, but it must be fun!

 

[buckethead]

In many cases you shouldn't need FB with steppers.
Many CNC machines use steppers without FB. The only issue is making sure the stepper doesn't loose a step. Making sure the max torque isn't exceeded is generally enough. Slouldn't be too hard with potentiometers.
Seems a tad overengineered to me, but it must be fun!

Yeah I think it's very overengineered - let's say it's a "learning project" .
I needed the feedback for the MCU to know where the stepper is. The plan was: manually turn pot+stepper to a position -> read and save that position -> recall different positions, much like a motorized fader. Sadly I didn't find any (affordable) smaller stepper motors with dual shafts (one side for pot knob, the other attached to the pot) yet, so I'm curious if we'll find some interesting alternatives here.

 

[abbey road d enfer]

Yeah I think it's very overengineered - let's say it's a "learning project" .
I needed the feedback for the MCU to know where the stepper is. The plan was: manually turn pot+stepper to a position -> read and save that position -> recall different positions, much like a motorized fader. Sadly I didn't find any (affordable) smaller stepper motors with dual shafts (one side for pot knob, the other attached to the pot)

That's a very frequent request, and I wonder why mfgrs do not answer that. Many use shaft coupler for that.

 

[dirtyhanfri]

Wouldn't be easier to use digital pots?

 

[abbey road d enfer]

Wouldn't be easier to use digital pots?

Digital pots have their own quirks. There is control voltage leak-through. Their resolution is not so good, particularly at the beginning of travel. Distortion is not as good as a pot.