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Resolved Resolved by jstarne1!

Doubling Up 2 Servos For More Power

Hi all,

I want to tie and Double up 2 servos for more power. I'd like have the shafts will be tied together and facing each other. This means they would be turning at the same time but in different directions. Also they would be controlled through scripts. Is this the best way to configure them? I can see this may cause challenges in scripting and wiring. Any suggestions on how to set this up in EZ Builder or have a better and different way?


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@Jason, I've been looking for the tandem servo controller from futuba you mentioned. Do you have a link?

Also, I was looking at the servo I'm using and it's a Digital "programmable" servo. One of the programmable features is reversing the direction from the stock setting. Wouldn't this do the same thing as your rewiring mod? Here's the link to the servo again:

HS-7950TH Servo

Thanks. Dave
By the way its Josh;) , to answer the first question you have a few options on sound.

That is a awesome servo by the way but that little dude is loud.

First option - you could use a analog servo instead

Why? Digital servos pulse at 300 times per second. This is so they physically move to the designated position quicker. This is important in flying models , like helicopters. Acrobatic models must respond to inputs quickly! Or die a fiery death. 300 pulses per second makes audible noise and gets louder with loud. A analog pulses 50 hertz which is audible but much lower frequency....and less annoying.

Second , in either case digital or analog you can apply dynamat sound suppression material which absorbs high frequency sounds and emits a lower frequency.

Third , you can cut out the servos original driver and implant a mosphet driver running a much higher frequency 20,000 hertz or more. This is wheelchair companies solution to quiet motor controller.

Lastly - you can relieve strain on the servo with a servo city gearbox and also increase your torque which may be needed for a large arm. I have two for shoulders myself. They have gear ratios from 3 to 5 .

This is the part you were looking for. This electronically does servo signal reversing and has a trim pot to adjust the second servo if you mix servo models and need a tiny bit of center adjustment. Realistically if you use the same servos for your dual servo setup then you wont need this box. The only advantage is center position adjustment.

These usually sell for $40 online
Woops, Josh. Sorry, I knew that. *blush*

Thanks for the quick reply Josh. ;)

I'll keep looking for a servo that will fit into the Gearbox I want to use. Maybe the ones you and Justin point to will work.

Not sure I understand your third option or how to do it.
@dave the reason the servos make noise is the motor physically resonates at the frequency the servo driver runs at. 300hz If you replace it with a higher end high frequency driver 20000-40000 then the sound it makes is is ultrasonic. I have read about several people making custom motor drivers for that reason. This isnt super easy

I love the gearboxes , they are great ! You could use a gearbox for the degrees of freedom closest to the body and tandem servo for mid joints. Math is your friend so calculating torque needed helps you plan.
My bad ! I forgot to answer the question about programming. If it has a servo reversing option it should act the same as the rewire. You can test it and see if it is the same , if not then manual rewire is needed. When they change settings on servos the manufacturer expects you to have trim control for adjustment.

Anyways if you have not already bought the digital servos you could get strong analog servos to reduce noise and the servo city gearboxes to multiply torque. A 5 to 1 gear box gets you 1200 ounce inch torque from a 1501mg :)
Justin, errr, I mean Josh, ;)

Thanks again for taking the time to assist. I'd love to use a analog servo but I'm having trouble finding a quality name brand analog servo over 450 oz-in that will fit into servo City's Gearbox. I'm not sure but I think these gearboxes will only accept Hitec standard size servos without modifying the gearbox but I may be wrong.
@ dave , ive tried , power hd , pololu , tower pro , futuba and hitec standard servos and they all just dropped in from the bottom. I have the "surface mount" version. If you are using a gearbox and a tandem setup you should have a large amount of torque to play with.
Wow, thanks Josh. Great that you have one of these gearboxes to play with. So I know how to proceed, which one of these are you using?:

Bottom Mount servo Power Gearboxes (With Potentiometer Feedback)

Gearmotor Gearbox (open loop made for DC motors)

I want to keep everything "inline" so I'll try to use the proper "L" brackets, servo City's channel and the Channel mounted gearbox. This design will help me save space. I only have a 4 &1/2" opening that this arm mechanism will be sliding in and out of. Even if I use them in tandem I should be able to do this. Once the arm is fully extended the elbow and wrist joints can move around in the wanted directions. Here's the type Gearbox I have now and am hoping to use:

Actobotics channel mount servo Power Gearbox

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You mentioned earlier that to get rid of the "whine" in digital servos I could actually remove the driver and replace it by "implanting a mosphet driver running a much higher frequency 20,000 hertz or more." So by this do you mean to actually remove the circuit board and replace it with a motor controller with specs at or above what you state? If this is the case I think your talking about a motor controller like this?:

Pololu Jrk 12v12 USB Motor Controller with Feedback

It states one of the features is "Ultrasonic PWM to eliminate switching-induced motor shaft hum or whine."
@Dave , Yes! That is an excellent example of a ultrasonic motor controller. This one is usb programmable and accepts either TTL or RC servo inputs so this is a very beefy controller. servos benefit from higher torque with higher frequency pwm too. The obvious drawback is cost but pololu doesnt sell any duds.

You would literally replace the original driver in the servo with this ultrasonic driver.


i have the top mount gearbox but channel mount may be best for your B9 arms.
Thanks again Josh. You really don't have any idea how much help you have been. If my B9 has any kids I'll make him name the 1st one after you! :D
Hahaha , this gives me a mental image something like the movie "batteries not included"

If you have any new questions make a new thread so it is easier to search for and the community to benefit from.;)
@Dave - if all else fails, there's the "Poor Man's Mechanic" solution for the servo noise: boost the volume of B9's Robot Sound channel until you can't hear the servo noise! :P
Yep, those mechanical sounds cover up a lot of annoying sounds.

Your avatar is a little freighting and disturbing. *sick*;)
Why, thank you! Just call me B(acon)9:D
Josh or anyone on the know,

As I stated above I want to tie two servos to each other and have them move exactly at the same time and speed (Tandem). Josh Suggested removing the control board in each and replace them with ultrasonsic PWM motor control boards. I found the below board that looks like a good choice.

Motor channels: 1
Operating voltage: 6 16 V
Continuous output current: 12 A
Peak output current: 30 A
Auto-detect baud rate range: 300 115,200 bps
Available fixed baud rates: 300 115,200 bps
Available PWM frequencies: 20 kHz, 5 kHz
Reverse voltage protection?: Yes
USB connector style: USB Mini-B

Pololu Jrk 12v12 USB Motor Controller with Feedback

Now, these little wonders are expencive at $100 each. I'd have to pop anouther $200 to control the two servos I want to run in tandem. Any reasion I cant run two servos from just one controller if the max current draw of each combined is below the 12A Continuous output current of the JRK 12v12 controller?

Here's how I see it working:
Remove the driver boards from each servo
Solder wire directly from the servo motor lugs and ground of each servo
Connect the new servo wires in parallel to the JRK's single motor output (A B).
Only attach one pot from the two tamdem servo's to the JRK's Feedback input.

My thinking is that with both servos being controlled by the same control driver board and only one pot that they should move exactly the same. Only variable would be the gearing in each servo. Hopefully being the same make high quality servo this should not be an issue. Here's the servo from servo City and the Robotzone gearbox I'm useing:


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HS-7950TH Servo

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Very impressive Anthony! Thanks for the imput. However sadly I have tried the invert method as I mentioned in your Lexi thread. My servos move at slightly different speeds and one travels farther than the other useing the same values. Oddly they are identical brand of high quality servos. *tired*
Dave, I think I figured out why your dual servos don't work. Take a look at the specs on the servo page

Specifically, the diagram that defines the movement. Neutral is at 12:00. Time to rotate 180 degrees clockwise is 1900 microseconds, 180 counterclockwise is 1100 microseconds. Evidently it travels slower in one half of a 360 degree rotation, so by reversing one they are going out of synch.

Weird? yes - but that's how I'm interpreting that diagram. Experts: am I correct?
Humm, It takes longer for the servo to go one way then the other. That would explain it. Wonder why that happens. Thanks, Nice catch. i dont like the answer but it makes sense. Well, back to the drawing board. *sick*