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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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Canada
#1  
A better way would be to use one stronger servo
I would not try doubling them up as it may be way more problems than its worth and could cost you your servos
#2  
Wolfie, Thanks for the suggestion. I've looked and there aren't many stronger servos available for what I want to do. In fact I had hoped to use a RobotZone servo Gearbox and servos doubled up to get enough power to lift the arms of my B9. I also have limited room.

Gearbox

Here's the servo that comes with and fits this Gearbox:

Gearbox Servo
Canada
#3  
How you considered making your own servos or using a more powerful motor in the servo you have

Here is a youtube channel list that has some good info on how to make powerful servos
HomoFaciens
Canada
#4  
If you are going to use two servos I would use some old ones you don't care about till you get the control scripts working right before you use good servos
Canada
#5  
Another thing to look into would be air pistons or hydraulics as they run a lot smoother than servos/motors
United Kingdom
#6  
You can get servo doubler upper thingys which do just that if you wanted a hardware version, I don't know of any off the top of my head but if that's the way you want to go I'll try finding the topic which they were mentioned in. Josh may have an idea, I believe it was him who mentioned them in the past (from memory).

Or, you can use multiple servos in ARC. Check out Relative Servos or the Multi Servos in the regular horizontal and vertical servo controls.
#7  
Thanks for the ideas and input guys. I'm always amazed at how much help and good advice one can get from good people if I just ask. I'm still in the R&D phase of my B9 arm design and it's great to get input.

Here's a good YouTube vid where a guy shows two ways to tie two small servos together. The second example uses a type of "clutch" using stops and springs.



The Gearbox I pointed to in my second post almost does the job but struggles to lift the extended load (this is what the servo will lift when arm is fully extended). When I ordered it from servo City I had it geared fro 3-1 and they offer all the way to 7-1. However the higher the gearing ratio the slower the movement. I may get a gear set of 5-1 and trade off some speed.

Another way I could go is what @Wolfie alluded to; Make my own servo. I could find a strong DC motor like a small wheel chair or windshield wiper motor. I could control it through a Sabertooth/Kangaroo combo of boards like I have done my waist and hip motors. However I'd have to figure out how to mount an encoder or pot on the shaft and the scripting would be more complicated. Also to operate this combo I need to use Serial commands. I'll have other servos farther out on the arm to operate two directions of the wrist and the open and close of the claw. The native servo scripting in ARC would come in handy in animating the whole arm if everything were servos and accepting the same type of commands.

Another annoyance I'm experiencing is the "Whine" of the digital servo when it's energized and holding position or load. These HiTec Digital servos I'm wanting to use are programmable. I'm wondering if programming in a wider Deadband would help with this? Everything I've read says this is normal and unavoidable. It comes from the servo moving back and forth very fast and trying to hold it's spot. *confused*
#8  
Hey guys i have been on vacation and my ear was itching so i logged in.;) yes double servos or more commonly called Tandem servos are a regular practice. There are some dos and dont's to follow here. A double servo is also the suggestion i made to Ant as well and he is now using them for lexis elbows.

Do-

- Only use the same model servos! I cannot stress that enough.

- try any use servos know for their power , here are some examples. Your budget will likely be your limitation.
-hobbico cs170 $90 over 400 oz in @ 7.4
-power hd 1500mg $20 240 oz in @ 6v
-hitec 645mg $20 133 oz in @ 6v

- in a tandem setup you can either use script or a tandem servo controller from futuba. This makes one servo move in the exact opposite rotation from the other servo.

Do test your setup to ensure they are moving in unison and the servos are moving to the same but opposite rotation position. Once you are sure of positioning you can move to testing completely assembled.

Donts -

- Dont even try mixing two different servos. Because of variations in position control , response time of the servos driver and the accuracy signal interpretation you are setting yourself up for failure to mix them.

- Dont order any knockoff / clone or china servos if you can afford otherwise due to quality control in some cases. ( most servos are made in China anyways but it has to due with the quality control of the dealer channel you get them through) If you get unbranded or tower pro 995 servos then at least use two servos that were bought at the same time so they are most likely to have the same quality.

Dont hook up two servos in tandem setup with any stiff or hard linkage when you first assemble them and test. If the servos are off by more than 1.2 degrees ( i believe thats the number) they will burn up within the first couple min.
#9  
Perminant and most reliable solution-

HARDWIRING A servo FOR REVERSE MOVEMENT

This is a hard wired way to make sure the second "reversed" servo moves in unison with the first servo. I realized i posted dos and donts first but this is the truly solid way to reverse the servo in unison. The reason why is both servos would receive the same signal from the same connection.

This is a modification you should only do once you have a bit of practice using a soldering iron. 4 wires will need to be desoldered and resoldered back inplace.

The first two wires to swap are the potentiometer wires on the driver board. There are three lined up usually all in the same color wire. Of these three you will swap the outside two and leave the center wire alone. User-inserted image


The second two wires we will swap are the leads to the motor itself. These also may be the same color. Carefully desolder and resolder them in reverse orientation.


User-inserted image


standard rc servos are all this wiring format , i just used a tower 9g servo for the example. So this will resolve your reversing needs:)

I guess i should repost this in tutorials section
#11  
Nice find sir! And looks like they carry replacement parts parts for repair of the 1501mg too.

So for those who had questions about how to reverse a servo for unison rotation , did i answer all your questions?
#12  
Yes @Jason, this was exactly what I needed to know and what I was looking for. I thought I remembered someone mentioning this process on this forum a few months ago. Thanks for taking the time to guide me on this.

I'll be ordering another servo to match the one I've already got and wire the servo pots as you show. I may have more questions so I hope you don't leave town again. ;)

I do have one other question. I also remember you commenting on the "whine" the digital servos make when holding load or trying to keep position. Have you found a way to quiet them?

I'll also post my progress with pics. Robot arms always seem to spark a lot of interest.

Thanks again!
Dave
#13  
@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
#14  
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 .
#15  
http://www.futaba-rc.com/accessories/futm4150.html

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
#16  
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.
#17  
@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.
#18  
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 :)
#19  
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.
#20  
@ 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.
#21  
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


User-inserted image
#22  
@Josh,

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."
#23  
@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.

https://www.servocity.com/html/robotzone_servos.html

i have the top mount gearbox but channel mount may be best for your B9 arms.
#24  
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
#25  
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.;)
#26  
@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
#27  
Yep, those mechanical sounds cover up a lot of annoying sounds.

Your avatar is a little freighting and disturbing. *sick*;)
#28  
Why, thank you! Just call me B(acon)9:D
#29  
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:

Gearbox


User-inserted image


HS-7950TH Servo


User-inserted image
#30  
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*
#31  
Dave, I think I figured out why your dual servos don't work. Take a look at the specs on the servo page
www.servocity.com/html/spg7950a-cm-360_servo_gearbox.html

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?
#32  
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*