How to modify a micro servo for continuous rotation
Here is how I've modified cheap e-bay servo for continuous rotation,
Thanks to Oddbot for his explanations :)
Servo is a SG90 , equivalent to HXT900... and many others
Tools you will need : A sharp knife, a PH0 screwdriver, some sandpaper, soldering iron (or glue)
+ Nice to have : A microcontroller that continuously sets the servo to its center position ( 1.5 ms , position 150 on picaxe)
below : the package I recieved from Hong-Kong (about a week after the order was placed)
Step 1: Cut the stickers and remove the 4 screws
Step 2: Gently remove all the gears (remind that you will have to put them back !!!) and Pull the circuit away
Step 3: Push the potentiometer out of its casing (as you can see, the output shaft is the potentiometer itself)
Step 4:
Connect the servo to your controller
The motor will rotate until you put the POT to its exact center position.
When the motor stops, you have found the good position, DO NOT disconnect the controller for the moment
Step 5 : SOLDER (yes SOLDER!) the shaft of the POT, from the back
Oddbot recommends to use hot-glue
If you decide to use glue, you will have to remove the white plastic cover sheet of the shaft-side, then fill with some glue
The shaft will never be able to move again, try to make sure... and power-off the controller if OK.
Then put the POT back to its casing
Step 6 : It's time to make the shaft thinner. (the outer gear as to turn freely)
I use sandpaper.
Step 7 : Cut the "stop finger" of the output gear
Step 8 : The finger is far away now, identify the 'notch' that prevents the gears to turn around its shaft.
Step 9 : I've used one of the screws to drill the notch out,
Then I've used the same screw to drill the plastic gear (Not sloppy on the shaft --> just be sure that it can turn with ease)
Step 10 : Put all the gears back to the shafts
Step 11 : You can put everything back together
Step 12: Write some code to test... and enjoy ;)
(note that servo0 is used to center the POT , and servo1 is used to test turning forward then backward for 3 seconds)
main:
servo 0,150
servo 1,100
pause 3000
servo 0,150
servo 1,150
pause 1000
servo 0,150
servo 1,200
pause 3000
servo 0,150
servo 1,150
pause 1000
goto main
@ Thu, 2009-06-25 08:24
pot position?
I'm wondering - you use the driver to find where the centre position of the pot is - but is that the most accurate?
Is it not more accurate to use an ohmeter to measure the maximum possible, across the outside legs of the pot, then tweak the pot till the resistance across one of the outer legs and the inner leg becomes half of that? -Given the comments by BoA, I'd have thought that an ohmeter would be the most accurate and best way to centre the pot.
@ Thu, 2009-06-25 22:15
Maybe
Nope. Couple of reasons:
1) you don't really want to find the middle of the pot, you want to find the servo's centre position and that may well depend on your controller. If there's a tiny innacuracy in your controller, then the middle of the pot might not be the servo centre. By adjusting the pot until the servos stops, you're matching it to the controller. My fixed potential divider only worked because I (typically) over-engineered my controller and it was uber-accurate. So accurate, in fact, that it could really only operate 3 or 4 servos at a time before it all fell over in a heap.
2) I haven't measured them, but the chances are the pots are logarithmic rather than linear. That is to say that the electrical middle is different from the mechanical middle.
@ Wed, 2009-03-04 04:27
i wish i found this link
@ Thu, 2009-02-12 22:32
Hey ndupont, I needed this
@ Sun, 2009-02-08 11:10
instead of gluing or
@ Sun, 2009-02-08 13:00
Tuning
Good plan. You could even fit a horizontal one and drill a hole inthe side of the casing for live adjustment.
Did you not find it difficult to get two servos adjusted sot hey're going at identical speeds for a given PWM?
@ Sun, 2009-02-08 13:10
I'll tell you, when I have
I'll tell you, when I have modified servo two :) I just got the idea yesterday, when I was looking at an old PCB with two smd trimpots. I'll modify no. 2 later today and maybe post a picture or two.
Good idea with the vertical mouted. I think it will work for normal sized servos, but I don't think there is room for it in the micro servos. But it would make it easier to sync two servos.
@ Sun, 2009-02-08 21:11
I'm afraid the answer is no.
@ Wed, 2009-02-11 00:40
Really?
Ha! How did I get horiz and vert mixed up? I'm an engineer, you know!
I would have another go at tuning them. The servo has its own PWM controller which is always trying to adjust the speed of the motor based on the resistance input. I honestly think the problem is that the two trimpots aren't matched.
....unless you're talking about when teh motors are both going flat out they still aren't matching?
@ Thu, 2009-02-12 11:05
I haven't had the time to
@ Wed, 2009-02-11 06:22
I've found that temperature
I've found that temperature can affect continuous rotation servos. SplatBot uses a servo driver board that allows larger motors to be driven like a servo. I replace the 10K pot with 2 x 5K metal film (1% tolerance) resistors. The off position drifted towards slow reverse when the temperature was cooler and slow forward when the temperature rose. I've had to increase the deadband to reduce this problem and ended up turning off the servo when I wanted it to stop. I suspect that under normal conditions when a feedback pot is used that this problem would self correct with only a minor change in centre position.
Because of this I think some form of speed feedback would be required. Perhaps a frequency to voltage converter taking pulses from an encoder with the analog output correcting the centre position voltage?
@ Wed, 2009-02-11 08:44
I'd seen this same action,
@ Fri, 2009-02-06 03:01
Great Walkthrough Ndupont!
I liked your use of the logo. I'll have to do something similar with my next walkthrough. Iam a bit confused by the code, why are you centering the servo on a different output pin to the pin used to control the servo?
Since servos vary greatly you will probably hear heaps of alternative methods like BOAs. I never considered solder although I'd be careful of a pots plastic housing. too much heat and your shaft could end up at an angle depending on the pot construction.
The Hextronic servos I used on Bot08M had a splined shaft instead of a slotted shaft like yours so the plastic gear was easier to drill but more work had to be done on the shaft to get it round.
@ Sun, 2009-02-08 12:58
Slotted
@ Fri, 2009-02-06 13:22
the code has two functions
the code has two functions :
port0 to keep everything centered while modding
port1 to test the final result
About the soldering, yes, it has to be done fairly quickly (but the plastic is made to allow soldering the 3 pins of the pot')
For sure, don't try so solder any iron, aluminium or other non-solderable alloys
The guys from Parallax/Futaba also keep an externally adjustable pot to tweak their continuous rotation servos.
http://www.parallax.com/Portals/0/Downloads/docs/prod/motors/crservo.pdf
@ Fri, 2009-02-06 00:27
Nice - and my extended tip
Not too sure how well the solder is going to hold on the pot spindle, though. Mine are made of steel or aluminium, neither of which solders. I suppose if you've thinned it enough that there's absolutely no torque on it then the hardened flux might just be enough to stop it rotating.
I've tried a couple of other approaches. With the pot centering technique described above, I could never get all my continuous servo mods centered at exacly the same pulse width. Consequently, most of my small bots can't go in a straight line.
One solution I tried was hacking the knob right off the pot (there was anough shroud let on the servo housing to be the load bearing of the main output shaft).
The solution I ended up with was to desolder the pot altogether and replaced it with a pair of resistors in a fixed potential divider. They MUST be of a very low tolerance. If you use +/-5% pots, their resistances are too unpredictable and might be too substantially different. Try to get 0.5% or lower. They're more expensive (like 20 cents instead of 10!) and well worth it.
(Hahaha - I just realised after talking about ally and steel shafts (as in SOME of my servos) the ones in my picture have brass shafts. Brass, of course, solders particularly well!!- D'oh!)
I'll chuck in a few more pictures of my partially disassembled servos. This one might be useful if you can't rememeber the order the gears go back on:
This one's got its knob cut off (TWSS). Note the slot I cut in the top to allow it to be tweaked with a screwdriver:
And this is the circumcised pot hidden back in the casing....
@ Fri, 2009-02-06 03:07
Circumcised pot
@ Sun, 2009-02-08 12:58
Nope
I don't think so. It may be a feature of my particular (Acoms) servos, but the main drive gear is a sort of flange on the drive shaft. The center of this "flange" sits over a shroud on the motor casing (the shroud can be seen on the reassembled gearbox). The pot was only attached by 3 gobs of solder and didn't fit particularly well in the middle of the shroud anyway, so I don't think it was affording much mechanical strength.
If your servos gain strength from the presence of the pot knob, I suppose an aluminium or plastic piece could be turned on your lathe (ha ha) to fit.