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[stevesteve]

Ive just seen topic about building a home made ESC and thought about something i built a while back for a large scale RC truck Ive been working on.

you can make your own servos out of pretty much whatever DC motor you want using a old servos and a small home built circuit called a "H bridge"

This circuit takes the signal from the original servo motors power supply and use it to control a circuit that can drive a much much more powerfull motor. the motors position is achieved by linking the motors shaft to the pointometer on the old servo this can be done ether by adding a bar on the large servos horn to the small one or you can do what i did and connect to the rear of the motors shaft directly. now your large high powered motor will follow your transmitter stick wherever you tell it to go, return to neutral when your stick does and will resist forces applied to it just like a standard servo even when in neutral position, it will behave exactly as a standard servo does just with a hell of a lot more pull to it. my home made servo has so much power it could cut your finger off, you have to be very carefull lol. there is however another use for this circuit, a esc of sort!

If everything is wired up but you do not link the pointometer from the old servo up but set it to zero instead you now have a speed controller! you will get progressive control over the motors speed as you advance the stick on your transmitter and because you have no sensing attached the motor can be run continuasly and when you return to neutral the motor will just stop where it is, reverse your stick and your motor will reverse in the same controlled mannor

heres pics of my controller, its spaced out and you can reduce the size a lot, adding heatsinks would also allow you to run much higher amps. i forget what this is rated at but it drives my motor under load with only a bit of warmth comming from the transistors. i did not use the exact same components as listed in the diagram i used better and more powerfull transistors for the switching. i replaced Tip120 and Tip125 with Tip142 and Tip147  The N2222's, the capacitor and the resistors were all left as is printed in the diagram .

heres some pics for you.




top view of controller circuit

my large servo motor with old servo acting as sensor

underside or controller



the transistors I used on this board are more powerfull than in the diagram



The circuit diagram for you to look at

[stevesteve]

This pic aint blurred

[Yamaha1]

Mmm - that looks interesting, Steve - maybe some more details could be useful - nice to see some experimenting going on, though.

Bet you get through some solder, looking at that Vero board!!!   

Mike

[stevesteve]

what info do you think would be usefull?

i believe the high side of this circuit can handle anything up to 100v but unsure of the amp capacity. Like i said, i changed the transistors in this for better ones but anyone who built this could replace what i used with ones that are even stronger! the small silver pot like components are the n2222 transistors. they have a pretty low power handling but only need a microscopic amount of power to switch them to open. these use the signal from the old servo and trigger the bigger transistors that need a more powerfull signal than the servo provides. these then supply what ever power you have connected to the high side of the circuit to the outputs. the low side is if from the old servo, it triggers the transistor gates but the power that flows through them comes from the high side.

there are 3 sets of connections on the board, the input from the old servo for the control of the circuit, high side power input and high power output

when modifying an old servo what you need to do is open it up, chuck away all of the gears but keep the final drive as its what connects to the pointometer. carefully remove the circuit from the servo and locate the power leads for the little motor inside. you need to cut and connect to the two motor wires, these will be your inputs A and B. then you need to find the ground or negative in the servos circuit. this may well have a black wire going to it but usually theres a border round the board that is neg that you can hook up to, a multi meter would tell you whats what. your original servo contains a H bridge circuit, what  were doing is extending it to another that can handle a big current pull

as i was typing away i remembered i have a old servo so i opened it and took a pic for you to see what i mean. all servos are slightly different inside but still use the same system so its just a case of using your eyes to find the connections.
also, when all is built and connected up do not attach your new servo to anything, you need to test run it first. if it spins the wrong way you simply reverse the wires ether from the servo to the board or the power from the board to the motor, if it were connected to you model and did this in testing it could damage your models linkage by wrenching it the wrong way.

if anyone needs info of setting up the position sensing just ask.

[dodgy geezer]

A useful donor servo might be this one: http://www.giantcod.co.uk/towerpro-sg90s-micro-servo-p-403861.html 

A miniature servo from Giant Cod - new price £1.94p!

What you would get is some internal electronics and a wired plug (the plug alone is probably worth £2) for making an ESC

AND ... a small gearboxed motor which is ideal for a radar mount installation.

All that for less than 2 pounds - how can you go wrong...?    O0l

[stevesteve]

yeah, that would be perfect as all you need is the control circuit inside, its pulling power is not relevent in this.
as long as the board inside is not too small to soldier to the units tiny size would make it a lot easyer to mount!
To use it as a radar motor what i would do is open it up drill out the bit on the final gear that allowed it to lock to the poinometer so it can spin freely on it and theres also a lug on the final drive that stops the final gear at a certain point, that would have to be filed down also, about 5 mins work at most. Then to drive it just use a resistor of the right rating to restrict the motor speed and bypass the servo control board connecting directly to the motor.
here is a bigger image of the circuit diagram, once you have opened it, if you click it again it will go full screen

[Yamaha1]

Hi Steve

This is beginning to make sense to me gradually - but I can't quite understand your pic 160a - the small bit on the back of your ( to me ) strange looking large servo doesn't look like a donor servo - it looks more like a small motor.

Also the large servo appears to me in two right-angled parts - maybe you elaborate a bit on these items for this new thicko!!

I can get to grips with the circuit - I could build that without too much trouble - it's the mechanical bits that I'm having trouble understanding !!

Mike

[stevesteve]

ok, in pic 160 you have a standard sized servo (the green bit) with two lots of white, red and back wires comming out of it. one lot go to your reciver the other to the h-bridge board you make as does the thicker black wire you also see  in the pic, that is the power cable for the new motor. how you design the actual motor is down to choice but here we have in the black right angle a motor with a worm drive turning a shaft. the pointometer of the old servo is linked directly to the rear of the shaft for position sensing and the "arm" of the new servo is at the other end.

heres some more pics, hope they clear things up a little








attatching to the rea of the shaft was just easy in this setup, in others i would have the old servo with a horn on it in a bracket level with the new servo motor, again with a horn on it and a bar between the two to move them in sync for the sensing. i just found that as i could attatch to the rear in this case it was  just easyer

[Yamaha1]

Ahh-soo, I think the penny has dropped at last - I get what you are saying - just that strange-looking motor assembly that had me confused!!

Thanks for taking the trouble to explain it so well - bet you'd be useful on Scrapheap Challenge !!

I am just getting started into boats - and especially electric drive - so it's a steep learning curve.   

Mike

[stevesteve]

other geared motors that are very powerfull yet smaller and lighter are electric window motors and wiper motors. some wiper motors are wound funny though and can run at a different speed in reverse, e windows are ment to go both ways however. the motor i have used comes from a electric armchair actuator capable of lifting 203kg by extending a pole out of the shaft on a corkscrew drive. obviously because i have removed 1 of the 2 gears so its not that strong but i still think its in the 100+ range! i cannot stop the servo arm from turning, even with both hands, its a beast!!

another thing i have just remembered, cordless drill gearboxes have too much play in them and as a result if used, many of them will cause the servo to twitch as its trying to find neutral in the play.

[nick_75au]

I've ordered one of these, It should suit those people with not so crash hot electronics skills

http://www.vantec.com/rbsb.html

for a similar project, I haven't received it yet, probably after Christmas
Im using a 24 volt wiper motor from a boat at 12 volts, be very carefull with these motors, they are powerful enough to break fingers, I severely bruised my thumb attemting to do a stall test but I could not get the motor to stop and the peak amps were less than 2 A, I hate to think what a 15 amp peak motor could do

[derekwarner]

SBN.........says......."some wiper motors are wound funny though and can run at a different speed in reverse"

Not sure about that    ...but they are certainly are arratic in reverse but there is a reason

The armature shaft has a single thrust ball in the motor casing to accept the induced EMF & force in the chosen/design direction >>>>>>>>>>>>>>>

Reverse the direction & the armature floats according to the enduced EMF which is also relative to the load on the shaft & as it has no method of constraining thrust

Same applies to motor vehicle ventilation fan motors although to a lesser extent as they do not have the helical worm & worm wheel design

nick_75au is certainly correct that these wiper motors provide sufficient low speed torque to break human bones   .........Derek

[Martin (Admin)]

( Not sure if this the right place to ask this. I'll split the topic if needs be. Martin )

Has anyone used a big servo as a drive motor for a smallish model, something like a Graupner Pullox?

[stevesteve]

the motor I'm using is rated for 24v and 3,4a at that voltage. i made a mistake about its pushing power when it was still an actuator, it was able to lift 3000n, thats a massive 305kg! before i modified it i had great fun putting it in various objects and watching it tear them apart from the inside! the motors are slow spinning torquey things when running by themselves but put through the worm drive they are arm jaw droppingly powerfull and the great thing about this is that its designed to be run under load in both directions and turns the steel arm i welded to the shaft at the same speed both ways and turns a touch faster than a regular servo. i think this design could be used in model boats where you need a moderately powerfull servo and a regular type just wont do. these don't have to be this strong and heavy to be usefull in boats, just got made like this cos i need massive pulling power for my truck steering (it has a mini quad chassis in it). as soon as the warmer weather is back I'm going to get working on that truck again, its about 4ft long!

[stevesteve]

the motor I'm using is rated for 24v and 3,4a at that voltage. i made a mistake about its pushing power when it was still an actuator, it was able to lift 3000n, thats a massive 305kg! before i modified it i had great fun putting it in various objects and watching it tear them apart from the inside! the motors are slow spinning torquey things when running by themselves but put through the worm drive they are arm jaw droppingly powerfull and the great thing about this is that its designed to be run under load in both directions and turns the steel arm i welded to the shaft at the same speed both ways and turns a touch faster than a regular servo. i think this design could be used in model boats where you need a moderately powerfull servo and a regular type just wont do. these don't have to be this strong and heavy to be usefull in boats, just got made like this cos i need massive pulling power for my truck steering (it has a mini quad chassis in it). as soon as the warmer weather is back I'm going to get working on that truck again, its about 4ft long!

quick note, the motor is rated 24v but i run it on 12v

[nick_75au]

( Not sure if this the right place to ask this. I'll split the topic if needs be. Martin )

Has anyone used a big servo as a drive motor for a smallish model, something like a Graupner Pullox?

I've used standard servos as a self contained propulsion unit direct drive, I tried one step of reduction and it barely moved the boat, so a large servo should be fine, just need to do the continuous rotation hack.

Nick

[stevesteve]

I Know theres other ways to do this but the easy way to make a servo spin continuously is to open it up, lock the pointometer with something like glue.
the pointometer shaft is keyed at the top as is the final drive gear that sits on top, carefully drill out the plastic so that it is a full circle or lop off the key from the top of the pointometer shaft.
now that the pointometer is locked and does not move the servo is no longer sensing its position and will run continuously in what ever position you move the stick on your transmitter and as long as your controller trim is set right, the servo will stop wher it is when you release your stick.

if you do this, remember the final shaft is just a bit of plastic spinning on a metal shaft so make sure theres no sharp edges or you will get wear very quickly. i would grease the pointometer shaft to reduse friction also.