tortoise machines are easy to control since you simply apply voltage to move the motor until it stalls and don’t have to worry about removing power.
has anyone tried the modifications in Operate turnouts with servo motors and Turnout Motors from Servos.
seems like a servo is a cheap source for a geared motor that can be controlled as simply as a Tortoise
The Tower G90 servo draws high current in stall!!! A S90 servo in more than a slight pressure would be toast in a couple of minutes. I’ve seen them draw close to 200ma. 50ma to 60ma moving the points then 20ma with a tiny bit of pressure, idle current is 14ma at 5 volts.
Mel
My Model Railroad
http://melvineperry.blogspot.com/
Bakersfield, California
I’m beginning to realize that aging is not for wimps.
the articles describe bypassing the servo electronics and driving the motor directly either with a small voltage (~0.5) or with higher voltage (9-12V) with 200-300 Ohm resistors which limit the stall current
The only mod I’ve tried is changing one resistor to get them to go the advertised 180° swing and the 360° mod, making it a motor. Working inside the servo isn’t easy for Shaky Mel.
That does sound interesting, when I think I can try that I’ll give it a go. Won’t be today, I had a bad go with my diabetes this morning and I’m doing good to type this.
Mel
My Model Railroad
http://melvineperry.blogspot.com/
Bakersfield, California
I’m beginning to realize that aging is not for wimps.
I never took one apart to measure the resistance of the motor to see just what current it draws, but it’s pretty high. That high current draw when one stalls because it can’t move to the commanded position isn’t the electronics. I’ve seen spikes of close to 1 amp on my power supply if I deliberately hold the servo. That too is the little SG90 size ones. A bigger one would only draw more current.
If anything, using a servo as a stall motor would be like some of those Tortoise alternatives that DO safely work as stall motors, but instead of 15ma stalled, they draw 50ma or more. I guess if you are just controlling them with a toggle switch instead of electronics. Since it’s so cheap to drive a servo the proper way using an Arduino, I don’t really see a point in tearing them apart and then trying to use them in a manner they were never intended. It doesn’t have to be anything fancy like my controller, Geoff Bunza has the circuit and code for a super simple one that just uses one pushbutton per servo and drives a bunch of servos.
–Randy
the PCB is left in place to hold the motor in place, but is disconnected from the motor.
the motor is driven thru external resistors that limit the current to < 50ma.
you don’t need an arduino, just a toggle switch
I would think 50ma is too high, the SG90s I have get pretty warm at 40ma when working as a stalled servo. I try to keep the current under 25ma. At 25ma it keeps a bit of pressure on the point rails but the servos buzz a bit.
The circuit draws 14ma so the motors on my servos are drawing about 11ma, no heat at 11ma.
Mel
My Model Railroad
http://melvineperry.blogspot.com/
Bakersfield, California
I’m beginning to realize that aging is not for wimps.
just pick resistor values that allow the motor to turn without damage
What I was saying is, it’s cheap enough to drive them the porper way anmd not stall them, is it worth it to use them in a mode they weren’t intended for? Even if they are cheap, the effort involved in crawling under the layout to repalce them when they burn out seems to make it not worth the risk.
Are those special unidirectional resistors that there are two in parallel? [:D]
–Randy
OK
I took a Tower SG90 apart and checked the motor resistance, 3.7Ω.
The motor turns pretty good at .5 volts, yes that’s ½ volt. The arm travel is 180° at .5 volts @ 50ma no load, stall at .5 volts is 103ma.
I think I’ll stay with using an Arduino.
EDIT:
Travel time at .5 volts is 180° in 5 seconds.
EDIT:
It still works after I put it back together.
Me
parrallel to split the wattage. For the 9V and 80ma case, a 0.7W 100 Ohm resistor is required or 2 200 Ohm 1/3 W resistors
A continuous 80ma will cook the motor in nothing flat, they get hot at 50ma.
Mel
My Model Railroad
http://melvineperry.blogspot.com/
Bakersfield, California
I’m beginning to realize that aging is not for wimps.
More and more this just sounds like a bad idea. And I didn’t even pull one apart and test it like Mel did. I have a good candidate though, one of the ones on my workbench doesn’t seem to work any more - I thought my circuit was bad or my code, but I hooked up a different one and it works fine.
I know this was in MR a while back - I posted then that i didn’t think it was a good idea.
–Randy
In servo mode the SG90 can draw up to 80ma or more moving the points but with the points in position the current drops down into the safe range, in constant current mode it will draw the highest current the resistors allow. Not good. You would have to limit the current to a safe level of at least a max of 30ma or face motor burnout. At 50 ma the motor gets hot enough to melt the plastic housing over time.
Mel
My Model Railroad
http://melvineperry.blogspot.com/
Bakersfield, California
I’m beginning to realize that aging is not for wimps.
a 170 Ohm (5V/0.03A) will limit the current to 30 ma. Is there enough gearing to move points with that little current? if not, maybe a bigger servo
It should be, unless you have particualrly stiff points (might not move a non-hinges #4 with code 100 rail, for example). I have one stuck to the bottom of a Peco turnout and while I haven’t hooked my peak reading meter in line to see what peak current I hit, unless I try to hold it back, it throws the points, spring still in place, without exceeding much more than that, and that’s my entire circuit with the ATmega328 and a relay, plus two LEDs on the control buttons.
–Randy
Should be, when stalled the current rose considerably on my bench power supply set to .5 volts. I didn’t check the amount of pressure on the moving arm at .5 volts.
Mel
max current is always with the motor stalled. there’s no BEMF so it’s just the winding resistance that limits the current.
presumably the gearing amplifies the torque
The gear ratio appears to be about 12.8:1.
Four sets of 32Tx10T spur gears.
I’m a EEE not mechanical at all but I ran the gears through the formulas and came up with 80% efficiency with 4.6 gain.
You must figure in that it’s been 60 years since I received my degree so ±50% accuracy.
Mel
My Model Railroad
http://melvineperry.blogspot.com/
Bakersfield
so when turning an arm < 1 revolution, the motor is turning < 13 revolutions