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
greg - Philadelphia & Reading / Reading
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
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
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
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.
rrinkerSince it's so cheap to drive a servo the proper way using an Arduino,
you don't need an arduino, just a toggle switch
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?
rrinkerAre those special unidirectional resistors that there are two in parallel?
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
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.
RR_Mele 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.
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.
gregc RR_Mel e 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. 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
RR_Mel e 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.
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
RR_MelThe gear ratio appears to be about 12.8:1.
so when turning an arm < 1 revolution, the motor is turning < 13 revolutions
Is that 4 ets counting the output gear on the shaft and the ger ont he motor shaft? In an hour I'll be able to look at one myself and tell. 4 sounds right though, 3 sets of gears plus the motor gear and the final output gear.
Somewhere I learned how to calculate gear ratios - I think it was an MR article on remotoring/regraring, actually. In an issue that might have been before my time. Or possibly in the old Machinist's Handbook which I guess was my Dad's, I also have a Carpenter's Handbook that was my grandfather's. That one is all wrong now, actual vs nominal measurements for lumber have changed over the past 70 years or so.
And if I want to learn to tie knots or sail - I have two US Navy Bluejacket's Manuals - one from 1943 and one from 1951.
So what is the potential goal here? Save money over the Tortise?
Or save space?
Sheldon
RR_Melshould be 104:1.
so ~50 rev in 0.3 sec (0.12 sec/60°) to turn the arm 180 deg. 150 rev/sec or 9,000 RPM seems reasonable
ATLANTIC CENTRAL So what is the potential goal here? Save money over the Tortise? Or save space? Sheldon
gregc RR_Mel should be 104:1. so ~50 rev in 0.3 sec (0.12 sec/60°) to turn the arm 180 deg. 150 rev/sec or 9,000 RPM seems reasonable
RR_Mel should be 104:1.
ATLANTIC CENTRALSo what is the potential goal here?
for me it's interesting. i'd like to understand what makes the tortoise a stall motor. and knowing multiple solutions to a problem can save time and avoid unnecessary hardship.
The Tortoise works like it does because the motor is wound such that it consumes 15ma at 12V with the rotor locked. Gonna make me remember all that DC motor theory now.... It is in itself a very low torque motor, so there are a nice pile of gears to serve as torque multipliers so it has enough power to move switch points.
I used to have one that was me "experiment on" unit - doing such nasty things as getting a rhythm going and it will bounce off one stop and nearly coast to the other side, try to fore it too hard and the gears strip past one another, and I took it apart countless times. One thing about them is, they are pretty much indestructible. That poor abused one would still work fine when put back together - a little noisy, but ti would move back and forth just like it was supposed to.
Pretty sure I even ran the motor with no gears installed, it's very quiet - allt he noise is the gears meshing. I never put a meter to it and checked the curent draw running free like that, or tested the winding resistance. Nor do recall how many poles the motor had.
That low current means it's within the output level of many logic ICs, and other generally low power parts like comparators or op amps. And of course 15ma is generally well within the safety margin to run an LED. Those first Tortoise knockoffs drew a lot more current, meaning no simple series LEDs, and any drive circuit needed an extra stage to handle the current. SOme of those that are still around have newer models with much less current draw, but while better than their original model, is still 10-20ma more than a Tortoise.
Servos are nice, when used as intended, because the power supply is independent of the control signal. The control signal draws nearly no current (hmm, have to try measuring that, my one meter has a low burden low ma range just for such things), so a driver can control a whole lot of servos without any special high current signal considerations.
None of this really matters if you are simply using DPDT or SPDT toggles and no electronics, since even rather small toggle is usually rated for 1 amp or more. It all comes down to what the motor can stand without melting the windings.
I programmed early PLC's to control assembly lines, pumping stations and other industrial applications in the 1980's. And I see the value in solid state logic for complex operations. We used PLC's to replace relay stacks that had dozens of inputs and relays in a single function.
But when the logic function is simple, a chain of only two, or three, or even four conditions, I don't really see the point.
Multiple control locations require the same amount of input/output wiring no matter the logic method.
I control complex interlockings from two different locations, with just a few $3 relays and LED lighted pushbuttons, using the same power supply for everything. Just seems so much simpler to me.
And I already have nearly 100 tortoise machines.....
I will still be curious what Greg felt the goal here was, saving money?