Constant torque motor.

I want to construct a motor which exhibits constant torque up to a set sped, the torque should be variable , but will be constant during each use..

What type of motor and control system should i be looking for please ?

How accurate do you want this torque to be ? +/- 10% ? 1% ?

Over what speed range?

In principle a brushed or brushless motor has torque proportional to the current supplied - but only very approximately.

If you want great accuracy you will need to measure the torque, which isn't easy

regards

Allan.

The amount of power you send to motor - 10%(aprox.)= the output power od motor, if you put in that power some formula inclunding rpm of motor you can get torque.
So if you want to control torque all you need is to control power, that would be job for PWM.

But if you need constant speed, let's say engine run about 3000 RPM with no load, when you put load on engine it will decrase RPM and you need program that will increse RPM to 3000 under load. For that my friend you need RPM sensor on engine shaft, it can be hall or optical sensor.

But you will need explain exacly what are you building, so we can help you

Its for a winch launch.
Bungee launch has to be tailored to the weight of each model, with selection of diameters and lengths of bungee cord.

Linear speed of 0 to 20 ms.
Rotational , max 100RPM on the spool.

Model weight from a few ounces up to 10 Kg.

I am guessing a BLDC motor with VFD drive would fit the bill but despite commercial availability and learned articles i can find little in the way of practical circuits.
I doubt my skills are up do designing a VFD.

I wondered if using a BLDC controller chip with an acceleration control input may work.

Winches i have seen appear to have geared starter motors but i suspect that 500W will be suffuicient.

Just a regular DC motor with a PWM control should work fine. It may take some experimentation to find the right setting to give the torque you want but it doesn't need to have any feedback.

An H bridge motor controller will allow you to reverse the motor too.

May or may not be relevant in this instance but winches used in many countries associated with organisations like MAAA disallow the use of any electronic control of the winch motor. We used series resistors (allowed)made from "suck-it-and-see" varying lengths,width and thickness of stainless steel switched by solenoids. Motors and batteries also had strict parameter limits. From memory, common use motor was slightly modified(allowed)A40 motor. Even club comps had to comply.

My opinion is to use a DC motor with a DC ESC (electronic speed control),
using the servo library you can control the speed easy without problem using a
potentiometer or digital button, the torque will be pretty high.

I only know about full-size gliders but constant speed is very bad for them. Constant torque is the best. Constant power is in the middle.

MorganS:
I only know about full-size gliders but constant speed is very bad for them. Constant torque is the best. Constant power is in the middle.

Yes, my local club has a twin drum, from the engine note it seems to run at full speed with some sort of clutch mechanism to control the pulling force.

To clarify, this is not for competition use.
The ability to launch gliders is a bonus but the principal use is for launching large EDF jets.

High torque is not needed.

The model provides the thrust only a small amount of pull is needed to keep it in a straight line until sufficient airflow across the control surfaces is achieved.

The model ones seem to be regulated by the cable on the drum building up and increasing circumference causing increasing speed.
I think that either torque or acceleration control is required.

MorganS:
It may take some experimentation to find the right setting to give the torque you want but it doesn’t need to have any feedback.

Not a practical method unfortunately , these are £600+ models.

Feedback is likely to be a broken plane or ten.

To clarify, this is not for competition use.

Looks more like a competition use or a the best, nr1...etc competition, if it's not
then start small, do some real research about how real airplanes works..etc

Building an expensive airplane as I understand you want to build with lack of experience you will
end at the end of the day with tears on your eyes because the airplane crashed.

What's your experience in RC modeling and electronics?

As I read you want to build a airplane for winch launch.
For that you need to calculate the weight of the 2nd airplane as total weight that means you need
a bigger and stronger motor to lift up the weight you need, even if the 2nd airplane is drugged on
the run way you will need a stronger motor because using the regular motor at 100% thrust you
will end up burning it.

I'd start by driving the motor with a constant (and adjustable) current given that the motor current is directly related to the torque it produces.

If I really wanted to measure the torque of the motor I'd look at the tension on the tether instead wtih some type of spring-loaded roller pushing or pulling against the tether. I don't think it'd be practical to electronically measure that tension using a strain gauge, however, given the short span of time during the launch; might be able to do it with microswitches to test the position of the roller instead.

Might be simpler to mount the motor on a moving/spring loaded platform to test the tension.

Since you’re building a plane launcher you need a steady acceleration profile up to full speed. I helped my son do this many years ago for a school project with a dc motor. My method was to put a negative temperature corefficient thermistor in series with the battery to give a gentle start . Arduino’s weren’t available then!

You could use the arduino to output a steadily increasing PWM value to the motor - updating the profile from a look up table say twice a second.over a period of a few seconds.

To deal with different loads, either make up different profiles or run it slower for heavier aircraft.

regards

Allan.

Domino60:
Looks more like a competition use or a the best, nr1...etc competition, if it's not
then start small, do some real research about how real airplanes works..etc

Building an expensive airplane as I understand you want to build with lack of experience you will
end at the end of the day with tears on your eyes because the airplane crashed.

What's your experience in RC modeling and electronics?

As I read you want to build a airplane for winch launch.
For that you need to calculate the weight of the 2nd airplane as total weight that means you need
a bigger and stronger motor to lift up the weight you need, even if the 2nd airplane is drugged on
the run way you will need a stronger motor because using the regular motor at 100% thrust you
will end up burning it.

I only fly small stuff and have several gliders i would like to play with.

Couple of the venues i use have short grass strips , good flyers find them challenging to get EDF off the ground although some can.
Plenty of experience in electronics, and some RC going back 30 years.
Just one of a couple of ideas being chucked around.

Chagrin:
If I really wanted to measure the torque of the motor I'd look at the tension on the tether instead wtih some

I looked at that first.
The angle that the wire leaves the winch increases as the model rises however, very difficult to implement although it could work if using a return.
A return is a pulley at the upwind end, the winch is at the launch point next to the model.
With gliders though the tension can increase as the glider rises.

I am trying to find out more from my local gliding club at the moment as to how big stuff works.

allanhurst:
You could use the arduino to output a steadily increasing PWM value to the motor - updating the profile from a look up table say twice a second.over a period of a few seconds.

To deal with different loads, either make up different profiles or run it slower for heavier aircraft.

regards

Allan.

Worth a try as it should be relatively easy to implement.
There is the problem of getting the data without wrecking some models though.

I've done a fair bit of real gliding ( good fun!) and the man on the big winch
1/2 a mile away does it by feel - don't know how - I haven't been trained.

ask a winch driver

regards

Allan.