Mechanism to move a platform at a constant velocity

[graceless9]

Hi all,

I'm new here on Physics Forums and can't wait to delve into some juicy topics!

I have a very practical problem that I need help with. I am doing a home project where I want to move a platform on wheels (loaded to a maximum of 70kg) in a 1.5m long straight line across the floor at a constant, and known velocity . There will be some trial and error before I can define exactly the velocity, but I'm guessing it will be around 1km/hr.

So far, I am thinking of attaching the platform to a rope, which can be passed through a roller anchored on the floor to direct the rope to a pulley on the roof which could then be attached to a weight (something like my dodgy diagram!). I'm guessing that the velocity will be somehow determined by the mass of the weight but I'm unsure how to quantify this.

Thanks so much, looking forward to hearing your input!

 

[anorlunda]

You do not say how precise the constant velocity must be. If it was me, I would look to use a stepper motor controlled by an inexpensive Arundino or Rasberry Pi computer, to achieve a very precise and controllable velocity.

If you want to do it mechanically, then think of how similar your project is to that of a clock. Clocks try to keep the hands moving at constant velocity. You might visit a clock museum to see the large number of mechanical solutions to this problem.

 

[Hesch]

1.5m long straight line across the floor at a constant, and known velocity .

If you want it to be exact ( exact velocity, no follow-error, etc. ), you must use steel wire/band ( not rope ) around some pulley connected to a servo motor. That's how they do it within graphical industry.

 

[A.T.]

I'm guessing that the velocity will be somehow determined by the mass of the weight but I'm unsure how to quantify this.

The acceleration will be determined by the masses of the weight and platform. The velocity will not be constant.

 

[graceless9]

You do not say how precise the constant velocity must be. If it was me, I would look to use a stepper motor controlled by an inexpensive Arundino or Rasberry Pi computer, to achieve a very precise and controllable velocity.

If you want to do it mechanically, then think of how similar your project is to that of a clock. Clocks try to keep the hands moving at constant velocity. You might visit a clock museum to see the large number of mechanical solutions to this problem.

Awesome idea with the clock! Although I'm looking for precision within 5% so maybe the motor is the way to go and a lot easier. Excuse my ignorance, this is very new to me, but I've looked up the stepper motor and it seems so small - how would I connect it to drive this fairly large load? Were you suggesting to the wheel axle underneath the platform or connected to a wire (as Hesch suggested) that passes through the pulley?

 

[Hesch]

Although I'm looking for precision within 5%

That's a lot.

I thought you meant within say 0.05%. The accuracy of the rotational speed of a (digital controlled) servo motor depends of the accuracy of its crystal oscillator, which could be within 10 ppm ( parts per million ). So how accurate can you make the diameter of the pulley wheel?

 

[graceless9]

That's a lot.

I thought you meant within say 0.05%. The accuracy of the rotational speed of a (digital controlled) servo motor depends of the accuracy of its crystal oscillator, which could be within 10 ppm ( parts per million ). So how accurate can you make the diameter of the pulley wheel?

Yep! This is just a rough home project! Not sure about the accuracy of diameter of the pulley wall as yet, my focus at this early stage is just finding a way to move this platform at a reasonably constant rate and have the ability to adjust this rate. What would be the advantage of using a servo motor over a stepper motor?

 

[Hesch]

What would be the advantage of using a servo motor over a stepper motor?

If a stepper motor is loaded by a heavy mass/big inertia, it could easily loose the synchronization ( it will "fall out" ). Even a synchronous servo motor will never loose synchronization as the digital controller will prevent that. It will just yield its maximum torque when overloaded, and will try to come up in speed.

Furthermore a (synchronous) servo motor is not turning "step by step". It turns completely smooth.

 

[anorlunda]

Hesch is correct in what he says about servo motors, and about wire rope. But a servo motor and controller powerful enough for your project can be quite expensive.

A low cost way, is to find a used sewing machine, and adapt it to your project. The good ones use powerful servo motors and they include built in controllers.

The mechanical way could be fun too. A wire rope, a pulley, a weight, plus an escapement gear and a pendulum would work well, and it would entertain your children and guests.