MegaSack DRAW - This year's winner is user - rgwb
We will be in touch
Son is working on an electronic control for an old TACX magnetic turbo trainer (to give intervals). He's after a way to pull cable using an electric motor and some Raspberry Pi style controller he has. Problem is that the cable is fairly hard to pull - would a pulley arrangement provide enough mechanical advantage? He's bright and can do the Maths etc, but very stubborn and doesn't accept help readily (wants it to be his own work), so it would be good to just float a couple of ideas with him. He seems certain gears or a cam won't work. I'll take the trainer apart and see what the cable does - see if we can make actuation easier.
Lever? Long pull at small force giving short pull at large force.
2 diameter spool - ratio of spool size gives increase in force.
Gears - see electronic mech with stupid high speed motor giving tiny movements
I'd go for a motor with a gearbox. When I was his age I'd use an electric window motor from a car but these days you can buy stepper motors with a gearbox very cheaply on eBay and they are easy to drive with a pi. Go for a very high ratio of the cable needs a lot of force
First thing to do is work out the force and amount of travel required, then look at standard off the shelf servos (radio controlled modeling would be a good start) that exceeds that requirement. If that doesn't work for budget look at the mechanical advantage that can be gained by using a longer stroke system that can be reduced (lever, gear or hydraulic) to produce a multiple of force with a reduction of throw.
How much force do you need to pull the cable?
What distance?
Be aware that electric motors don't like producing force in a static position for extended periods. This will probably rule out a standard off the shelf servo (as used for rc models) unless you use it to activate something else, e.g hydraulics.
A motor driving a worm gear would work and allow the use of whatever type of gearing or rack you prefer to pull the cable. You would need to figure out how to stop and start it at the end of its travel.
Thanks all. Some stuff I'd considered and some new ideas there. It was the start stop I was concerned about too. Cables currently has a few indexed click stops. Only really need two settings and taking a few seconds to shift between the two is not an issue.
This sort of thing?
He'd need to do some engineering to get correct cable pull
It's an arduino that's controlling it
That'll probably do it. I've used a few of those. For an arduino it can easily be driven with a pololu A4988 board. A couple of quid on ebay: click. Use this to make wiring up easier: click or an arduino header: click
If you want to do it mechanically use a two pulley system.
Number of 'falls' or loops you can calculate.
You reduce effort but have to pull a lot more cable to cover the distance.
You need to measure the force first. Get a spring balance (cheap thing to weigh your luggage) and hook something up. Also need to know how quickly it needs to pull. I'd also look at lead screws, but maybe too slow. If I was doing it, I'd use a pneumatic cylinder, but not really practical at home.
Thanks all. The shift can be fairly slow and gradual. I'll measure the force needed and go from there.
A bit more practical would be an electric screwdriver. Lots of torque.
RC servo? they are usually PWM controlled, so should be easy enough to connect to an adruino, and usually have a decent enough of grunt for the size?
I don't know what level GCSE would be but if you used a geared motor with a pulley for cable pull you could incorporate timing 'teeth' on the outer of the pulley. In conjunction with a proximity or photocell sensor would act and demonstrate closed loop control/feedback.
Telling a PWM servo how far to move is fine but in the real world of engineering - feedback can be essential as you need to know if the gearbox has failed or the pulley has fallen off type of scenario rather than knowing you told the servo to move and then presuming everything downstream is fine. Again might be more than is required at GCSE level.
Have a look on RS - they have a range of geared motors.