step speed regulation problem

[tomstudios]

hi all,
I have been given the project of converting a step that the company i work for fit into the side of vans for disabled access from manual to automatic, Anyway i have converted the step to chain drive and am using the http://www.bosch-ibusiness.com/boaaelmoocs/category/CHP/283/product/803 bosch chp 12v Motor I am unable to find a motor that has the apropriate 5/6rpm as the motor i am using at the moment  so i would like to stick to the motor present wich brings me to my question, how could i bring the rpm right down without sacraficing torque i have tried using a 20a rated pwm but once wired up to a car battery the rpm could be achieved but looses all torque and cant succesfully lift the step! has anyone got any ideas on speed regulators e.t.c, my second question is as the nature of the step being driven on the pivot as the motor rotates the step is working against the motor until it reaches a certain point and then starts working with the motor speeds up and slams shut! is there anyway of regulating that! p.s there is not allot of room for gear reducions/increasement.
(pics of step https://flic.kr/s/aHsk4TgeyG)
many thanks
tom

[SeekingVision]

If you are using a commutatorless motor then you can pulse and break with PWM at a slow frequency and get good low speed torque. If it is a standard DC motor it would need a pretty hefty commutator and brushes to last long with this method. I've no idea of the longevity you can achieve if such methods are employed, are you planning on making your own PWM?

[tomstudios]

thankyou for your reply,
its a motor i picked up from the spares pile at work haha
i believe its just a standard dc brushed motor!
i was planning on designing and building a mosfet based pwm wich i am assuming would need voltage regulator pot and frequency adjuster ? it would also have to be able to take 20a as a max current do you think a gearhead motor like so would be able to be reduced to such an rpm and torque not be sacraficed ?
cheers
tom

[bdeuell]

you could use a feedback controller (with an encoder or other sensor to measure the motor speed) to ensure constant velocity by enabling the PWM controller to ramp up the duty cycle when the load increases. this is not a particularly simple solution so it would not be my first choice.

On a related note you could look at back EMF speed control. essentially the motor windings are used to sense its own speed and adjust the PWM control accordingly. 

If you are truly stuck with the current motor i would try to find a mechanical means of slowing the speed down. can you fit a smaller sprocket on the motor?

perhaps a cable drive would work for this application, it looks like you only need 180 degrees of motion for the step. im picturing a cable that is attached to the outside of a pulley at the step. then a threaded rod at the motor for it to wrap around. ideally you would custom cut a helical groove with a rounded profile to match the cable but a standard thread can work. che cable would then continue back to the pulley at the step and attach to the same place or near where it started.

[mklrobo]

  Hello!
If speed, cost, and design time are of concern, I would suggest the following;
Aquire a DC electric wench, and work that into your design. Cheap and fast,
the gearing is already there, it is made for vehicles anyway.(enviroment, power, etc)
Your cable could be run through a pully system, to equal out forces on both sides of the
step. Limit switches must be installed, to limit the mechanisims range, as a failsafe.
These electric wenches are built pretty well, and offer peice of mind in securing a
step when somebody really needs it. Good Luck!