Re: Re: Whoooosh..thud..oh bugger

[Martin Howell <martin.howell@xxxxxxxxxxxx>]

Hmm, this unit from RS gives the 'soft start' I'm looking
for, but not the 'soft stop, which is where the real problem
lies.  I agree with your analysis of the drive train
issues, but I'm concerned that strengthening the cables and
applying more tension will just move the force to the next
weakest part of the drive - the cable fails at present
because of a joint, inserted partly to prevent breaking a
more difficult to fix part of the drive.  I'm looking at a
more simplistic system of forcing a slow down at each end of
the movement, watch this space to see if it works, or if the
whole contraption bursts through the wall into the garden :-)

Martin

David Cole wrote:
> Martin
>
> Sounds to me like you need a PWM (Pulse Width Modulation) drive to
> efficiently control the speed of the motor.
>
> Something along the lines of 510-1209 at RS should do it, this would
> give the acceleration and deceleration whilst also limiting the
> current if the door jammed. Alternatively google for PWM DC drive
> circuit
>
> You want to ensure that your drive train (cables) are as stiff as
> possible, this is why industrial applications usually use rack and
> pinion drives. This may mean increasing the diameter of the cable to
> ensure that you don't have any stretch which causes jerk. It sounds
> like you have a lot of inertia in your system it may help if you add
> additional gearing which will reduce the speed, increase the torque
> but also decrease the amount of reflected inertia seen by the motor.
>
> Hope this helps
>
> Regards
>
> Dave
>
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