Sail winch servo

[David Norris]

I’m sorry, I know this has been covered elsewhere but I can’t find it right now. 
I can turn the drum and servo with my finger and thumb with minimum pressure. If I try to hold my boat by the keel and flop it from side to side, to assess the twist, the sail goes out. When I switch off the boat, the winch winds under the tension of the elastic (too tight?). It’s a V2 servo with only light usage and seems to be performing well under sail.

Any thoughts?

David

[Peter Baldwin]

David

I take it that you are referring to a DF65 or 95 winch (as this isn't mentioned and is in the generic RC Equipmemt and Electrics area).  If so, in my experience then the eleastic should not turn the winch drum even when set to a close hauled position when power is turned off.  Either the tension is too tight or the elastic is too strong or even both of these.  When sheeted out there should only be a slight tension left in the elastic which stops the winch line jumping off the drum and no more.

If the elastic is too strong or too tight then you will increase battery drain (as the winch is always working harder especially whilst close hauled) for no end benefit.

Peter

[John949]

If the winch is moving under the pressure of the elastic then it is way too tight.  What elastic are you using?  Fishing pole elastic is the best stuff as it can extend 6 times its original length without damage.  The only problem with it is that it degrades under UV so needs changing regularly if exposed to sunlight.

I suggest you compare your servo to someone else's to see if it's faulty. 

Servo position control is probably too complex to explain in a few paragraphs here but nearly all the servos I have tried are actually very badly designed for use as a sail winch.  For most applications it doesn't matter much if the servo position moves a bit as the load increases as you are interested in the effect of deflecting, say the rudder, not the precise angle to which it moves.  This is the opposite to a sail winch where the you want the fully sheeted in position to be the same regardless of the load being applied to the winch.  The control law to do this needs to integrate the error over time (the I term in PID) but most servos I've played with don't do this.  Analogue ones increase the current in proportion to the error between the set point and the feedback signal hence the torque is proportional to error and it's not until the error reaches quite a significant value that the winch will produce its full torque.  It seems that most digital winches just implement this very crude algorithm digitally.  Some sail - winches get over this problem by simply being way too powerful (e.g. RMGs) -  If the servo can pull 10Kg then it doesn't move much if you pull it with 1Kg, others just 'give' noticeably if you pull hard on the sheet.  

Try twisting the drum on your winch and hold it at a slight deflection.  Does the force required to do this stay the same or does the winch fight you and increase the force trying to turn it back to the original position?  The only winch I've found that does have an integral component in the control law was one of the latest RMGs, which is ironic as with this better control law they could have made it much smaller and lighter.  

[David Norris]

Thanks both.

Yes, DF95.

I did increase the tension on the elastic after having the line jump off the drum twice during racing on the same day. Sounds as if I have overdone it. I originally used the thin black elastic supplied by RC Yachts, but put a stronger piece on recently. I will ease it off.

I’ve not heard of fishing pole elastic but my neighbour is a fisherman, so I will tackle (no pun intended) him.

David

[John Bennett]

I use 1mm or 1.2 mm pole elastic and only have a little tension on it when the sails are sheeted right out. I buy it in my local GO Outdoors fishing section, needs changing about once per year.

[David Norris]

Thank you, John

D