Linear actuator/solenoid

[pandos]

I am putting together an old autopilot chain drive. In other threads I have sorted the electronics so that it will be run by the gubbins from a wheelpilot,

I still need to push a lever within the drive mechanism to engage a gear. I only require about 20mm throw and very minimal force.

Originally this was achieved by a plethora of old electronics and a large solenoid and a spring. I suspect this would be heavy on battery power as it would be engerised whilst the pilot was switched on.

I thought about a linear actuator but in a power cut/ fault I think I would require power to disengage the drive from the steering system.

I have thought using an electromagnet to hold it in place (latch) after it is pushed into place using a solenoid driven by a momentary switch.

It would then fail open if power was cut via the clutch circuit on the smart pilot.

Does anyone have any experience of using electromagnets for long periods? Or have any suggestion for alternative approach?

 

[William_H]

Your desire for power fail release really means power needed continuously for operation. You may be able to conjure uopa circuit to reduce current in solenoid after actuation. If you can cope with needing power to deactivate then an actuator as used in car door lock mechanism might suit. Having a small motor drive a rack to actuate reverse power to drive release. ol'will

 

[pandos]

Your desire for power fail release really means power needed continuously for operation. You may be able to conjure uopa circuit to reduce current in solenoid after actuation. If you can cope with needing power to deactivate then an actuator as used in car door lock mechanism might suit. Having a small motor drive a rack to actuate reverse power to drive release. ol'will

Yes I think you are correct; that failsafe pretty much requires power maintained during operation.

I am hoping there is an option that I don't know of...Kind of an unknown unknown....


My fear is having an autopilot go hard over, blow a fuse and require power to release the steering system for manual steering. (Been there done that on a 44 foot boat in strong winds many years ago with a hydraulic ram)

 

[KevinV]

Yes I think you are correct; that failsafe pretty much requires power maintained during operation.

I am hoping there is an option that I don't know of...Kind of an unknown unknown....


My fear is having an autopilot go hard over, blow a fuse and require power to release the steering system for manual steering. (Been there done that on a 44 foot boat in strong winds many years ago with a hydraulic ram)

Could you power the actuator from a seperate circuit - then the autopilot fuse blowing wouldn't affect your ability to disconnect it? I think it would even be possible to make that happen automatically - in the same way an alternator light works, that when the voltages are balanced nothing happens, when they're out of balance current flows

 

[pandos]

Could you power the actuator from a seperate circuit - then the autopilot fuse blowing wouldn't affect your ability to disconnect it? I think it would even be possible to make that happen automatically - in the same way an alternator light works, that when the voltages are balanced nothing happens, when they're out of balance current flows

You may be onto something.

I think what you are suggesting is essentially a secondary circuit to reverse the actuator, if the main power is lost, may be an option.

Electromagnets coming from Amazon Monday so I'll have a better idea of how they might work then.

 

[William_H]

Use 2 diodes for power supply to actuator to disconnect. One feeds power via normal circuit one feeding power via a separate circuit for back up. ol'will

 

[B27]

The current needed to hold the solenoid may be a lot lower than the current to move it into engagement?
Maybe as it reaches the end stop, it could operate a micro switch cutting the current to the 'hold' level.

 

[barryhall]

I am putting together an old autopilot chain drive. In other threads I have sorted the electronics so that it will be run by the gubbins from a wheelpilot,

I still need to push a lever within the drive mechanism to engage a gear. I only require about 20mm throw and very minimal force.

Originally this was achieved by a plethora of old electronics and a large solenoid and a spring. I suspect this would be heavy on battery power as it would be engerised whilst the pilot was switched on.

I thought about a linear actuator but in a power cut/ fault I think I would require power to disengage the drive from the steering system.

I have thought using an electromagnet to hold it in place (latch) after it is pushed into place using a solenoid driven by a momentary switch.

It would then fail open if power was cut via the clutch circuit on the smart pilot.

Does anyone have any experience of using electromagnets for long periods? Or have any suggestion for alternative approach?

What you need is a car overdrive solenoid, they switch to a light holding coil once engaged.

 

[B27]

I sometimes used to sail on a boat with a wheelpilot.
A simple manual lever engaged the motor drive.
Or if the electronics were switched off, it was a handy way to lock the wheel.

Maybe it could be done remotely by a bowden cable or something?

 

[pandos]

What you need is a car overdrive solenoid, they switch to a light holding coil once engaged.

That looks like the solution..thanks.

I sometimes used to sail on a boat with a wheelpilot.
A simple manual lever engaged the motor drive.
Or if the electronics were switched off, it was a handy way to lock the wheel.

Maybe it could be done remotely by a bowden cable or something?

I had thought of that but the motor will be under a bunk in the rear cabin so a good long cable would be needed.

The current needed to hold the solenoid may be a lot lower than the current to move it into engagement?
Maybe as it reaches the end stop, it could operate a micro switch cutting the current to the 'hold' level.

There's only one connection and a negative body on the overdrive solenoids that I have seen so I guess they drop their consumption automatically.

.8 of an amp per hour is quoted on some sites for an MG solenoid..

Thanks to all for the suggestions... 40 years ago I recall a mechanically minded friend of my father had a very dodgy wire and switch stuck on the dash of his car which "put in the overdrive" I'll finally see what was on the other end of that wire....we live and learn....

 

[alahol2]

Search for Dual Coil Solenoid. The Pull coil uses a big current but the Hold coil uses low current.

 

[Alex_Blackwood]

Search for Dual Coil Solenoid. The Pull coil uses a big current but the Hold coil uses low current.

Or something like this. Latching Solenoids - Strong Hold Position No Power Required

Portchester up early this morning!

 

[William_H]

Or something like this. Latching Solenoids - Strong Hold Position No Power Required

Portchester up early this morning!

As I read that link it will still require a current in reverse direction to unlatch the solenoid. Not what oP wanted. ol'will

 

[Alex_Blackwood]

As I read that link it will still require a current in reverse direction to unlatch the solenoid. Not what oP wanted. ol'will

Even if it does, I don't think that is beyond the wit of man to fit and extra "Switch" to do that. One button for "On" one for "Off"

 

[Cariadco]

On the Raymarine Electric Ram, there's an electro clutch, so if power fails for whatever reason, the ram will be freed off, and not jam the steering. In passing, this clutch is also an emergency release, for the helmsman to apply a large turning force on the helm will release the clutch.

 

[pandos]

Even if it does, I don't think that is beyond the wit of man to fit and extra "Switch" to do that. One button for "On" one for "Off"

The object is to achieve a system which will be safe in use in the event of a power loss.

This could be achieved with a linear actuator and a complex series of relays and a secondary power supply but I'd prefer the simpler "fail safe" of a device that simply let loose if it is not powered up.

The locking solenoid is similar to the actuator in that it will require juice to disengage it

On the Raymarine Electric Ram, there's an electro clutch, so if power fails for whatever reason, the ram will be freed off, and not jam the steering. In passing, this clutch is also an emergency release, for the helmsman to apply a large turning force on the helm will release the clutch.

I am not sure if actuators generally have this facility, I think not.

What I don't yet know is how much force?/ friction?/inertia? the un-powered motor would put on the system. If this is not substantial then even if it stayed connected physically, but powered off I may still be able to steer manually, so the simpler solution would be ok, but if the wheel is effectively locked, the solution will require to be closer to fully failsafe..

 

[alahol2]

Try this, see if it matches any of your requirements...
Dual coil solenoids

 

[pandos]

Try this, see if it matches any of your requirements...
Dual coil solenoids

Interesting document. Plenty food for thought.

 

[Alex_Blackwood]

Try this, see if it matches any of your requirements...
Dual coil solenoids

Possibly his best bet as they will "auto release" on power failure. Loads of different ones on the market if you do a search for "Dual coil solenoid" Other possibility is to use a relay and fit an "Economy Resistor". Again do a search for that. One thing to be careful off is vibration. With any of the above the lighter current may not hold if heavy vibration. You need to look at the circuit and installation as this could possibly be a problem. Probably not but just raising awareness!