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Linear Actuator
#1
Love the Pypilot project. Have been following it for some time.

I have always wanted a below deck linear actuator but have not wanted to pay for it. Over the last few months Pypilot has inspired me to try to come up with a "homebrew" unit.

This has crossed my mind in the past but each time the biggest challenges always seems to be how to implement a clutch system that doesn't drive the price up to the cost of a commercialy available unit.  Recently there has been a sell off of Ogura micro clutches on eBay for about $25 USD. Ogura sells high quality electromagnet clutches of all sizes.

Using the specifications of the Raymarine Type I linear actuator as a general guide I pieced together an electrical linear actuator using a ball screw, planetary geared motor, Ogura clutch, stainless tubing and some miscellaneous aluminum parts/hardware.

           

The current configuration is for bench testing purposes and has the long lower square tube to allow for load testing. The aluminum mounts for the motor and clutch are crude and will be refined in the next revision.

Here are the test results so far:
  • Free travel requires about 22-25 LBS of force to manually move the actuator with the clutch disengaged.
  • The clutch will not slip until it reaches 850 LBS of force (measured) at 130 watts of power (The clutch is the limiting factor for power output at this point)
  • With the clutch engaged and the actuator at rest it consumes 7 watts
  • No load movement is 17 watts
  • When the unit is producing 130 LBS of force it consumes 32 watts
  • Stroke length 11"
  • Stop to stop travel time 10.5 seconds
Lots of testing still to do but it is looking good so far.

So far it has cost a total of about $325 USD to piece together. Pretty sure it the pricing could drop with better parts sourcing.

Any ideas/comments greatly appreciated.
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#2
It would be interesting to see a price breakdown. What about waterproofing? The sliding seal can add a lot of friction.
  Reply
#3
Hi Sean,

Not sure about the waterproofing, it will certainly have a splash guard, but Raymarine does not waterproof their linear drives that are used below the decks.

Here is the breakdown of the parts:


Qty         Part #                       Item                                         Amount               Where Purchased
1              AMC 10-29            Clutch                                          $29.80                   Ebay
1              MT3608                Step Up Converter                       $9.56                     Amazon
1              SFU1204               1204 Ball Screw                           $57.85                   Amazon
1              M6 x 65mm          Cap Screws                                  $10.09                   Amazon
1              260-5M-1             Timing Belt                                    $8.77                     Amazon
1              210-5M-15           Timing Belt                                    $8.85                     Amazon
1              5M20T                  Timing Belt Pulley - 8MM               $8.55                     Amazon
1              5M25T                  Timing Belt Pulley - 8MM               $8.09                     Amazon
1              5M20T                  Timing Belt Pulley - 6MM               $7.09                     Amazon
1              608RS                   Bearings                                      $5.98                     Amazon
1              6546K55               1" x 3" x 3' 1/8 thick Alu             $44.47                   McMaster
1              5025-2                  1" SS Round Tube .065              $9.99                     Defender Marine
1              44965K493           304 Stainless Adapter                  $9.57                     McMaster
1              93712A400           Partially Threaded Stud 3/8-24    $7.29                     McMaster
1              638284                 HD Preimium Planatary Motor      $39.99                   ServoCity
1              555188                 Motor Mount                               $5.99                     ServoCity
1              IY-5PinRelay-5    Waterproof Relay (5)                     $24.99                   Amazon
1              L298N    Motor Controller                                         $5.00                     Amazon
                Misc Parts & Hardware                                              $20.00  
                                                               
                                                 Total                                      $321.92
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#4
Very interesting.
Also thinking of installing under deck actuator but concerned about the price.

Any particular reason your using a 24v clutch?
Setting current limit to about 10 amps should give plenty of power and keep it from self destructing.

From my perspective:
Concerned about my ability to fabricate some of the pieces.
May be too large to fit in my available space.
As a plus can afford to have spares of any probable failure parts.
I doesn't think waterproofing is a big issue where it is mounted but I'm in fresh water.
Then again in a severe storm water can get in places you would never expect.
Motor is probably the most vulnerable part.

John
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#5
(01-12-2020, 03:05 PM)johnm Wrote: Any particular reason your using a 24v clutch?

Price, I can get one of these for $25 on eBay. Using a boost converter right now. It draws about 7.5 watts engaged including the boost converter.
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#6
(01-12-2020, 05:19 PM)Opie91 Wrote:
(01-12-2020, 03:05 PM)johnm Wrote: Any particular reason your using a 24v clutch?

Price, I can get one of these for $25 on eBay. Using a boost converter right now. It draws about 7.5 watts engaged including the boost converter.

7.5 watts is a huge amount of power just for a clutch.   The pi zero uses about 1 watt, and often the average motor consumption is less than this.    What about using a resistor and capacitor in series with the clutch?   This can limit the power it consumes (to hold) but still allow a larger current to lock.  The usual clutch current of raymarine drive is also around 6 watts so this is really a lot of power consumption.

If you are using a motor which can actually be driven from the L298N: "2) Large output current, the instantaneous peak current can be up to 3A"   it isn't enough for the motor.

Why do you have toothed pulleys and belts if you are using planetary gears?

Also, the cheap ballscrews on amazon may work but are usually 400 grade stainless which means they will rust a lot more than normal stainless.   I am not sure where to find 316 ballscrews, or other alloys though.   I think these ballscrews may work well if there is enough grease and they can stay covered.

I am still not sure which tillerpilot you want to replace...   The belowdecks units such as raymarine type 1 and 2 linear drives using ballscrews have a much larger brushed dc motor than what you selected.     This is a much more powerful and efficient motor.   It uses something like a 3.5:1 reduction from this motor using a toothed belt to the ballscrew and magnetic clutch.   In raymarine units,  these toothed pulleys wear out especially if the autopilot is stressed a lot and eventually this belt skips which acclerates the wear of the toothed pulleys.   I would consider this and either use hdt5m belts at least 15mm wide, gearbox which is rated for 4x the torque you will ever drive it, or have the magnetic or other clutch capable to slip before the the gearbox or belt would.
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#7
[quote pid='11720' dateline='1578879718']

7.5 watts is a huge amount of power just for a clutch.   The pi zero uses about 1 watt, and often the average motor consumption is less than this.    What about using a resistor and capacitor in series with the clutch?   This can limit the power it consumes (to hold) but still allow a larger current to lock.  The usual clutch current of raymarine drive is also around 6 watts so this is really a lot of power consumption.

If you are using a motor which can actually be driven from the L298N: "2) Large output current, the instantaneous peak current can be up to 3A"   it isn't enough for the motor.

Why do you have toothed pulleys and belts if you are using planetary gears?

Also, the cheap ballscrews on amazon may work but are usually 400 grade stainless which means they will rust a lot more than normal stainless.   I am not sure where to find 316 ballscrews, or other alloys though.   I think these ballscrews may work well if there is enough grease and they can stay covered.

I am still not sure which tillerpilot you want to replace...   The belowdecks units such as raymarine type 1 and 2 linear drives using ballscrews have a much larger brushed dc motor than what you selected.     This is a much more powerful and efficient motor.   It uses something like a 3.5:1 reduction from this motor using a toothed belt to the ballscrew and magnetic clutch.   In raymarine units,  these toothed pulleys wear out especially if the autopilot is stressed a lot and eventually this belt skips which acclerates the wear of the toothed pulleys.   I would consider this and either use hdt5m belts at least 15mm wide, gearbox which is rated for 4x the torque you will ever drive it, or have the magnetic or other clutch capable to slip before the the gearbox or belt would.
[/quote]

Thanks for the input Sean.

My goal is to build something out of readily available items and try to keep the cost down making it worth the effort. That has driven many of the choices made to this point. I am certainly no expert, just have always thought about giving it a try.

Definitely will try to lower the clutch draw and adding a capacitor and resistor sounds like a good start.

The electomagnetic clutches that I can find are either very expensive or very inefficient. This made the $25 eBay surplus clutches the most reasonable choice at this point. With that in mind it limits what the unit can be and how it is constructed. If anyone has any suggestions that could work better that would be great.

The L298N was just something I had laying around. Will uses something more robust for sure. Again another thing I'm no expert at. Just started messing around with motor controllers and Arduino for that matter.

The toothed pulleys are just what the clutch came with. It could certainly change, but just wanted to make sure that it could handle the power that was desired. I tried spur gears below the clutch at first (lower set of gears) but there was too much resistance and it caused a much higher resistance to movement of the piston when the clutch was disengaged.

I bought that motor with the planetary gears because it had enough torque to get the job done and turned at the right RPM allowing the proper hard over to hard over time. This motor was also fairly inexpensive for the amount of torque it produced. Any recommendations?

The belts are HDT5M 15mm wide.

At this point the actuator is more of a proof of concept than an actual usable unit so any input is greatly appreciated.
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