Plunger designs and using Leobodnar load cell modules #1
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Hi @Locoblade. I have to confess that I don't remember exactly why the tolerance was so big. I think my friend with the printer suggested that the model could bleed about 0.2mm, but that doesn't quite explain it. I can say however that the slack is not noticeable in practice. I think partly this is because the amount of movement the plunger undergoes is very slight. I realise now that I forgot to include the original Fusion files in the repo. I'll add them later this week to allow you to experiment if you like |
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Thanks for getting back to me, yeh you're right about the small amount of movement probably helping but having the Fusion files to play with would be great, many thanks! |
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Just thought I'd update as a friend did a test 3d print for the base of the piston for me today and I had a couple of hours to play around making and trying a variation for the plunger with bits I had lying around which may be an additional source of ideas for others. I used a 10mm bolt with the head chopped off as the basis to give a smooth section for passing through the lower housing, and I decided to try and dampen the plunger from below using some rubber (an old MTB handlebar grip I cut up) rather than use a spring. Initially I had the top of the plunger full length inside but I found with the rubber compression alone there was too little travel and it was too stiff so I found a small spring, chopped the top of the plunger down by about 10mm then put a sliding collar and a couple of washers on top which pushes against another larger ring of rubber in the top housing (a cut piece of car radiator hose). This way when the pedal is depressed the collar and spring start to compress first to give an initial soft-ish pedal (the rubber in the top is primarily there to give the top of the plunger some space to move into, it doesn't add much to the damping), then after about 10mm of pedal travel the top of the plunger touches against the top of the housing at which point the rubber at the bottom starts getting compressed to ultimately push out of the bottom of the housing and against the load cell. I haven't actually received the load cell yet due to postage delays so may need to adjust the damping and the throw a little so once that's nailed down I'll get rid of the upper washers on the plunger and make a single collar exactly the right length, but I put everything back together as-is still working with the original potentiometer and the pedal feel in-game is already a load better.
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oh, that looks pretty cool! I'll knock up something similar over the weekend and give it a go too |
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Also, I forgot to mention, but I have some spare PCBs if you want one |
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Cheers, just sent you a PM on RaceDepartment rather than fill Github up with our chat :) |
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Hey @Locoblade - I started to get the parts to replicate your idea, but I noticed something I wanted to check with you. Is the lower rubber tube intended to be compressed? If so, I am not sure how mechanically this will work since the bottom of the plunger needs to be in contact with the top of the load cell. I suspect trying it without the cell is giving a nice feel because the plunger is moving a fair bit. This is really a downside to the design and in hindsight I would have ensured the loadcell was offset to the axis of the plunger to allow it to travel. With the design as it stands, you need the compression at the top of the plunger, but I think you could simply increase the length of the top rubber to allow it to compress once the spring has compressed? |
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Without having the load cell to hand I wasn't sure if it would provide sufficient resistance on its own or how far it travelled plus I also thought having something there to push the plunger back up could give the option to set it up not quite touching the load cell at rest creating a short dead zone with zero output before it starts to activate much like you get in a real car taking up the gap between pad and disc. I see what you mean though and you may well be right as that amount of rubber in addition to the load cell may well be too stiff and once my plunger has engaged with the load cell the only thing providing any additional pedal movement is the remaining travel in the spring if the plunger hasn't already hit the top of the upper body, so I may need to add more rubber damping at the top like you suggest. |
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The loadcell deforms a little, but it really is a tiny amount and I suspect you will be unlikely to notice it via your foot. I'd just recommend having a bit of a play once you get the loadcell as it will make more sense once you get to experience the amount of flex it has, but I just wanted to forewarn you that your current idea might not work exactly as intended in it's current configuration. It kind of depends on how 'hard' you want the feel in the end though. Version one of the model https://github.com/GeekyDeaks/g29-load-cell/blob/master/stl/dual_holder.stl had very little space for compression and gave a very 'racey' feel, but it just felt too hard for me and I wanted a more spongy 'road car' feel - or as my friend put it - they need bleeding :) |
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Just to update what I ended up with for the pedal setup (with an extra locking nut on the bottom I put on after the photo was taken). Note I've got rid of the rubber under the plunger and flipped the bolt around so the spring and sliding aluminium part is now on the smooth section, I did this as I found the spring was grating against the threads when the other way around which made the pedal really notchy to use. This way although the thread is passing through the damper body hole, like you I find the movement is small enough that it doesn't have a significant effect on smoothness. This car radiator hose goes in the top section of the damper body to allow the sprung rod to move. It adds a small amount of give but you could use something more solid like wood or plastic with a hole in it if required The sliding aluminium plunger needs to be long enough to sit partially inside the lower body of the damper at rest to prevent it catching on the lip when depressed and locking up the entire mechanism Not a great photo but total length of the rod is ~63mm and the spring is just under 20mm. Not sure where the spring came from or it's strength unfortunately as I found it in my "box of odds and sods" in the garage, probably compresses fully with about 10-20kg of force at a guess. The aluminum plunger (not shown) is about 15mm long. Also for any PC user who's not confident enough to knock up your own electronics, you can use a Load Cell USB Controller from LeoBodnar instead (http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=94&products_id=183 or http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=297). To connect the load cell to the USB controller you need a couple of 1kΩ resistors to create a "wheatstone bridge" with the 3 wire load cell, this gives you the 4 connections required to connect to the USB module. |
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Quick update with completed photos of the Leobodnar install, there's not too many places to mount the board inside the pedals as there's lots of strengthening ribs everywhere, but found a home for it and hot glued it in place on it's side next to the brake pedal along with the little home made resistor circuit. I also made small notches in the plastic ribs for the accelerator and clutch wires to pass through and hot glued them in place so as not to be trapped when putting the lid back on and I had to make one small cut in a rib to allow for the USB cable end to pass through. I also took the carpet grip out from the bottom of the pedals as I mount them on a wheel stand and taking that out gives you a couple of openings to pass the USB cable out from inside without having to drill any holes in the exterior of the casing, and I used a short 30cm USB Type B male to female extension cable so I didnt have to trap a long USB cable inside the housing for connecting directly to the PC (full shopping list at the end of this post).
Here's a full shopping list for anyone wanting to go down this route, over and above making the load cell pedal itself
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Locoblade
changed the title
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Two 50kg cells in series would increase the maximum weight by around 100kg or just the precision? |
Just the precision as it will double the voltage range from the wheatstone bridge. You'll need to put them in parallel to also increase the range of the weight as that will distribute the load across the two. |
Hi, many thanks for sharing all this, I'm currently in the process of sourcing the parts I need to mod my G25 pedals and this has been invaluable.
Just one query I've got, looking at the STL file I noticed the 3D printed part has a 11mm hole which seems quite big for a 10mm bolt to pass through and I was just wondering whether you'd had any issues with the bolt binding on the plastic or not sitting square in the hole and whether there'd be any benefit to making the hole a slightly snugger fit, say ~10.2mm?
cheers
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