Need help with design of electromagnet

[queenidog]

I'm making a Phantom Chess game where a computer and stepper motors move players around on a chessboard. The pieces are made with N and S magnets, and an electromagnet is to pull them around. I have a Gauss meter and tested a number (about 30) solenoids and found one that gave pretty good results and gave the highest flux reading of 1452 Gauss. (Commercial EMs that had a 5kg rating were around 300 Gx). The EM (it is a solenoid with metal core) easily pulled the players around with a 2mm sheet of aluminum between them. HOWEVER...there is more to go between the player and EM: a PCB with Hall sensors on it and a 2mm acrylic sheet. I tried it...doesn't work, distance too great, so I need more oomph! The solenoid pulls 250 mA at 12 volts and any attempts to raise the voltage only heats the solenoid up. I will be using an H-bridge to change polarity and likely PWM from an Arduino MEGA. Everything is in place and ready to go as soon as I can get the EM working, so I'm at the point where I want to build my own EM. I bought a roll of #38AWG magnet wire at 2381 ohms/km, I can make bobbins with my 3D printer and EM is only slightly constrained physically because it has to fit in the trolley that moves the players around, roughly 30mm x 30mm. (My current EM is 16mm dia x 30 mm. It sits on a spring to keep the magnet surface in contact with the bottom of the board. My players have metal surround base to concentrate the magnetic field downward to minimize side-to-side attraction (the moving player has to move between other players. Square size is 40mm. My project is here: https://www.raiderracing.com/Engineering/PhantomChess.html Skip down to 4th Build, because that is where I'm at.
WHAT I NEED: Information on how to wind the coil, a calculator so I can determine how many turns I need for the bobbin size. Calculating the flux reading is not necessary: I'll just make the EM with as many coils as I can, then TRY IT. I've also got some #42 wire coming from China. Everything I've seen on the web is how to make an electromagnet with a nail that attracts paper clips....

 

[Lightium]

https://coil32.net/download.html

 

[queenidog]

I have more info... #38 wire has max 130 mA current, so more coils requires higher voltage to maintain that current. So my 12 volt power supply may not work. My small coil of #38 I bought is 5300 feet, or 1 mile long. the thickness of the wire is 0.101`6 mm. So I'll look for a calculator that may be able to tell me approximately how much magnetism I will get...as a start. If it's higher than my solenoid, that would be good. I still wonder about magnet parameters, is short and fat better than long and narrow? How long does a pulse have to be to get max magnetism (without burning out the coil?).

 

[rjenkinsgb]

Look at "cup" style electromagnets - they have an outer cup jacket so the opposite pole is around the centre pole.

From what you say, that should fit well with the chess piece magnets have an outer ferrous body (hopefully also "cup" style to give a field path from the top of the magnet).

Example:

Electromagnet with 10-32 UNC Mounting Hole - 12V DC/2W - 3/4 in. dia x 1-3/16 in. thick - 9.92lbs Pull

*Measurements have been rounded to the nearest fraction inch - please check the product description for specific measurements.*Easy to use electromagnet for lifting and holding applications. These magnets require a power source; when electricity is passed through them they produce a strong...

www.first4magnets.com

 

[queenidog]

I have three of these cup-style magnets, in fact is the one I started with. A couple other guys tried using the same thing, but we all failed. One my magnets says it has a 25kg load!! Yet, using my Gauss meter, the commercial magnet has a value of about 500 Gauss at 1.35A compared to my much smaller solenoid of 1452 Gauss at 250mA. The only difference I can see based on these values is number of turns of the coil (more with the solenoid).
Using various calculators and the information I have, I calculate 163 Gauss for my electromagnet which is 10x too low!!

 

[rjenkinsgb]

If both the electromagnet and chess piece have similar diameter cup magnets or armature surrounds, the effect should be to repel as the magnet moves off centre and add some lifting force to the chess piece, helping it move.

If the outer part of the piece is just an isolated ring, I'd expect the effect to be very different, attraction rather than repulsion, making them harder to move.

I'd also suggest using some PTFE wear tape such as Tygaflor or similar glass fibre reinforced self adhesive PTFE tape on the bases of the pieces, to minimise the sliding friction.
(Amazon sell some cheap ones).


The internal field strength of a coil is simply Ampere-Turns. 100 turns at 0.1A = 10 ampere-turns.
1000 Turns at 10mA is the same strength.


Magnet "lifting force" is how strong it is with a thick ferrous object in direct contact with the poles.

The distant effects are vastly less, and very much down to the design of the polepieces, to give the optimum interaction between the objects.

What magnets are you using in the pieces?

 

[queenidog]

I can test the EM with one polarity and just change the piece I'm testing. White has North magnets and Black has South magnets. I'm using neodymium 8x3mm magnets, not sure what grade they are, but they have strong attraction to steel. I like the idea of PTFE tape. At one time I was using felt. Yeah I understand direct contact is a lot different than not! With just the 2 mm aluminum plate, all is well, but add the combined 6mm of the sensor board and top acrylic sheet, magnetism is lost. Photo shows the electromagnet which rides close to surface (due to spring tension)of the aluminum sheet above. I have videos... According to one online calculator, 3000 turns of wire carrying 130 mA of current, with coil length of 30mm, the field strength is 13000 A/m with a resulting flux density of 16336 uT or 163 Gauss! That value is 9 times LOWER than the old solenoid I'm using.

 

[rjenkinsgb]

The magnet you have in that photo is not a proper cup cased style like the link I posted - it only has some small fingers at the outside.

A quick google search just found this - is uses a cup magnet, at 7mm separation according to the drawings:

Automated Chessboard

Automated Chessboard : Welcome, I like playing chess game and up to now I played a lot on the Internet or with different Apps. But I was frustrated to pass all this time behind a screen. I started to look into the electronic chess games, but I rapidly realize that the b…

www.instructables.com

 

[queenidog]

The one you show at that link (where all this started) is exactly the one I used at the beginning and it sucked. The one I showed in the photo is an old solenoid with 3x the magnetism. Attached are two of the button type (I have a 3rd) beside an old solenoid.