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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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V3 Coil Carbine

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Yandersen
Sat Dec 08 2012, 08:04PM
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Aha! Here is the fixed schematic:

So, initial state:
C1 is charged to the [battery voltage-0.6V];
C2 has no charge;
IGBT' gate is at the ground level, as well as SCR2 gate and cathode.

When the projectile approaches the coil, you trigger your IGBT by charging it's gate to around dozen of volts, right? Then current starts to rise in the coil. When projectile is inside the coil, you are switching off the IGBT by lowering it's gate to the ground. During that process voltage across IGBT rises, making SCR2 able to handle the current flow and redirect it to the C2. When current depletes, all unused energy is stored in the C2, and SCR2 closes, capturing the energy inside that cap.
When projectile is outside the coil, you trigger SCR1 to return energy back into C1, so next shot coil will start charging from higher voltage, thus reusing some of the energy from last shot. When current depletes, voltage across C1 is higher than the battery voltage, and C2 is 0.6V below the ground level. SCR2 will not allow the current to flow back into C2, because it's gate at this moment is at the ground too. So we returned to the initial state with only one difference - next shot coil will be charged faster and use little less of battery.
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Ash Small
Sat Dec 08 2012, 08:08PM
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Steve Conner wrote ...

I'm pretty sure some brushless ESCs for RC cars incorporate regen braking. If your theory was right, that wouldn't be possible without destroying the batteries.


Well, there are several factors to consider here, firstly, in the case of regen. braking for RC cars, maybe the batteries are configured so that they only re-charge at the designed rate?...Although, as far as I'm aware, all the Lithium based batteries require the charge rate to be reduced to a few % of the initial charging rate as charging progresses (from what I read when I was designing a simple charger that runs off a 12V car battery for a Nintendo DSi, although they are lithium ion batteries in the DSi.)

I read quite a few stories about peoples cars going up in flames while they were re-charging LiPo's 'in the field', and the insurance companies not paying up because the chargers were faulty.

As others have pointed out, all the 'authorative sources' say that the 'proper charging procedures' must be adhered to for safety and reasonable battery life.

I assume a 'momentary burst' won't overheat them sufficiently to cause them to burst into flames, but it will still degrade them, I'd assume, and repeated bursts will presumably overheat them sufficiently to eventually cause a fire.

Maybe someone should run some tests to see what effect this has on them?

(I'm still of the opinion that, if they are near fully charged, as they would presumably be at the beginning of a day's 'shooting', they are likely to suffer some degradation.)

EDIT: I've just read the three previous posts by Hanzie and Yanderson. I think Yanderson is probably on the right track, placing a diode in series with the battery, etc. As far as Hanzie's point is concerned, I'm still trying to get my head round exactly how recupreated energy is used to slow the car down. Most of the KERS systems in formula 1 use some form of lithium battery as far as I'm aware (Williams and, I think, one other team use flywheels to store the recuperated energy) Red Bull consistently has problems with their batteries overheating, and several times in the past couple of seasons have had to disable the KERS system until the batteries cool down again. Maybe all you need to do is monitor battery temperature?

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Yandersen
Sat Dec 08 2012, 09:11PM
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Sorry for the multiposting, I've just realized that IGBT will actually start to close some time after it's gate will drop down to 0 volts. If so, then this schematic will ensure that SCR2 opens after IGBT is closed:

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ben5017
Mon Dec 10 2012, 01:42PM
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Got some work done on the X3CC this weekend. Both accelerator stages are up and running. As of now, I have no way of compiling performance data. How do you guys estimate velocity? Am I better off buying a cheap chronograph?

Anecdotal results: I was pleasantly surprised with the power from one stage + injector. The results from adding the second stage showed a noticeable increase in power, although I was expecting a bigger jump in performance after how well the first stage worked. This leads me to believe that it is nowhere near tuned at the moment.

Since I finally have a multi stages coil gun, I can now start experimenting and work out ways to optimize the design.


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Hanzie
Mon Dec 10 2012, 03:52PM
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Ash, those RC cars slow down by giving very little reversed throttle like they would when they drive backwards. As the car still drives forward, this works as a brake. I don't know how else to explain it, but as far as I'm aware there's no energy going back to the batteries. This wouldn't really be a whole lot anyways, so it's not really worth the trouble of putting such a system inside. Real-size cars may use the technique though.

As for the coilgun; Looking awesome so far Ben. ^^
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Yandersen
Mon Dec 10 2012, 10:26PM
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I check speed of mine by the oscilloscope and a piece of pipe with two optical gates distanced by 10cm from each other.

Ben, try a shorter projectile - it will give your coils a little more time to dessipate energy before suckbacking. Saz at least puts a bunch of serialized diodes to deplete current faster. BTW, I've tried a comparably little 1N5408 diode as dempher diode for the coil which had 100A running through - it sustained the surge pulse without even mentionable heating. Consider using small diodes - for a single surge you don't need iron monsters.
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ben5017
Tue Dec 11 2012, 09:54PM
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yaz: I am trying to gain some aerodynamic stability with the longer thinner projectiles, and hopefully improve the effective range of the carbine. The scar 1000's projectiles were shorter and fatter, and were unacceptably unstable, some would shoot stable and straight, others would buzz end over end. for the x3cc I made sure I found a barrel that fits absolutely perfect. I need the long heavy slugs to hit tip first in order to have sufficient penetrative power, hopefully these two changes in addition to the increased velocity will do the trick.

-As for my diodes. I just put those on there to test everything is working as expected. Now I will begin upgrading the various systems. Finding the optimal diodes, and flux augmentation are the first things on my list.

-Should I be looking for diodes primarily by there amperage and speed ratings?

-You had mentioned adding 5W zeners in series with my diodes? Can you explain how those would help and where the energy will get dissipated? What voltage zeners should I be looking for? 25V?(same as my battery)

-So IGBT half bridge, Nonpolar caps in parallel , and zeners in series with my diodes are 3 different ways to limit suck back?

Saz: did you ever measure the velocity of your stages with out external iron? Where did you get your powered iron?
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Yandersen
Wed Dec 12 2012, 04:24AM
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Ben, what is the voltage rating of your IGBT? Let's say it is 600V. Then put 3 zeners of 180V rating IN SERIES to get a little below 600V (540V you will get). The leftovers of your current will dissipate on those zeners almost instantly. You may parallelize few such strings of zeners to be sure it is not overwhelmed. The idea is to higher the voltage drop. When IGBT is switched off, current is circulating through the coil and zeners expiriencing R*I*I dissipation power running through the coil and V*I power dessipation over zeners (V is a voltage drop over the single zeners' string).
Just do this and forget about any suckback. But make sure nothing touches the coil during the voltage pulse shot... :)
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ben5017
Wed Dec 12 2012, 03:09PM
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Yan: Good guess, they are 600v, just ordered (24) 5w 180v zeners. Should I still be putting more dempher diodes in series also? Or with the addition zeners is this now not necessary? Will my IGBT experience any significant different conditions, like a spike in amperage or voltage that I should be concerned about?
Can you explain what will happen if something touches the coil, I assumed the enamel wire had enough dielectric strength that 600v would not be a problem.
Those 5W zeners can dissipate all the current in the coil? Over one hundred amps? Should I expect them to experience significant heating?
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Yandersen
Wed Dec 12 2012, 09:28PM
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No, just one dempher diode. It will dissipate almost nothing. Your IGBT will be scared by voltage spike, but I doubt it will be over 600V, so their pants will stay clean. Good question about the wire. Hardly it will blow inbetween the layers, but keep ends wrapped with additional insulation and away from each other. 540V is pretty dangerous, but you want to dessipate energy the fastest way possible, right?
Zeners will dissipate almost all energy stored in a coil (it is E=L*I*I/2, so you can estimate it if you know your coil inductance and current). The rest will be dissipated on coil resistance. Current will linearly decrease, and voltage will stay constant while the last electrone will die. When current stops, voltage will drop instantly from 540V to 0V. It will not be long. In fact, there are no any faster way to dissipate energy rather than let the coil shot a HV spike.
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ben5017
Thu Dec 13 2012, 04:30PM
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If my wire can blow between layers at 540V, how do some of you guys play with a couple thousand volts? Do you get wire with a special HV insulation?
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Yandersen
Thu Dec 13 2012, 09:44PM
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"...about the wire. Hardly it will blow inbetween the layers, but keep ends wrapped with additional insulation and away from each other."
I play with 780V. No accidents since I started to insulate iron washers holding the coil from both ends. Used just 1 layer of paper glued to the washer by epoxy.
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ben5017
Tue Dec 18 2012, 05:46PM
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Yan, I miss understood your previous post.

Update: I got the zeners set up on both the accelerator stages. It seemed to give a slight increase in velocity, since the sensor is now right up against the coils while minimizing suckback. I have no way (as of yet) to measure the velocity so I have no hard data to provide. However I will put together some data and share it showing various different configurations and average results for each once I do.
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Yandersen
Tue Dec 18 2012, 10:13PM
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I just highlighted th word "ends" which means you have to pay special attention only on insulation of wire ends sticking out of the coil bobbin, as it is 540V inbetween them.

You can roughly estimate the difference in kinetic energy by shooting into a modelling clay by the deepness of penetration.

If you are ready to be more precise, you can mount two optic gates on a piece of pipe, sensors distanced by, say, 10cm from each other, and check the time it takes for projectile to pass from one sensor to the next via an oscilloscope connecting probes to the optic sensors.
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MrFlatox
Sun Jan 20 2013, 09:37AM
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Posts: 102
Hi !

I am also planning to build a LiPO powered coilgun, so I've naturally read this thread. You are talking about putting a zener diode in series of the damper diode to hasten the current drop in the coil, and thus preventing any suckback effects. As I plan to use mosfet to switch main current, I must choose a zener diode with voltage rating just a little below my maximum drain to source voltage ok ? I tried to simulate this , and I got that :



Graph shows the voltage that the mosfet will see. For this simulation, I chose a 51V zener diode. My question is : Why do I get 130V peak voltage with a 51V zener diode ? Is my schematic wrong ?

Thanks in advance
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Ash Small
Sun Jan 20 2013, 11:25AM
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MrFlatox wrote ...

Graph shows the voltage that the mosfet will see. For this simulation, I chose a 51V zener diode. My question is : Why do I get 130V peak voltage with a 51V zener diode ? Is my schematic wrong ?

Thanks in advance


Try putting a capacitor accross the zener. If the zener is taking a few uS to 'kick in', this should stop the voltage rising until the zener starts 'doing it's job'.

I can't be certain I'm right, but it's worth adding a cap to the simulation to see. Maybe ~500uF, but I'm guessing here. Try a few different values.

EDIT: It may be better to try a capacitor accross the MOSFET instead.
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Yandersen
Sun Jan 20 2013, 11:42AM
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Please, don't listen to the suggestion above.
Voltage is higher because of the coil's resistance and zener parasitic resistance - both add voltage drop due to the current running through. Coil resistance should be considered as unavoidable, but zener can be found with better parameters. Voltage drop for different currents are usually given in a datasheet table for TVS diodes.
Simulation seems to look right.
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MrFlatox
Sun Jan 20 2013, 02:09PM
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So how could I protect the mosfets, and have a quick de-energization of the coil at the same time ?

For example, let's say I am using a 75V mosfet, (or multiple stacked in parallel) and my battery provides 33V, what should I do to prevent the mosfets from blowing up, and have the fastest current decay in the coil.

Another question : in a battery powered coilgun, the maximum current that could circulate is dictated by the discharge rate of the battery, so the load (the coil) should be designed so that its internal resistance matches the max current the battery could supply ? Or is there another way, for a given coil and battery, to limit the current the coil receives and thus the current the battery is giving ?

Thanks
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Ash Small
Sun Jan 20 2013, 02:27PM
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Yandersen wrote ...

Please, don't listen to the suggestion above.


Surely a capacitor accross the MOSFET will absorb the spike and protect the mosfet?

It's a fairly standard snubber device.
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Yandersen
Sun Jan 20 2013, 05:11PM
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Hard to predict the exact capacitance value.
Flatox, just put more powerfull zener with a bit smaller clamping voltage.
For 33V battery and 75V MOSFET you have 42V of maximum drop at a coil (so ideally current may decay just a little bit faster than it's rizes). If you limit the current by coil' resistance, than you will have the same 33V drop on coil and zener may be up to 42-33=9V. Zener will not do much difference. The less V drop is on resistance opposite to the zener, the more linear and fast current decay will be. Limiting current by resistance is bad anyway.
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MrFlatox
Sun Jan 20 2013, 05:37PM
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Ash Small wrote ...


Surely a capacitor accross the MOSFET will absorb the spike and protect the mosfet?

It's a fairly standard snubber device.


I have just tried this, and it makes an ugly oscilating LC circuit. I do not think that it would make things better at the end.

If my simulation is correct, then using zener diodes will never make the mosfet live because voltage spike is never under 160V, no matter the Vzener voltage I choose.
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Ash Small
Sun Jan 20 2013, 06:44PM
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MrFlatox wrote ...

Ash Small wrote ...


Surely a capacitor accross the MOSFET will absorb the spike and protect the mosfet?

It's a fairly standard snubber device.


I have just tried this, and it makes an ugly oscilating LC circuit. I do not think that it would make things better at the end.

If my simulation is correct, then using zener diodes will never make the mosfet live because voltage spike is never under 160V, no matter the Vzener voltage I choose.



Did it keep the voltage 'within limits'?

You could try putting a fast diode in series with the capacitor, to stop the oscillations. You'd could also put a high value (mega Ohms) resistor in parallel with the capacitor, to drain it after each shot. (there are other ways to drain it).

(It's just another solution that you could try simulating, and compare it with other suggestions. Did you try other capacitor values?.)

EDIT: This capacitor, diode and resistor circuit I've described is a standard snubber circuit used to protect silicon and prevent oscillations. There are other variations used, but this is one of the more common ones. It could also protect the MOSFET in the event of TVS failure.
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MrFlatox
Sun Jan 20 2013, 08:42PM
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It lowered the voltage spike when using high value capacitor, but the oscilations are just too big. I will try your suggestions about the diode and resistor.

As you are talking about TVS, does a TVS diode with a voltage rating of 60V would do the same thing as my diode plus zener diode in series ? It could be intresting because they can pass a lot of current, and are cheap.
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Ash Small
Sun Jan 20 2013, 09:46PM
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MrFlatox wrote ...

It lowered the voltage spike when using high value capacitor, but the oscilations are just too big. I will try your suggestions about the diode and resistor.

As you are talking about TVS, does a TVS diode with a voltage rating of 60V would do the same thing as my diode plus zener diode in series ? It could be intresting because they can pass a lot of current, and are cheap.


TVS's (transient voltage suppressors) and zeners do pretty much the same thing,as far as I'm aware, but I can't quote any specs or anything for either. I'm still learning this stuff too.

I've researched snubbers quite a bit though, for protecting transistors of all types from voltage spikes. Adding a fast diode kills the oscillations. A high value resistor will discharge the cap each time you fire, and not really affect the rest of the circuit,. You'd need to research'experiment/simulate the exact values/specs yourself, though. It should also protect the MOSFET in case of zener/TVS failure, etc.
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Yandersen
Mon Jan 21 2013, 06:25AM
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Guys, I'm sick hearing about your ridiculous computer simulations for circuits with 3 parts! Do any of you ever hold a soldering iron in a hand and an oscilloscope nearby? Your simulations are either wrongly done or simply away from reality. What do you think you are "learning" without practicing? For the whole month 4HV are dancing around TVS diodes antiparallel to the coils and seems like noone understands a thing.
Ah, I'm just in a bad mood. What are we talking around here?..
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Ash Small
Mon Jan 21 2013, 01:15PM
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Yan, I don't do simulations either, I once downloaded a free version of SPICE, but couldn't be bothered to spend hours and hours learning how to use it. I just use a 'scope and a breadboard and see what happens in reality. I was working on snubbers a few months ago, but had to stop because I didn't have any fast diodes. I have some now, but my breadboard now has another project on it, so I've not got around to continuing with my snubber 'experiments'. I also use the minimum amount of maths that I can get away with. I also try to use the best quality components which are generally over rated for what I need, although sometimes I just use what I can get or already have.

I do take an interest in other peoples simulations though.

All that is required here is a capacitor with a high enough value to reduce the voltage spike to 'acceptable levels', and a diode that is fast enough to 'kill' any oscillations and handle the current, plus a resistor that only has to discharge the capacitor before the next shot.

Just using 'trial and error' will kill quite a few MOSFET's in the process, so a simulation should reduce the number of MOSFET's that have to die before the correct values are found.

All I'm doing here is suggesting an alternative to what you've suggested, and which should also protect the MOSFET in the case of TVS/diode failure. Your solution is simpler because it uses less components, but mine is arguably more reliable.

I'm simply pointing out an alternative method to achieve the same thing, ie, to reduce the voltage spike accross the MOSFET.

It could be argued that the best solution would be to implement both, if space and availability of components allow it, certainly at the breadboarding stage.
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Yandersen
Tue Jan 22 2013, 03:57AM
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Okay...
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ben5017
Tue Mar 05 2013, 04:05PM
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Update:

-Added 3rd accelerator stage
-Modified coil geometry (removed 1/2 layer from each subsequent stage to compensate for decreased "on" time due to higher V initial (trying to keep amps approx same each stage to maximize power through the igbt))
-Removed diode/zener protection (replace with (3) 5kw tvs diodes in series (540V totalbreakdown voltage))
-Minimized circuit size and moved all to one board
-Remade sensors into interchangeable units
-Added Fuse box (separate fuses for each stage + injector + cap bank)

Additional Changes to be made:

-Add cap bank to increase discharge rate (from 45C (lipo) to 85C (Caps))
-Add selector switch to change between monostable and astable 556 chip to drive injector
-6s lipo instead of 5s

Test shot results.

(3 stage no injector) approx 75mm penetration into solid Styrofoam block.
(3 stage no injector) through beer can, 35mm penetration Styrofoam.

Current Bugs:

First stage works perfectly when just testing the sensor, however when I actually shoot it, it stays on, blows the fuse and the 421 inverting gate driver??? Having trouble figuring out the issue...

Also I cannot get my 556 edge triggered monostable to work correctly (I have made this circuit numerous times with no issues....)

I am starting to think my breadboard is F***** up, it has been doing some weird stuff.....







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Yandersen
Tue Mar 05 2013, 09:00PM
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I had both of those bugs. Latchup of optical gate may have many reasons. The one I have encountered was sensitivity of photodetector to the EMI from coil - when coil shuts off, sensor gives driver the white noise which ends up in continuous on-off oscillations blowing the driver and possibly switch.
The issue with 556 timer is a strange coupling between output and trigger. When I connect OUT directly to mosfet gate and mosfet is switching off, the parasitic capacitance between drain and gate forcing the gate and timer's OUT in turn. For some reason that spike acts like a triggering signal for timer. Had no such issue with NA/NE/SE/TLC-555 timers though... Add 100 Ohm resistor for the output of 556 to prevent current overloading - worked for me.
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Maxwell
Wed Mar 06 2013, 12:38AM
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@Ben5017
Since you have optical gate triggering - why not buy a cheap arduino and count the time between gate triggers - will provide you with a pretty accurate velocity?
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