Author Topic: kWeld - "Next level" DIY battery spot welder  (Read 48045 times)

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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #475 on: December 06, 2017, 08:52:39 PM »
Not sure I got it correctly or not. So the kWeld determines current draw by measuring voltage drop across the welding cables and electrodes? Does the welder rely on the length and thickness of welding cables used to be the same as those provided by Frank? I apologize if I am asking too much.

To be really honest, I am trying to replicate one for myself to learn electronics. Being a undergraduate freshman who don't have a major yet, I definitely dare not sell my potential product to others, I might even blow up my dorm room in the process of trying to build one :-DD Also I am too broke to buy the real deal from Frank.
kWeld is measuring current through the on-state resistance of the MOSFETs, using them as a shunt. This is not very accurate as you suspected, you end up with something in the order of +-30%. And there is quite a temperature coefficient. But absolute accuracy is not that important here, the idea is to have repeatable results. The user will adjust the pulse energy to his needs anyway. The temperature coefficient is more important, and that is one of the reasons why I have chosen the most powerful MOSFETs available (at quite a cost), because they do not heat up much. Even during my torture tests, I never managed to heat them above 50°C.
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Online anishkgt

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kWeld - "Next level" DIY battery spot welder
« Reply #476 on: December 06, 2017, 08:59:04 PM »
Well if you want accurate current measurement, one way would be to measure the resistance of the cable via 4 wire method and measure voltage on portion of the cable like 10cm apart then use ohms law to calculate current.

You could add these to the code also.
Hope this helps.


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Online Fraser

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #477 on: December 06, 2017, 09:13:16 PM »
On the matter of current measurement accuracy, I concur with Frank.

It is not necessary to have absolute accuracy of current measurement in a spot welder as there are many variables that can effect such, including the performance and contact area of the probes combined with battery tag strips. What is important is repeatability of weld. Franks design is excellent in this respect because of his novel approach of using Energy measurement rather than just a timer, as found in some other designs.

It would be possible to create a highly accurate current measurement for a spot welder, but one has to ask, is it needed and what advantages does it offer to offset the added complexity of the design ? I would argue that Franks approach is both novel and relatively minimalist for the required task (the KISS principle). The operating principle is well thought through and produces the desired repeatable results   :-+ A far better design than my £80 Chinese spot welder that just uses a transformer and timer controlled triac to set the weld current duration.

Fraser
« Last Edit: December 07, 2017, 02:17:19 AM by Fraser »
 

Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #478 on: December 06, 2017, 09:17:06 PM »
Well if you want accurate current measurement, one way would be to measure the resistance of the cable via 4 wire method and measure voltage on portion of the cable like 10cm apart then use ohms law to calculate current.

You could add these to the code also.
Hope this helps.


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The tempco of copper is a in the same order as that of a MOSFET (4m / K), but the wiring also heats up much more. So there is no advantage with that respect. The absolute accuracy could be better, but at additional cost and mechanical complexity. This is why i decided against that in the beginning. I also thought of adding a dedicated shunt resistor, but didn't like the additional cost and complexity either.
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Online anishkgt

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #479 on: December 06, 2017, 09:18:26 PM »
I wouldn’t argue about franks design but just sharing idea for current measurement.


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Online anishkgt

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #480 on: December 06, 2017, 09:23:34 PM »
Quote
The tempco of copper is a in the same order as that of a MOSFET (4m / K), but the wiring also heats up much more. So there is no advantage with that respect. The absolute accuracy could be better, but at additional cost and mechanical complexity. This is why i decided against that in the beginning. I also thought of adding a dedicated shunt resistor, but didn't like the additional cost and complexity either.

Yes a bit of work too but reading voltage “ 10cm apart” is like using a shunt. Since you also provide the cable too the resistance would be a constant but as mentioned by others “how important is current accuracy?”


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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #481 on: December 06, 2017, 09:36:00 PM »
the resistance would be a constant
No it's not. It is copper, which has a huge temperature coefficient  ;)
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Online anishkgt

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #482 on: December 06, 2017, 09:38:19 PM »
Hmmm yea. Oooopsy again that wouldn’t be accurate but would give an idea of it.


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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #483 on: December 07, 2017, 02:26:09 AM »
Hmmm yea. Oooopsy again that wouldn’t be accurate but would give an idea of it.
Nothing that the on-state resistance of the MOSFET's wouldn't give - except better absolute accuracy.
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Offline TopQuark

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #484 on: December 07, 2017, 06:36:41 PM »
Frank, really appreciate your answer :D using the mosfets as a shunt is quite a clever idea. How about using the fuse as a shunt? Does the resistance of the fuse change much from one to another, or does the temperature rise of the fuse contribute too much to resistance instability?

If I end up building one for myself, my OCD will probably force me to place a 'real' shunt resistor to get a slightly better current reading. Maybe a 100 microOhm resistor with an amplifier circuit similar to what Dave used for the uCurrent project. But now I am quite occupied with another project I have on hand, so maybe I'll just place Frank's welder on the top of my Christmas wishlist... :P
 

Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #485 on: December 08, 2017, 04:28:05 AM »
How about using the fuse as a shunt? Does the resistance of the fuse change much from one to another, or does the temperature rise of the fuse contribute too much to resistance instability?
Both, as I don't always buy the fuses from the same place. And they heat up significantly, way more than the MOSFETs.
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Offline JohnG

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #486 on: December 09, 2017, 02:14:48 AM »
For what it's worth, I have found that Infineon MOSFETs have pretty accurate resistance values, maybe within 10% or better, and the tempco is well specified. My experience has mostly been 100V FETs and above, so it may not be directly applicable here.

I would guess that if you know the FET temp, you can do a pretty good job. For those who have to know, there could be an option to calibrate the current using an external shunt. Not sure if it would be worthwhile, because I suspect the current varies a lot during the weld process.

John

 

Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #487 on: December 09, 2017, 10:21:37 AM »
For what it's worth, I have found that Infineon MOSFETs have pretty accurate resistance values, maybe within 10% or better, and the tempco is well specified. My experience has mostly been 100V FETs and above, so it may not be directly applicable here.
Did you find this by measuring, or were these datasheet figures?

I would guess that if you know the FET temp, you can do a pretty good job. For those who have to know, there could be an option to calibrate the current using an external shunt. Not sure if it would be worthwhile, because I suspect the current varies a lot during the weld process.
I was considering calibrating Rdson during production testing, but as mentioned before the absolute accuracy is not that important for this application. And as the MOSFETs only heat up by 25K (accounting for ~10% of error), I didn't go for measuring their temperature either. If I would add temperature monitoring, then to protect the power stage in first place.

In the end, kWeld has proven to make very repeatable welding results, and that's all it has to do  ;)
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Offline ovnr

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #488 on: December 11, 2017, 12:34:08 AM »
In the end, kWeld has proven to make very repeatable welding results, and that's all it has to do  ;)

Let's have a massive round of applause for tatus1969. Congratulations, you've avoided the feature creep hole I invariably spend my days digging even deeper...
 
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Online Fraser

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #489 on: December 11, 2017, 01:06:46 AM »
Yes, the scourge of many projects. Frank has released a perfectly functional design that he is now enhancing in a controlled manner to provide sensible improvement. The wish list for many projects can become a nightmare as the BoM and design man hours can spiral out of control.

Great work Frank  :-+

Fraser
 
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Offline rhb

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #490 on: December 11, 2017, 05:12:55 AM »
I went back through the thread quite a bit, but it's a long thread, so please forgive me if this was already addressed.

As I understand it, you're discharging a capacitor bank and you're measuring the energy of the pulse by measuring the current.  The accuracy of the  measurement is limited by the difficulty of measuring a large current under dynamic conditions.

If the charging current is disconnected prior to the pulse and the capacitor bank voltage measured before and after the pulse that should give an accurate measure of the energy from first principles.  There is likely some voltage variation after the pulse is cutoff related to the degree to which the capacitors do not discharge uniformly.  However, this might provide a means for assessing the condition of the capacitor bank.

Is there a problem with using the relationship among capacitance, charge and voltage to determine the energy of a pulse that I have overlooked?  This seems to me a good bit easier and more accurate.  One still has heating losses, but those should not be too hard to measure or estimate using suitably placed thermistors.
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Offline JohnG

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #491 on: December 11, 2017, 09:17:54 AM »
For what it's worth, I have found that Infineon MOSFETs have pretty accurate resistance values, maybe within 10% or better, and the tempco is well specified. My experience has mostly been 100V FETs and above, so it may not be directly applicable here.
Did you find this by measuring, or were these datasheet figures?



At one point I did (i.e. my tech did) a LOT of voltage drop measurements on Si and SiC MOSFETs and IGBTs at 25, 75, and 125C. The Infineon ones all measured within 10% of that specified in the data sheet. Note that they were easier to measure than high current, low voltage FETs since the Ron was 10s to hundreds of milliohm, but I was surprised at how close they were to the data sheet. Interestingly, so were the measured CV curves.

Not all the FETs were like this... 

John
 
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Offline ovnr

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #492 on: December 11, 2017, 10:02:51 AM »
Is there a problem with using the relationship among capacitance, charge and voltage to determine the energy of a pulse that I have overlooked?  This seems to me a good bit easier and more accurate.  One still has heating losses, but those should not be too hard to measure or estimate using suitably placed thermistors.

That only estimates how much energy you drain from the capacitors - not how much gets delivered to the weld site. A majority of the energy consumption is in the capacitors themselves (ESR).

Also, kWeld supports both supercaps and high-energy LiPo cells; the latter are more cost-effective, IMO, and would not work with your proposed measurement method.

Using Rds(on) to estimate current is actually a really common method. In fact, if you're reading this on a computer you're already using a device that does it, namely the CPU core voltage regulator on your motherboard.

 

Offline rhb

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #493 on: December 11, 2017, 12:18:05 PM »
ESR is measurable as is the cable resistance.  The relationship between charge and voltage for a LiPo is more complex than the linear relationship of a capacitor,  but I don't see why it is harder than the time variant  Rds(on) of a MOSFET passing a large current for a short period of time. 

A comparison to the measurement of CPU drains is fatuous at best.  That does not involve going from 0 to 1000A and back in a fraction of a heartbeat.
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Offline ovnr

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #494 on: December 11, 2017, 12:58:43 PM »
ESR is measurable as is the cable resistance.  The relationship between charge and voltage for a LiPo is more complex than the linear relationship of a capacitor,  but I don't see why it is harder than the time variant  Rds(on) of a MOSFET passing a large current for a short period of time. 
Wait. You think that estimating how many joules - with a precision of 0.1J or better - remain in a LiPo based solely on the pack voltage is easier than Rds(on) sensing? Really? Besides, as tatus1969 has said: The weld energy is not calibrated in any way. You're supposed to find the correct energy level by trial and error, at which point the welder will produce consistent results.

Also, the welder does the joule counting during the welding process, cutting off the current the instant the correct energy level has been reached. That means you can't very well measure the voltage before and after.

A comparison to the measurement of CPU drains is fatuous at best.  That does not involve going from 0 to 1000A and back in a fraction of a heartbeat.
True; 1 kA CPUs are still a bit out. Still, we do have 200+ amp CPUs like AMDs Threadripper (with a healthy overclock). And the VRM switches at 500+ kHz.
 

Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #495 on: December 11, 2017, 05:54:30 PM »
As I understand it, you're discharging a capacitor bank and you're measuring the energy of the pulse by measuring the current.  The accuracy of the  measurement is limited by the difficulty of measuring a large current under dynamic conditions.
The kWeld firmware uses a measurement cycle of 10 microseconds, that is enough bandwidth to sample the current waveform.

If the charging current is disconnected prior to the pulse and the capacitor bank voltage measured before and after the pulse that should give an accurate measure of the energy from first principles.  There is likely some voltage variation after the pulse is cutoff related to the degree to which the capacitors do not discharge uniformly.  However, this might provide a means for assessing the condition of the capacitor bank.
There are quite a few CD welders that use a similar mechanism. But instead of measuring voltage before and after, they let the user dial in the desired stored energy, calculate the corresponding charging voltage for them, and firing all the energy at once until the capacitors are drained completely. This has the advantage that you can use an SCR as the switch, as it does not have to have the capability to interrupt the flowing current again.

Is there a problem with using the relationship among capacitance, charge and voltage to determine the energy of a pulse that I have overlooked?  This seems to me a good bit easier and more accurate.  One still has heating losses, but those should not be too hard to measure or estimate using suitably placed thermistors.
The idea behind kWeld is to control the energy that goes into the weld. (To achieve that, measuring the flowing current is just one requisite.) With your idea (or the way standard CD welders work) you can only control the total energy consumption. A large part of that goes into the capacitor (or battery) itself, the connecting leads, and the switch.
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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #496 on: December 11, 2017, 05:59:08 PM »
ESR is measurable as is the cable resistance.
Yes, but measuring ESR is technically easier, as the MOSFET that you use as the current shunt is typically GND referenced. This way you only need to measure one ground referenced voltage, instead of having to measure a voltage differential. In this way I am following the same rationale that has brought up ESR current sensing in DC/DC converters.
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Offline rhb

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #497 on: December 12, 2017, 11:44:33 AM »
Interesting.  Thanks for the explanation.
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