Rds_on current sensing

On Sat, 17 Oct 2015 09:57:14 -0400, krw Gave us: snip

Wow... krw is actually helpful for once.

As always, AlwaysWrong is wrong.

Then it's a question for Thompson I guess.

You must also have ruled out sensing a stall by feedback pulses from an optical sensor or something. You could make an accelleration cutoff with a reed switch and a movable magnet that just flies away from the switch, but maybe you ruled that out because it's too slow. OTOH it might even switch before the stall happens. I'm probably not the only one curious about why those were ruled out.

d

,

yep,

-Lasse

id

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h n

the high current is not through the reed, the high current wire is wrapped around the reed so that is it closes the contact when the current is high enough

-Lasse

They don't actually use Rds as the sensing element... on-chip FET's have multiple source connections... one, of perhaps hundreds, sees a fraction of the current and that is measured.

Yep. I thought of that too... just a photo (or magnetically) tripped one-shot.

Possibly, I don't know these motor types... if they are brushed (perhaps even electronically commutated) use those current spikes to trip a one-shot. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
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I love to cook with wine.     Sometimes I even put it in the food.

Suggests an electronic fuse scheme... what values for run current versus stall current? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
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I love to cook with wine.     Sometimes I even put it in the food.

You lost me on that one. If the motor is stalled and the current is high, how does that current not flow from the supply?

--

Rick

I didn't know those existed! It's something to think about, certainly.

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www.wescottdesign.com

h

I've also seen an appnote on using them for high speed switching, "abusing" the kelvin connection for the gate drive, bypassing the source inductance

-Lasse

There is a switching regulator, so most of the current comes from the regulator inductance and free-wheeling diode if the output voltage is low.

--

-TV

gh

t-

but that indicates that there is already some kind of current sensing going on

also a BLDC driver must track the rotor with sensors or bmf so it is rather surprising that it doesn't go into some kind of error mode if it is stalle d

-Lasse

Huh? A buck regulator, when loaded that heavy, will go to a high duty cycle, thus the supply current will rise. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

It's not regulated, but yes.

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www.wescottdesign.com

If you're not at full throttle then the ESC acts like a buck regulator. The duty cycle is fixed, but that doesn't keep the current in the coil from exceeding the current in the supply.

I suppose you could monitor the supply current and modulate the threshold

-- I need to think on that.

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www.wescottdesign.com

First, the commutation frequency is significantly less than the lowest reasonable switching frequency, so if you're waiting for a commutation event then at best you'll know it's too late.

And regardless of what should be happening, it doesn't -- crashes that stall the motor burn up motors and ESCs.

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It's electric Mossie...agree about IC surviving...

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** Wouldn't the blades normally break off the prop or the whole thing come loose from the drive shaft when suddenly made to stop like that ?

Ought to be enough spinning inertia in the rotors of BLDC motors to do some damage.

When racing RC power boats, using 2-stroke glow plug engines, one of the fe ars is losing control of the model at full throttle and having it and trave l off the water into weeds or grass. Because there is suddenly no load on t he prop, the engine sound rises to a never before heard pitch - followed by sudden silence.

.... Phil

They end up breaking every which way. If there's ten possible ways for them to break, Nature will have found 20.

I have a lot more direct experience with IC engines. I've seen one prop blade break, two break, planes bounce of the ground, planes bore in with identifiable spiral groves in the mud where the engine took about 1/4 turn to stop, etc. I've even seen one prop blade break and the engine NOT stop (that'll tear an airplane apart -- in this case the pilot realized what happened and bounced it off the ground again to stop the engine).

--
www.wescottdesign.com