Controlling actuator

[drkidd22]

With the circuit below I want to be able to control a linear actuator.
Basically there is an input 24Khz PWM signal that's LP filtered to the non inverting input of U13.
When PWM duty cycle drops below 75% (below 9V Vref) the output of U13 goes low and output of U27 goes high to engage the actuator via U15.
When the PWM goes back to 100% the actuator is reversed, simple right? At least that's what I want it to do.

I'm not familiar with BA6285FS and was wondering if someone here is. I need to limit the current to the actuator so that it doesn't go over ~100mA and I think it can be done with BA6285FS, but it's not clear to me from the datasheet. The actuator has internal circuitry (switches) to stop the drive power, but only when it is fully extended or retracted, my concern is if it's stuck in the middle, it will most likely overheat and burn out thus why I need to limit current or some other safety measure. Will adding like a 120Ohm resistor to VM and tie it to +12rail accomplish this?

 

[dknguyen]

You can get rid of U27 by putting some hysteresis on the U13 and swapping the connection on the inverting and noninverting inputs.

EDIT: didn't notice you needed both a inverted and non-inverted output. But you still want to put some hysteresis on U13. The schmidt trigger on U27 comes too late in the signal chain to deal with potential variances of the LP PWM signal near the threshold.

 

[drkidd22]

You can get rid of U27 by putting some hysteresis on the U13 and swapping the connection on the inverting and noninverting inputs.

I don't think I can do that, because logic for U15 will be incorrect.

 

[dknguyen]

I don't think I can do that, because logic for U15 will be incorrect.

I noticed that and modified my post. Re-read please.

As for your actual question, I see no current mechanism inherent to the driver IC. You can make your own current limit by putting a current sense resistor in series with the motor, and reading it with a comparator and then having the comparator gate the FIN/RIN signals going to the IC.

You will also need a differential amplifier or current shunt amplifier to amplify the voltage across the resistor before feeding it into the comparator. The one you choose will be dependent on whether you choose to place the currenrt sense resistor on the high-side or low-side of the motor.

Or you can choose an IC that combines a current shunt amp and a comparator:
https://www.ti.com/amplifier-circuit/current-sense/comparator/products.html

Since your circuit is bi-directional you will have to take into account that there is both a positive and negative threshold for overcurrenet which may mean you actually need two comparators, depending on how you want to go about it.

 

[drkidd22]

Yes will add hysteresis to U13.

As for your actual question, I see no current mechanism inherent to the driver IC. You can make your own current limit by putting a current sense resistor in series with the motor, and reading it with a comparator and then having the comparator gate the FIN/RIN signals going to the IC.

That sounds good way of doing it or using a regulator with shutdown option, but, I'm limited in space. Can I just current limit the regulator? U14?

 

[dknguyen]

Yes will add hysteresis to U13.



That sounds good way of doing it or using a regulator with shutdown option, but, I'm limited in space. Can I just current limit the regulator? U14?

Not with the regulator you are using. You either need to get Vm it's own regulator that is set by a resistive divider and wire that regulator up as a constant current source or just use a different regulator that has a 100-200mA internal current limit.

I suppose you could just put a PTC in series with the motor if you can find an appropriate one.

 

[drkidd22]

Not with the regulator you are using. You either need to get Vm it's own regulator that is set by a resistive divider and wire that regulator up as a constant current source or just use a different regulator that has a 100-200mA internal current limit.

I suppose you could just put a PTC in series with the motor if you can find an appropriate one.

I think one of these little guys will work. Not familiar with PTCs, but It will be good enough, just gotta find the right one.
How fast do they normally reset when the current trips them?

**broken link removed**

 

[dknguyen]

I think one of these little guys will work. Not familiar with PTCs, but It will be good enough, just gotta find the right one.
How fast do they normally reset when the current trips them?

**broken link removed**

Not sure. Depends on the device and ambient conditions I guess. Maybe 5 seconds or less for a small device? It just resets however fast it cools down.

 

[drkidd22]

Found this little guy with current limit feature.
https://www.ti.com/lit/ds/symlink/drv8871.pdf
DRV8871

 

[drkidd22]

So on the BA6285FS how is the current limited on the Vm input by using a resistor? It also looks like it's dependent on Vref, not clear from datasheet.

 

[dknguyen]

So on the BA6285FS how is the current limited on the Vm input by using a resistor? It also looks like it's dependent on Vref, not clear from datasheet.

View attachment 110939

I dont see that on the datasheet anywhere

EDIT: oh, its a mult-soucred part so there are multiple dtatsheets from different manufacturers. R1 is just a resistor in series with the motor if you look st the internal schematic.

 

[dr pepper]

I've used the drv chip before, you can get modules from china with that device on, theres also a big brother version.