Hi all,
I have a Pololu "Baby Orangutan" controller board with a Toshiba TB6612FNG dual H-bridge IC.
The PWM mode of operation of this chip (see truth table) seems to be VCC/SHORT rather than VCC/OPEN as I would expect. I'm sure I can drive it differently to get VCC/OPEN operation, but my question is, should I do this, or run it as-is (VCC/SHORT)?
To me, VCC/SHORT seems wrong.
Opinions?
(click image for full size)
If you have vcc/open, you create a boost converter that generates many volts. The chip would not last long.
Vcc/close is what you want.
This refers to the outputs....it is not shorting your Vcc input...it is just shorting the motor leads together.
This refers to the outputs....it is not shorting your Vcc input...it is just shorting the motor leads together.
If you have vcc/open, you create a boost converter that generates many volts. The chip would not last long. Vcc/close is what you want.
Thanks for the replies. The "VCC/SHORT" refers to the motor. That is, the chip will either apply VCC to the motor, or else short the motor leads together (acting like a dynamic brake).
I understand the idea of the motor snapping back a nasty inductive kickback with the leads go "open" (which could destroy the H-bridge chip), but a short across the motor seems... well... wrong to me.
Also, in all my decades of electronics work (that is 45+ years), I've never seen PWM doing POWER/SHORT to the motor.
I don't want to sound like I'm doubting you, but could you show me an example where this is done?
Thanks!
Can't provide an example, but, in former lifetime, that was our standard operating mode. We wanted the motor to stop, not continue spinning.
Shorting vs open has TWO consequences, not just one:
Shorted: Motor stops quickly AND there is very little transient voltage back-applied to the driver
Open: Motor coasts AND there can be a very significant transient voltage back-applied to the driver
These effects come in pairs; with one, you have no choice but to get the other, also.
The shorted choice does, however, usually have two options:
1. Shorted means connecting both to ground
2. Shorted means connecting both to Vcc
Usually, there is not a big difference in behavior but your mileage may vary.
Jim
in former lifetime
I used to race slot cars, shorting the motor was a common way to get brakes for a faster stopping action, so this seems the right way to do it.
i also used this trick in a safe application, the motor tripped the locking lever to open the safe, and stop at the location needed for the next operation.
jim
For a decent power motor (drawing several amps at 12V from a car battery) , we actually stop it quicker by throwing it in reverse for several milliseconds, then applying the shorting brake...this works to give a dead stop for positioning mechanics with zero coasting. This is also known as plugging. Need good drivers and cabling for this surge!
https://www.motioncontroltips.com/what-is-plugging-for-electric-motors/
Plugging — sometimes referred to as “reverse current braking” — is possible on both DC motors and AC induction motors. For DC motors, plugging is achieved by reversing the polarity of the armature voltage. When this happens, the back EMF voltage no longer opposes the supply voltage. Instead, the back EMF and the supply voltage work in the same direction, opposing the motor’s rotation and causing it to come to a near-instant stop. The reverse current produced by the combined supply voltage and back EMF is extremely high, so resistance is placed in the circuit to limit the current.
We have a PWM control circuit (largely for historical reason) which open circuits one pair of the driver transistors and PWMs the other pair (diagonal pairs, of course) - when the PWM is high the transistors are on, when low, everything is high impedance. Every now and then we take everything high impedance for a couple of milliseconds to measure the voltage generated by the motor, which is proportional to the speed. That lets us keep a very good control of speed via the PWM and also lets us know if the motor is stalled.
Neil
I also used this trick in a safe application, the motor tripped the locking lever to open the safe
Geraldo wants to speak with you!
Every now and then we take everything high impedance for a couple of milliseconds to measure the voltage generated by the motor, which is proportional to the speed.
Yes, that back-emf sensing seems to work quite well for generic avg speed control....even with a single transistor circuit (single direction driver)...you can sample after a slight delay to avoid the switching transients. If you have current monitoring (for overload detection), you can also monitor the torque.
Can't provide an example, but, in former lifetime, that was our standard operating mode. We wanted the motor to stop, not continue spinning.
Shorting vs open has TWO consequences, not just one:
Shorted: Motor stops quickly AND there is very little transient voltage back-applied to the driver
Open: Motor coasts AND there can be a very significant transient voltage back-applied to the driver
These effects come in pairs; with one, you have no choice but to get the other, also.
The shorted choice does, however, usually have two options:
1. Shorted means connecting both to ground
2. Shorted means connecting both to Vcc
Usually, there is not a big difference in behavior but your mileage may vary.
Jim
Well, I'm convinced. I wish there was some way of throwing that energy back into the battery instead of just shorting it into heat in the H-Bridge. 
No reason why you can't - though whether it would be worth doing is a separate question; you'd only be recycling the rotational energy of the motor and associated drive change.
When the motor isn't being powered, diodes can carry any generated voltage to the rails. It won't be quite as high as the drive voltage (generator losses and the diode losses) even at full speed so you'd have to isolate the drive voltage, feed what you generate into a temporary storage/DC-DC converter and feed that to the charging circuit.
I haven't investigated, but I suspect that an electric vehicle with regenerative braking does something essentially similar - running and braking are essentially different regimes. When you're running, you're essentially driving at a reduced voltage anyway; you wouldn't want to drive for the on-cycle and then brake for the off-cycle.
Neil
