So, do you allow for the frequency-dependent (i.e. RPM scaled) eddy currents in the commutated inductors? A DC brushed motor is actually pushing AC through the electromagnets, and there are AC resistances involved. Higher internal impedance than simple winding resistance results from the transformer coupling with these eddy currents in the core (and other material losses as well).
I think a flyback diode could easily be included in the calculations. Discontinuous mode would pretty much kill off any chance of being able to figure things out without direct measurement -- fortunately I don't have to worry about that in the least.
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
MOSFETs, dead-boring diagonal switching (around here, "clever" usually means "Tim is outsmarting himself again").
Vrail = 24V
torque constant ~ 0.15 N-m/A
Torque (and hence current) is highly variable in normal operation, as are speed commands.
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
How do you measure motor current, with all that h-bridge flailing going on?
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John Larkin Highland Technology, Inc
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The flyback diode does NOT effect the calculations. There's a subtlety there that _even_I_ overlooked at first ;-)
Could you share your schematic? Privately to me if you prefer. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| 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.
Not per se. But this sounds kind of like a two terminal R versus 4 terminal R type problem. Is the wiring resistance between the controller and the motor enough to account for this?
And my mind went ding. I have seen an awful lot of two brush DC motors that have an odd number of commutator segments. Thus most of the time one of the brushes is spanning two segments. It is a thought.
The clue, I think {;-), is that the offset is at all commands EXCEPT at stall... "stall" I presume means you're running 100% duty cycle, so no flyback diode involved?
That's why I keep asking for a schematic, or at least a command equation... I suspect Vdiode is in your equation, when it doesn't belong. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| 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.
That doesn't explain the "offset". ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| 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.
power rail --o--- | | fet driver --> FET | o--------- MOTOR -----> other half bridge | fet driver --> FET | current sense --o | .-. | | current sense R '-' | current return -o | --- GND
Current, Im, is sensed with a Kelvin connection from the current sense resistor to an op-amp (to reduce ground issues) to a pair of ADC channels which sample at the center of the lower FET on time. Then the ADC readings are subtracted to get motor current.
Voltage is sensed at the rail, and computed as
Vm = 2 * (1 - rho) * Vsupply,
where rho is the duty cycle from 0 to 1 (50% being off).
Speed is computed as Wm = (Vm - Ra * Im) / k_t
Speed is regulated with a PID loop that commands motor current, motor current is regulated with a PI loop that commands duty cycle. That's lots-o-loops, but it's nicely stable and I have a need to interpose current and speed limits, which fit very nicely into those loops.
The problem is not with the loops, or at least not something overt with the loops -- I can look at the computed speed, and its average matches the commanded speed quite nicely.
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
If the "average" speed matches the commanded speed, I don't understand the problem... that's what is should be. Is the ripple confusing your computation?
I've used a full-H-bridge to drive motors, but I use one side to set direction (held low) and PWM the other side... then change direction by swapping side functions. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| 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.
The average _computed_ speed matches the commanded speed. The computed speed reads about 30RPM high of actual. That's the offset.
That works well if you're mostly driving in one direction or another, but it makes for a hiccup around zero drive. I could do it that way with a few lines of code, but experience has shown that if you're reversing voltage a lot it's not smooth.
(I dunno if the "reversing voltage" concern makes a difference here or not -- I have to admit that banging both half bridges at the same time is a long-time habit.)
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
A habit I avoid, at least in my video iris controls ;-)
I often independently control all 4 devices in a full-H-bridge... to contain body currents to safe levels. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| 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.
That's interesting. Is it something that's necessary when you're using discretes, or just an integrated-driver sort of things?
Mostly what I'm doing is driving the H-bridge the way that the microprocessor is set up to do. It's easy enough if you stay within the confines of what the hardware can handle, hard if you go outside of that.
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
It's not clear to me how you extract Vbemf that way. Maybe it'll register... sometimes I tend toward slow ;-)
I'll behavioral model the situation and play it. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| 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.
This _is_ a PM motor, right? No field winding, either parallel or in series? ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| 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.
Are you sure you're not getting an effective DC offset as a result of the dead time control of the PWM?
Best regards, Spehro Pefhany
--
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