Regenerative Braking - links pls
I'm still at the design stage·in building·a bot that uses Everest & Jennings (wheel chair surplus) 24V motors and Curtis 1505 PWM control boards. I'm looking for some way to implement regenerative braking with parts of this scale. As a reminder, regenerative braking involves recovering the energy being generated by a turning motor (that you are trying to slow down) and putting it back into the batteries.
In my case·I'm using sealed·lead-acid·batteries·wired to produce·a no load terminal voltage of about 26 volts. I'm stuck at the point of thinking I need more than 26V coming from the motor in order to "drive" current back into the batteries as it slows down. I'm having trouble with the idea that if you put 24V (pulses) into·the permanent magnet motor to get it spinning, that you can then get more than 24V out of it when it's acting like a generator in the slowing down phase. This no doubt has something to do with the impedance that the voltage is being developed across but since it's DC (not even pulsed in this phase) you can't use a step up transformer to get the voltage back up. On the other hand, I've measured 28V+ on the motor power terminals (and almost cooked the drive transistors in my test jig) when I turn the speed pots down too fast.
I've read about how the Toyota Prius works, and various other automotive systems but I'm having trouble finding anything that resembles·what I'm trying to do. Does anyone have any links to suitable circuits?
Here's a couple of pics of the early going:
A mock up
Same with big wheels and Ametek motors
The power test unit
TIA
Post Edited (Hobbyist) : 5/11/2010 3:40:34 AM GMT
In my case·I'm using sealed·lead-acid·batteries·wired to produce·a no load terminal voltage of about 26 volts. I'm stuck at the point of thinking I need more than 26V coming from the motor in order to "drive" current back into the batteries as it slows down. I'm having trouble with the idea that if you put 24V (pulses) into·the permanent magnet motor to get it spinning, that you can then get more than 24V out of it when it's acting like a generator in the slowing down phase. This no doubt has something to do with the impedance that the voltage is being developed across but since it's DC (not even pulsed in this phase) you can't use a step up transformer to get the voltage back up. On the other hand, I've measured 28V+ on the motor power terminals (and almost cooked the drive transistors in my test jig) when I turn the speed pots down too fast.
I've read about how the Toyota Prius works, and various other automotive systems but I'm having trouble finding anything that resembles·what I'm trying to do. Does anyone have any links to suitable circuits?
Here's a couple of pics of the early going:
A mock up
Same with big wheels and Ametek motors
The power test unit
TIA
Post Edited (Hobbyist) : 5/11/2010 3:40:34 AM GMT
Comments
These two companies make ESC's for radio controlled cars and some of their documentation is superb. All r/c ESC's that I know of utilize "regenerative braking" I also have a book around here somewhere that discusses that topic and tells how to build . . . it was way over my head but I'll try to find it if you'd like.
Unless you are coasting down hill, there is no way that the motors can generate >more< power than what you put into them during normal operation from your power supply. Otherwise it's just shunt braking. Regenerative braking requires a DC-to-DC converter that steps-up the voltage generated from the motor (which is acting as a generator) to a level suitable for battery charging through an appropriate charge controller. The load required for the step-up converter and charging is sufficient for braking or slowing down the motor. For hard fast stops it might still be desirable to have shunt braking.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 5/8/2010 10:04:34 PM GMT
Then you would need the handset program it. It may be expensive but Curtis controllers
are easy to set up.
curtisinstruments.com/index.cfm?fuseaction=cProducts.dspProductCategory&catID=10
-dan
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Arguing with myself--sometimes me, myself, and I don't always agree.
(Former) Caterpillar product support technician
I really appreciate the your help as well Beau.. perhaps you could help me understand a bit more. I've spent the last few hours trying to test out the answer to this myself but nothing is ever easy : -) I'm giving up till tommorow... Here's what I'm wondering:
My permanent magnet motors are rated at 24 Watts. They came out of a 24V (wheelchair) system so I'm running them at that and sure enough they draw about 1 amp from my regulated supply. If I use one of those motors, connect it with a simple belt drive to turn another one as a generator, what determines the voltage that the generator motor produces? If I short the terminals together that would be "by definition" 0V and I guess there would be 1 amp flowing through the system. If I put a resistance of some sort between the terminal then there would be a voltage developed across it - no? Wouldn't that mean the input "resistance" of a circuit powered by the generator would determine the voltage produced? Wouldn't I have something a bit less than 24 Watts to work with and if my charger system preseted an appropriate resistance across it's input terminals that could give me an input voltage of say 48V with a current approaching 500mA or any other "ideal" voltage and corresponding current?
Again,
I really appreciate the help all!
Per Beau, a real regen braking system (as in the Prius) is pretty dang complicated. And quite frankly, not cost-effective in most applications. Unless you are frequently braking from high speed to a full stop, (like a Prius in city traffic) it's probably not worth the complexity.
That said, I love getting the most out of simple designs. You could try a series/parallel battery arrangement with 6V or 12V SLAs to use in a 24-volt system. You could use heavy-duty relays to switch four six-volt batteries (24V) in series to drive the motor, then for braking, switch them all in parallel (6V or 12V) and reverse polarity to the motor for recharging. Once the motor/generator voltage falls below the battery voltage, you would disconnect them.
Years ago, many guys successfully converted regular cars to electric by replacing the IC engine with a 36V or 48V starter motor out of B-17 bombers and such! Yes, I'm serious. They had dozens of full-sized car batteries, all manually switched in series/parallel arrangements for simple speed control. Not sure if they used regen braking, but at least there's a precedent for wiring the batteries up as I describe.
If you go this route, you must exercise LOTS of caution and use VERY heavy duty switches & relays. Relay contacts have been known to weld themselves together under a heavy motor load. Switching a motor (an inductive load) is much harder on the contacts than switching a light, which is is a resistive load.
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·"If you build it, they will come."
http://forums.parallax.com/showthread.php?p=894383
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·"If you build it, they will come."
www.lynxmotion.com/Product.aspx?productID=565&CategoryID=10
www.lynxmotion.com/Product.aspx?productID=564&CategoryID=10
www.lynxmotion.com/Product.aspx?productID=562&CategoryID=10
Personally, the lack of on/off control for the fan and the lack of regenerative charging are two of the main reasons I'm not so fond of the HB-25s (hint hint Parallax!).
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When the going gets weird, the weird turn pro. -- HST
1uffakind.com/robots/povBitMapBuilder.php
1uffakind.com/robots/resistorLadder.php
see:
http://www.hamptoncontrols.com/vari_speed_r400_series.html
I'm not sure this is exactly what I want (but I get the feeling I may not have thought enough before making that statement : -) ). I guess a bit more on the purpose of this bot might help.. [noparse][[/noparse] img ] tags don't seem to work in this sub-forum so I'll just put up some photbucket links.
As you mention erco, plain-old brush motors aren't going to cut it. I have 2 possiblities for motors. Either Everest & Jennings (ex wheelchair) 24W permanent magnet motors (close up) or Ametek permanent magnet motors that are spec'ed at 30V. (close up)
For power I'm designing for the sealed lead acid batteries that I have on hand but am sure I'll be going to car batteries at some point (I'm already looking for deals : -) ).·The Panasonic ones I've got now are arranged as two by·6V units that are dead easy to split apart. (close up) They would work for the cell swapping you mention erco and I've been keeping my eye on 6V (motorcycle) auto-grade ones for just that reason.
OK.. the point.. This is my kids project as well so we've scaled back their suggestion of a battle-bot and scaled up my suggestion of a remote controlled lawn mower and we're building a treasure hunter.. It's got the hedge trimmer sweeping back and forth on the front followed by the metal detector on the back. When we detect something we use the camera to see what we've found.·The camera is basicly a port of a project from a few years ago that came out of an old "Radio Electroinics" magazine (73 de·VA4MGW BTW).·A small·on board "vacume" to suck·things up is a long range possibility.
I like these Curtis boards and I'm fooling around trying to control them with a digital pot chip. Thanks for the freebies Analog. I'm making a little test unit·to drive one of the Ametek motors and having it spin another one (with a clutch to explore braking functions) so I can develop an "add on" that would let my apply "dynamic" braking in such a way that the power would go back into the Panasonic batteries rather than being burned off in a big power resistor. I'm looking at using a relay·to switch to "braking mode" when nessasary. My very first effort I think will involve using an FET and an LM723 to manage the current flow·that the (now) generator produces. I'll attempt to·present·a variable "resistance" across the motor terminals·and as a result·stabalize·30+ volts across the input impedance·right down to the point where the current nessasary to maintain this voltage becomes useless to recharge the batteries.
Apologies in advance if some of these links don't match up.. I'm out the door to work 5 mins ago : -)
TIA
Post Edited (Hobbyist) : 5/11/2010 3:14:36 AM GMT
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·"If you build it, they will come."
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·"If you build it, they will come."
We sure think alike there erco. I've got a couple of 12V "mini" air compressors designed to blow up car tires from a car cigarette lighter and a couple of different "nozzel" units. My son was actually the one who thought we would need to blast away surface dirt before we bothered pointing the camera. That's on the back burner along with the on-board vacume unit though.
Here's an example of what I've been looking for... take a look at the Ametec link above and the motor in the pics on this guy's site... a perfect match.. 30V, etc.... now apply that cell splitting idea controlled by the "braking mode" relay... progress!
http://www.mdpub.com/Wind_Turbine/index.html
Thanks again.
Post Edited (Hobbyist) : 5/11/2010 3:21:46 AM GMT