Pulling slack taut on a wire with a constantly stalled motor (this must be bad,
Hey,
I have a project which involves taking up the slack on a 300 ft. long 1/8" thick wire and wrapping it around a spool as it is pulled in or out. The far end of the cable is attached to a robot with enough torque to pull the cable out. The other end is wound on a spool and is kept taut using a motor. As more or less line is paid out, I am able to manually vary the amount of pull (torque) of the take-in motor using a motor speed controller to vary the voltage. I do this with a potentiometer attached to the analog input of the motor controller. This all works well at low voltage/low torque.
I almost never need full power from the motor (in this case Dayton 4z144 from grainger, 24 V, brushed), and using the potentiometer, I usually feed the motor between 2-5 volts DC. But when I do need the full 24 volts, my motor controller and power source are unable to handle the motor's load. This is because the motor is almost always in a stalled state (as it is just keeping the wire taut, not consistently turning and spooling wire). Stalled, the motor delivers the necessary torque, but draws over 75 Amps.
My question is then what to do about this situation. I would sometimes like to use the motor's full voltage range, but I can't without blowing a fuse, or destroying my motor controller and power supply. Should I find a motor with a lower stall current, but the same torque curve? Control it differently? Someone suggested a feedback loop that would dynamically control voltage, and using a torque controller instead of a speed controller, but I would like another opinion, or an explanation of the difference.
My motor controller is a critical velocity SPD-315 D controller (15 amp continuous output), and my power supply is a TDK-Lambda DLP180-24-1/E (24 volts, 7.5 amps).
Any advice would be appreciated.
Thanks,
Blake
I have a project which involves taking up the slack on a 300 ft. long 1/8" thick wire and wrapping it around a spool as it is pulled in or out. The far end of the cable is attached to a robot with enough torque to pull the cable out. The other end is wound on a spool and is kept taut using a motor. As more or less line is paid out, I am able to manually vary the amount of pull (torque) of the take-in motor using a motor speed controller to vary the voltage. I do this with a potentiometer attached to the analog input of the motor controller. This all works well at low voltage/low torque.
I almost never need full power from the motor (in this case Dayton 4z144 from grainger, 24 V, brushed), and using the potentiometer, I usually feed the motor between 2-5 volts DC. But when I do need the full 24 volts, my motor controller and power source are unable to handle the motor's load. This is because the motor is almost always in a stalled state (as it is just keeping the wire taut, not consistently turning and spooling wire). Stalled, the motor delivers the necessary torque, but draws over 75 Amps.
My question is then what to do about this situation. I would sometimes like to use the motor's full voltage range, but I can't without blowing a fuse, or destroying my motor controller and power supply. Should I find a motor with a lower stall current, but the same torque curve? Control it differently? Someone suggested a feedback loop that would dynamically control voltage, and using a torque controller instead of a speed controller, but I would like another opinion, or an explanation of the difference.
My motor controller is a critical velocity SPD-315 D controller (15 amp continuous output), and my power supply is a TDK-Lambda DLP180-24-1/E (24 volts, 7.5 amps).
Any advice would be appreciated.
Thanks,
Blake
Comments
Depending on the material of the wire, you might have stretching/thinning issues over time.
I would think that you could sacrifice speed for torque to reduce the amount of required holding current with a torque converter (<--gearbox essentially)
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
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PG
AFA maintaining tension, simple solutions are usually best. How about a spring-loaded arm extending off your spool with a pulley on the end. Thread your cable through that. The arm swings through an arc trying to take up slack when the tension decreases, this can trigger a simple on/off switch to your spool motor to pay in or out. Could also rig a pot to measure the arm angle for continuous motor control.
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·"If you build it, they will come."
Another method is magnetic coupling of the motor and spool. This is used in modern cranes to keep power cords taught but not tight.
Google "magnetic coupling" for more info.
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The Truth is out there············___$WMc%___···························· BoogerWoods, FL. USA
-Phil
I'm sure you know that stalling a brushed DC motor is brutal. They are not intended for that type of service. You can burn brushes, commutator, or motor windings. Someone may just turn you in to the SPCM. [noparse]:)[/noparse]
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·"If you build it, they will come."
I guess the point is to be able to keep the motor running to apply enough torque to keep the line tight, but the viacous joint would keep the motor from stalling.
Duffer
Are you making robotic crop circles?
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·"If you build it, they will come."
Post Edited (erco) : 1/7/2010 5:17:37 PM GMT
@Blake, the only other suggestions I can make beyond a current controlled source is to match the gearing of the motor to the speed required to keep the cable tight. I am assuming the purpose is to take up slack as the robot moves back and forth and the only other idea I can come up with is to route the cable over pulleys so that a weight can do the job.
So in order:
@beau schwabe: I doubt I will have any thinning stretching issues. My robot robot weighs about 30 lbs, and has about 30 ft. lbs of torque. The cable is rated for over 400 lbs. of pull.
@erco: The issue at hand is not that I need to lock the line taut. If the robot is standing still the line does not pay in or out from the spool and thus the motor is stalled (still), being mechanically linked to the spool. Many motors seem to have incredible high stall currents, this one no exception. It's not really a matter of extreme loads really.
@$Wmc% and Duffer: I haven't heard of these before and they may work. It's probably like an automatic transmission, right? The engine keeps running when you stop and start the car. I'll look into it. But both of your ideas are basically coming up with a fix for an inherently mis-designed system: that the motor i'm using can't handle its own stall torque, right? Searching the web has led me to ONE company that advertises "tension control motors" designed to operate under full stall continuously. It's http://www.welco-tech.com/ but there must be other, CHEAPER (they want a couple grand per motor) solutions out there.
@kwinn and phil: I couldn't agree more. I will look into current regulated supplies, but even here, at 24V, 75 Amps, the dayton motor is simply not meant to handle its own stall conditions in a continuous load situation. I can stall this thing at 5Volts, but not at 24V. Therefore, my motor controller, power supply, and MOTOR all need to be changed.
@erco again: You are right, the motor does indeed spin backwards at times when the robot is traveling away from the spool. However this robot only moves about 6 inches/second, and I don't think a motor can draw more than its own stall current rating. Remember though, that for the most part, I only run this 24V motor between 0-5V. This totally enclosed, non fan-cooled motor can continuously handle stall conditions for hours (tested) with this voltage. I only pose the question in the first place because it may become necessary to run the motor at full power at some point in the future. And by this I really mean, "I may need more torque.."
About gearboxes: The motor is actually sprocketed to the spool with a 3.5:1 ratio. I am hesitant about any more gearing because my robot must pull the line from the spool as it travels. If I gear it too much, the tension on the line might be too much for the robot to pull out causing significant traction issues, etc. It has been my experience that gearhead motors are extremely hard to spin from the shaft end.
@kerryfenn: This is an idea, but then I have to coordinate my robot moving controls with my line tension motor controls, right? Like if I want my robot to move away from the spool, I have to tell the motor/winch to pay out exactly as much as the robot moves. Then what if the robot slips on the ground or doesn't move the same 5 feet that the motor pays out? Or maybe I'm totally misunderstanding you?
@kf4ixm: hydraulic might work, but the system needs to be electric for the requirements of the project.
finally, @kwinn again: Routing over a pulley is something I already do. Here's the big secret, this robot is wall-climbing and the cable is a "fall protection" measure in case power is lost. However, my spool's motor's pull on the line must be able to change so just having a weight on the other end isn't an option.
-Phil
Post Edited (Kerryfenn) : 1/12/2010 3:35:05 PM GMT
1. red axle
2. yellow spool
3. green electromagnetic brake
4. orange sprocketed motor run at a fraction of rated voltage and giving constant pull on the line to keep it taught
I propose to add:
5. purple extra-pull motor for lifting/lowering applications, and
6. blue electromagnetic clutch to engage/disengage purple motor from red axle
the extra-pull motor will be a gearhead motor with minimum stall current. This will be possible because the only time this motor will be used is when the robot is itself "out-of-service" and will not be pulling on the line. If it were to try to pull line out from the spool with the gearhead engaged, it would be unable to overcome the torque gear ratio of the motor.
This sounds like the easiest, albeit, not prettiest solution. I am still using the original motor in a way it wasn't intended, but i will not need to stall it at full rated power.
@kwinn: visit welco-tech.com for motors designed to be permanently stalled.
@kerryfenn: I like the winch idea in theory, but it really would require an encoder or something to exactly match robot speed and/or position and the amount of line paid out from the winch. Also, and more importantly, being a wall climbing robot, I have to keep total system weight down (to at least under 30 pounds) and I am trying to avoid mounting anything extra to the robot. I think the idea you are suggesting is similar to the "powered rope ascender" designed by some MIT students recently (google it, it's pretty cool).
Post Edited (Blake) : 1/12/2010 4:49:38 PM GMT