DC motor

[manz]

hi there,

I`m dealing with a dc motor to do some task. To make my explaination easier, the task is something similar to the power window for car, that is to lift the window up untill fully closed.

My question is that, how can I design a circuit to cut the current ( by simply activating a relay) supply to the dc motor when it is fully closed? is there anyway to measure the current flow that supply the current to the dc motor , just like the circuit of the power window of a car that is stop the current flow when it is already fully closed?

thanks alot.and by the way im dealing with a 12v dc.

 

[Papabravo]

I'm not sure, but I don't think there is a current sensor involved. On machine tools there are devices called "limit switches" which are activated when an actuator approaches the limit of its travel. This is called a "soft" stop because it just provides a signal to the controller that a limit is being approached. The next thing that happens is that the motor runs into a "hard" stop. Running into a "hard" stop is unlikely to damage the motor, but has the potential for bending metal depending on the size of the motor and the leadscrews and couplings it is connected to.

 

[manz]

yup i can see your point but im pretty sure that there`s no limit switch in car power window system and that`s why i`m using it as example in the 1st place.i`m not saying it is not a good idea using a limit switch,but for the project i`m dealing with now,it just have no place for any additional limit switches. the car power window system using some sort like a circuit breaker (electronically). thanks again.

 

[Hero999]

You can measure the current flowing through the motor using a hal effect sensor or low value resistor. The output will probably need amplifying which can be esailly done with an op-amp and using a comparator to detect current threshold at which you want to turn your motor off.

 

[Papabravo]

I thought Hall Effect sensors measured magnetic flux density, not current. Motors tend to be noisy and imprecise when trying to extract information from their waveforms, especially when they stall. It is an attractive and appealing myth that this can be done cheaply, easily and reliably. Many have proposed, but few have solved. If you find a way to do this, I'll certainly be interested in hearing about it.

 

[manz]

so do you mean there`s no way to measure the current like in the car power window unit to stop the motor, easily,cheaply,stablely? maybe a circuit breaker would do the job?5A circuit breaker maybe.is there such a thing?

 

[Papabravo]

A circuit breaker trips when there is an excess of current for some period of time. They have to be manually reset. Who is going to reset them and under what conditions? I know that when my radial arm saw hits a knot while ripping a 2 x 4 and stalls, it is a PITA to walk over to the breaker box and flip the switch.

You can measure current but the signal is noisy and difficult to extract information from. What this means is that you will stop the motor when you shouldn't and you will fail to stop it when you should. There may some clever DSP filtering processing voobah whizz bang algorithm that I haven't seen, but I doubt it. If you are convinced that this is possible, then by all means don't let me stop you from continuing the investigation. You have the benefit of my experience for what it is worth.

 

[Emantra]

well i dont know if it will work or not just try...

first of all calculate the time(say 't') needed for the window to fully closed then use the timer IC to cut down the current from the motor after the time 't'.

hope that it will work

 

[Styx]

I thought Hall Effect sensors measured magnetic flux density, not current. Motors tend to be noisy and imprecise when trying to extract information from their waveforms, especially when they stall. It is an attractive and appealing myth that this can be done cheaply, easily and reliably. Many have proposed, but few have solved. If you find a way to do this, I'll certainly be interested in hearing about it.

Yes Hall effect sensors measure magnetic flux BUT when current flows it generates a magnetic field. It is this field that the HE can detect and the strength of the field is proportional to the current flowing in the conductor

What exactly do you mean by extacting infomation is a myth? I do it every day be it with a low-value resistor to sense currents of a few amps in a Machine OR be it via a HE to sense a few hundred amps in a machine.

it is not hard and it is most certainly reliable, current-sensing for motor control is keeping airplanes in flight

 

[Papabravo]

Yes you can sense steady state currents. What he wants to do is use changes in current to detect things like a window meeting the door frame. If you and he can find a way to do it, then by all means do it and prove me wrong. Your knowledge and experience are just as valid as mine.

 

[Nigel Goodwin]

As already suggested, don't many car windows already do exactly this?, even stopping if a childs head is in the window?.

 

[manz]

As already suggested, don't many car windows already do exactly this?, even stopping if a childs head is in the window?.

that excatly what i meant.is that kinda circuit too complicated for a normal person with a limited knowledge in electronics like me?or is there any easier way but just as good as that is in the power window circuit to do the job? anyone please feel free to advise and give your opinion.

And for the circuit breaker thing,i`ve found it in the internet the circuit breaker that self-retract after the temperature goes down and waiting to trip for the next time without we manually resets it,but i just couldn`t find it in my local electronic`s shop.

so if there`s anyone that know maybe how to do the circuit breaker (electronically), please share that good peice of idea. thanks to all.

 

[Nigel Goodwin]

that excatly what i meant.is that kinda circuit too complicated for a normal person with a limited knowledge in electronics like me?or is there any easier way but just as good as that is in the power window circuit to do the job? anyone please feel free to advise and give your opinion.

To be honest, I've no idea how it's done - I presume it's by current monitoring?, but I've never even seen what one looks like!.

 

[giaosucan]

somebody knows how to control servo motor ???
I have to design a schematic which controls a servo motor.But
I haven't much experience about that.
thanks for helping me ..

 

[Optikon]

Why don't you measure the rotation of the motor shaft? Once you start the motor for operation, turn on the monitoring. You can use a shaft encoder or some similar scheme. When the motor stops (encountering any obstructing object be it the end or someone forcing it) you can sense that it is stopped and you remove the motor drive with that signal and then reset the monitor for the next operating cycle.

 

[phalanx]

Microchip has a "Window Lift w/ Anti-Pinch" board you can purchase. If you study the schematics and go over the included source code, you can figure everything out that you need to know without spending any money.

**broken link removed**

I think I remember seeing an Atmel app note about it too.

 

[giaosucan]

many thanks for your comment i'll try to do it

 

[manz]

if its all seems to be a bit complicated than i thought, maybe i just have to use the classic way by using the limit switch,just have to figure it out where to put it.

well i dont know if it will work or not just try...

first of all calculate the time(say 't') needed for the window to fully closed then use the timer IC to cut down the current from the motor after the time 't'.

hope that it will work

oo,forget to mention that the circuit cannot just being monitored by its time to fully closed because the timing differ with the load.so the time of each cycle might not be the same.thanks for the idea anyway.

 

[Papabravo]

In the microchip application note I found the phrase "filter the current" presumably to get rid of noise to be an interesting confirmation of my original comment. It is definitely a job for a processor to ignore the initial motor turn on spike which might be coincident with reaching the stop. What then?

It has been my experience in designing drive and control circuits for tape drives, disk drives, and printers over the last 35 years that these are difficult problems.

I think it is sad that the original poster lacked the determination to investigate and understand a solution that was beyond his ambition.

 

[manz]

i just found the circuit for the job.its a Fast Acting Electronic Circuit Breakers.but i dont know wether it can do the job or not (that is to cut the power supply to the dc motor).i need someone expert to look at it 1st and see wether is it suitable or not.thanks. here`s the add.

**broken link removed**