General question about motors

Other Hardware General Electronics
1

Hi, i've been working with arduino on a hobby basis for about 2 years, i've tried several projects, but i've been avoiding motors for the time being, with a view to getting into the motor side of things somewhere further down the line.. but a small emergency has prompted a push to get into it now, and i need some advice as i must be missing something along the way.

SO... i have a new baby, not my first, and we have a fisher price swing, which gently rocks the baby and allows us a little time to do things like eat and go to the bathroom... but it has died, the motor still works, but the electronic board is dead. i will get into fixing that later so i may be back for some support with that. but in the mean time i need to get the swing swinging..

So i started with suppying a few volts directly to the motor from a cheap power suppy, and it turns. but it doesn't seem to supply enough current, it just seems to touch the pendulum of the swing and not push it. or at higher voltage it just slaps against the side of the chair, without transfering any movement

So i tried batteries thinking that they could supply more current, and i found that this does work quite well, but if the batteries are too new, and the voltage too high, it is not synchronised with the speed of the baby pendulum, and often cancels out any gained momentum. It works at a certain voltage but not for more than a few hours and then the voltage is too low which is also not good..

So i next thought i should try a cheap buck controller from china with a controllable voltage output.. and i thought that as it said CC for constant current, that the max current would be available at whatever voltage i selected for controlling the speed.. but again now.. i have the same situation as above where the pushrod kisses the seat without really pushing it.

So i thought it was time to ask for some specific advice from the forum. The motor it contains seems to be as follows

Model: RF-500TB-18280
Nominal voltage: 6 V DC
Operating voltage range: 3 - 6 V DC
No load speed at 6 VDC: 5,150 RPM
No load current: 50 mA
Full load (maximum efficiency @ 6VDC) performance characteristics:
Speed: 4,300 RPM
Current: 260 mA
Torque: 21 g-cm (2.06 mN-m)
Power output: .27 Watts
Stall torque: 135 g-cm (13.2 mN-m)
Stall current: 1,870 mA

What options would i have and what would be the advantages/disadvantages of the different options?

I would be happy enough to just have something stand alone, or something involving the arduino, i've seen some boards from china with a knob to control it and this too could be a good option. but i clearly dont have the experience with motors to know what would or wouldn't work..

Desperately seeking answers to save my tired baby-carrying arms. :smiley:

2

It's hard to help when we don't know anything about the electronics or mechanics...

That's a rather small motor at 6V and about 1/4 Amp.

Did originally plug-in or did it run on batteries?

Is the motor geared-down?

Does the motor or mechanism reverse (electrically or mechanically) or does it just push in one direction and then release? Are there any sensor switches so it "knows" where it is or which way it's swinging, etc.?

I'm going to guess the trick is "synchronization", just the way you synchronize your pushing when you push a kid in a swing. That might just be a timed pulse that gives the motor a "kick" once per second or so. Or, it might have a switch (or two) to sense the position of the swing.

So i started with suppying a few volts directly to the motor from a cheap power suppy, and it turns. but it doesn't seem to supply enough current...

...I tried batteries thinking that they could supply more current, and i found that this does work quite well, but if the batteries are too new, and the voltage too high, it is not synchronised with the speed of the baby pendulum...

...So i next thought i should try a cheap buck controller from china with a controllable voltage output.. and i thought that as it said CC for constant current, that the max current would be available at whatever voltage i selected for controlling the speed.. but again now.. i have the same situation as above where the pushrod kisses the seat without really pushing it.

Motors (like most things in electronics) generally run from a known voltage. The current depends on the resistance/impedance of whatever's connected ([u]Ohm's Law[/u]).

When you use PWM to control motor speed, you are varying the average voltage.

In the case of a motor the impedance depends on the mechanical load... With the motor running free with nothing connected to the shaft, the impedance is high and only a small amount of current flows. The more work the motor does, the more current flows. The impedance is minimum and the maximum current flows when the motor is stalled, or when the motor first starts and it's "fighting" inertia to get started.

3

A couple of questions:-

What are the mechanics? You mention a pendulum, but don't go into further detail, so i'm having trouble picturing exactly how it works.

What type of batteries did you use when it almost worked properly? (Voltage & chemistry)

What were the ratings of the 'cheap power supply' that didn't work?

What was the 'constant current' supply you mentioned? Got a link? Constant current usually means that the output voltage will vary, to keep a constant preset current flowing through the load. This isn't really what you want for a motor.

For 260mA typical current, you'd get away with a pretty simple power supply - even a 9V plug-pack/wall wart and an LM317T regulator, although any switching regulator with more than 1A output and the right output voltage range should work. I think that using an Arduino would be overkill.

And finally, out of interest, how did you get such detailed specs for the motor? I would have expected it to be a 'cheapie', with an untraceable part number, as is the case with many similar mass-produced devices.

I see that Doug answered as I was typing, with similar questions, but I'll still post this because between us we've covered everything we need to know. :slight_smile:

And I agree with Doug - 6V at 250mA is only 1.5W, very low power to do what it has to do, I would have thought. Are you sure those motor specs are correct?

4

ok.. i'm quite confident with the specs, it operates on a type of crank action, it starts with a worm driving a gear, which is then connected to a springy rod which then pushes another pushrod which swings the swing.... complicated i know. It is an amazingly weak mechanism, but it works with the idea that there is a removable chair that hangs at the top, and then the push rod slowly adds momentum like a swing at a kids play park. it doesn't lift the whole weight at any time but just slowly brings it up to swinging speed and maintains it.

Interestingly there is a temporary hack where you can replace the motor with one from a automatic air freshener. (Yes, the little motor that presses the spray button).

SO.. the other questions.

I got the right kind of current and voltage with 2AA alkalines in series that were ok when they were running at a depreciated 2.4 - 2.6v and i think were drawing around 200 - 500 mA but my memory might be hazy..

The cheap power supply was the type of replacement wallwart style thing with 3, 4.5, 7, 9, 12v etc rated for 1.5A max at 12v

The other supposed constant current supply was LIKE THIS

The motor did have a pn on it and i guess they fail a lot, and a search for the part number RF-500TB-18280 showed that they are frequently listed with the "fisher price swing replacment motor" somewhere in the description.

I think that covers the questions... one more comment, all i need to be able to do in my mind is to supply enough current at these low voltages, and control the speed..(is this assumption correct)? i would like the speed to be controllable, so that i can tune it when the baby gets bigger and heavier.

Sorry if this rambles a little.. tiredness comes with the new baby and seems to make me waffle a lot!

Thanks again for any help!

ps.. would something like this do what i want it to do?

http://www.ebay.co.uk/itm/272002772511?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

5

but it has died, the motor still works, but the electronic board is dead....

...ps.. would something like this do what i want it to do?

http://www.ebay.co.uk/itm/272002772511?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I doubt it. Like I said, motors usually operate from a constant voltage source, not a constant current source.

We still don't have a clue to what the old electronic board did. Do you think it was just a power supply?

Let's try this - What kind of connections did the board have? I assume there are 2-wires to the motor and two wires to a battery (or to AC power)? And, wires to an on-off switch somewhere in the circuit? Anything else?

I got the right kind of current and voltage with 2AA alkalines in series that were ok when they were running at a depreciated 2.4 - 2.6v and i think were drawing around 200 - 500 mA but my memory might be hazy..

The cheap power supply was the type of replacement wallwart style thing with 3, 4.5, 7, 9, 12v etc rated for 1.5A max at 12v

I assume you set the power supply to 6V to match the motor's voltage rating? Did you try the other voltage settings? Did you measure the voltage? Since it's rated at 1.5A, and you were drawing less than that from the batteries, the current limit shouldn't be a problem. (If it can't supply the required current, the voltage will drop.... Ohm's Law again.) Maybe the motor's start-up current is causing the power supply to shut down?

it operates on a type of crank action, it starts with a worm driving a gear, which is then connected to a springy rod which then pushes another pushrod which swings the swing....

So, the motor doesn't reverse? And, it doesn't start & stop? It just runs constantly at the right speed right speed the thing works?

6

DVDdoug:
I doubt it. Like I said, motors usually operate from a constant voltage source, not a constant current source.

The link was actually for a voltage regulator not a constant current supply so in this case it would probably work.

It's really hard to get a motor to turn at a consistent speed from an unregulated battery pack.

The voltage regulator might be the easiest solution but a proper solution would be to use some sort of feedback to make sure the motor was turning at the correct speed.

7

DVDdoug:
We still don't have a clue to what the old electronic board did. Do you think it was just a power supply?

---->It was a small board with an opamp chip and a pnp transistor.. so maybe just a power supply?

Let's try this - What kind of connections did the board have? I assume there are 2-wires to the motor and two wires to a battery (or to AC power)? And, wires to an on-off switch somewhere in the circuit? Anything else?

Yes, two wires to the motor two to a 6xseries d-cell battery pack, also another board with power switch and buttons to play melodies and change through 3 different preset speed settings.

I assume you set the power supply to 6V to match the motor's voltage rating?

Well i started at 4.5v and already it was turning way too fast for speed of swing.

Did you try the other voltage settings?

Yes, but neither was quite correct in terms of speed... too fast/too slow

So, the motor doesn't reverse? And, it doesn't start & stop? It just runs constantly at the right speed right speed the thing works?

Yes, thats how it works.

8

DuaneDegn:
The link was actually for a voltage regulator not a constant current supply so in this case it would probably work.

It's really hard to get a motor to turn at a consistent speed from an unregulated battery pack.

The voltage regulator might be the easiest solution but a proper solution would be to use some sort of feedback to make sure the motor was turning at the correct speed.

My ideal solution would be to avoid battery pack and supply from AC -> DC by whatever method is recommended..

The Ebay link is for something that i ordered a few days ago. Is this more likely to work than the other supply that i used that didn't seem to have enough current? i assume the one i tried must have not been able to supply enough current at the voltage i set it to, whereas the ebay link has control to set the current and voltage.. whereas the first was just able to control voltage...

Could you recommend yet a better solution? what kind of supply would you use if you had to do this.?

Thanks again for all the comments and help. Even if i don't get this working i am learning on many fronts. :slight_smile:

9

All you need is to be able to control the voltage. Provided the power supply can supply enough current the speed will be proportional to the voltage. If you have an adjustable current limit, set it to just
above the full load current for the motor (260 mA) to give the motor some protection in case it is blocked.

Russell.

10

Gambituk:
The Ebay link is for something that i ordered a few days ago. Is this more likely to work than the other supply that i used that didn't seem to have enough current? i assume the one i tried must have not been able to supply enough current at the voltage i set it to, whereas the ebay link has control to set the current and voltage.. whereas the first was just able to control voltage...

You provided two ebay links in the earlier post. Here's the first one.

Gambituk:
The other supposed constant current supply was LIKE THIS

I was mainly correcting your statement about the LM2596 being a constant current supply. As you correctly identified in a later post, this is a constant voltage regulator.

This could do the job of controlling the motor reasonably well. As long as the power supply you're using can source enough current (and the current is within the regulator's limits), this regulator should keep the motor turning at a consistent speed.

The second link you provided looks like it's to a constant current regulator.

Gambituk:
ps.. would something like this do what i want it to do?

http://www.ebay.co.uk/itm/272002772511?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I'm not sure this one is needed. As russellz pointed out, you could limit the current so if the motor is blocked, the you're not as likely to burn it out.

I think the constant current regulator is more commonly used to charge batteries or power LEDs than it is used to power motors.

11

Do you have any pictures of the old board? I'm curious just what the old board did. My guess (based on your limited descriptions) - is that it is actually some kind of 555 timer board - either for PWM speed control, or maybe some kind of variable pulse/bump control (that is, it makes the motor run for a bit, then stop, then run again, over and over - kinda like a long-delay PWM in a way).

But I am curious what makes you think the parts are an op-amp and a PNP transistor? I only base my thought about in that a 741 op-amp is an 8-pin package, and the 555 is an 8-pin package (unless you have the part numbers involved; I suppose there might be some way to use an op-amp in a motor control situation for a swing). Also - I would expect the transistor to be an NPN type - not that PNP couldn't work, just that low-side switching is easier, and NPN (or N-channel - if a fet) is cheaper than PNP/P-channel devices.

Before you can make a solution, you need to know exactly what is needed for the motor (voltage and current needs) - which it sounds like you mostly have. But you also need to know how it was controlled by the swing's electronics - so you can replicate that.

Ultimately - you just need to get power to the motor (it doesn't need to be regulated in any manner - in fact, you want to avoid this) - and control it with a transistor (NPN or N-channel). Then, have the Arduino control the transistor in the same manner as it operated before.

BTW - did you try to figure out what was "burned out" in the original board? Likely it was the transistor, but who knows. Still, it might be easier and cheaper just to fix that part, if you think you understand enough to troubleshoot it (and you have the tools).

12

Hi, just a little feedback. The second psu finally arrived, the one with control for current and voltage, and i finally have a working swing again. So thanks again for all the help and assistance.

Gambituk