Motor spins

pkf

Member
Hi, I am trying to use a motor for a project. As it didn't spin at the beginning within an assembly, I used a multimeter to measure the resistance. When I set it to X1 scale and connected the multimeter probes, the motor started to spin. When I set the resistance to a X10 scale, I got a stable reading and the motor did not spin. Is it unusual for a motor to spin when connected to a multimeter ? Thanks
 

hippy

Technical Support
Staff member
It is probably a matter of what voltage and how much current a particular multimeter puts out and what the motor requires to make it turn.

I cannot say if it is unusual or not as I have never used a multimeter to measure a motor's resistance but I do use the continuity function of mine to light a LED dimly without any resistor to determine which lead is the anode and which is the cathode.
 

AllyCat

Senior Member
Hi,

Is it unusual for a motor to spin when connected to a multimeter ?
If it is a very small, low inertia motor and/or a rather "insensitive" multimeter then it is possible, but definitely unusual. What are the specified "full-scale" readings of the x1 resistance and the most sensitive voltage ranges on the multimeter?

Normally a multimeter is basically just a (milli-) Voltmeter with the other ranges switching in different resistances and maybe a drive voltage (battery). Typically, the most sensitive voltage range might be 200 mV and resistance range 200 ohms. In this case the meter could feed out 1 mA to the probes, so that a 200 ohm resistor drops 200 mV (Ohms Law). The next (x10) range might output only 0.1 mA so that a 2k resistor would drop 200 mV, etc..

It would be (very) unusual for 1 mA to be enough to spin a motor. Does the meter have a lower resistance range, or could there be a capacitor in the circuit? The meter might put out a much higher voltage into an "open circuit" which could slowly charge a capacitor and then then "kick" the motor when connected.

Cheers, Alan.
 

pkf

Member
Yes, the motor is a small one with length about 1.7cm and 'diameter' about 1cm.
The multimeter at X1 scale goes from 0 to infinity, but I would have switched the scale to X10, X100, or X1000 to get more accurate readings if needed.

The multimeter is an analogue one, and it records the DC AC value at 2000ohm/ V at the bottom left corner of the display so looks like the multimeter isn't of high precision (?), but how do we know when it (the analogue multimeter) is 'useful' (if it ever is) ? 1 reading comparison may not tell the whole story but I asked because I tested with a LDR. On a digital multimeter, it reads 17.3kohm and on this analogue multimeter it reads 16-17kohm.

On the X10 scale, the motor reading gave 10 ohms (inferred as a 10mV drop). I am not sure there is another way for me to conclude if there is a capacitor. The components are all surface mounted and each of them had only 1 number (not 2) on the surface.
 

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premelec

Senior Member
Analog meters are very useful - depending on what you are doing - for instance it is much easier to see if voltages are going up, or down compared to watching fast changing numbers on a digital meter... also the mechanics of a mechanical [d'arsonval] meter averages it's input. If you need precision and or accuracy you need to know your measuring instruments well - and the techniques to use them. All measuring instruments have some effect on what you are measuring - and sometimes this can be compensated for. In the case of motors about all you'll get with resistance measurement is possible stall current - which may be of interest. A running motor produces a back emf tending more toward constant current than a resistor.
 

AllyCat

Senior Member
Hi,

I am not sure there is another way for me to conclude if there is a capacitor.
My capacitor suggestion was only a "long shot" on the basis that I couldn't see how or why a DVM might be able to deliver enough current to rotate a motor. But the fact that it's an analogue meter, with rather a low sensitivity (20 kohms/volt is more common), is reason enough.

The operation of an analogue meter is rather different and in particular the "raw" meter is current-driven, not voltage-measuring as such. "Zero Ohms" corresponds to the Full-Scale Deflection of the meter, i.e. when the maximum current flows. The "limiting factor" is the "high resistance" ranges: Often the highest resistance range will use a higher voltage battery (to push more current through the meter coil) but the lower ranges all use the same battery. The next most sensitive range must push out enough current to drive the meter to FSD, so the lower ranges may need to have internal resistances that push out much more current than is needed to just drive the meter to FSD.

Cheers, Alan.
 

hippy

Technical Support
Staff member
I am not sure there is another way for me to conclude if there is a capacitor.
It may help to detail exactly what you are seeking to do, want to achieve, and perhaps why or what problem you have encountered or are trying to resolve.
 

westaust55

Moderator
From your multimeter test it is clear that the motor will rotate and has no mechanical (seized/jammed rotor) problem.

So not you need to determine why it did not operate when conencted into your PICAXE circuit.

How are you connecting the motor to the PICAXE?
PICAXE outputs are limited to 20 mA (25 mA absolute max) and many motor may draw more that that.
If you are connecting directly such higher currents will cause the PICAXE output voltage to drop due to internal resistance thus limiting the current and thus the motor may not run.
You should be using a transistor or a channel from a ULN2803 darlington array.

So let us know how you are connecting the motor.
A schematic diagram would be a great source of informaiton for folks here to make comment rather than guessing.
 

pkf

Member
What I wanted to do was a similar setup to page 42 of bot120.pdf.
I have already set up the picaxe without the servo and the ultrasonic sensor given in the diagram. I have a picaxe ultrasonic sensor but I do not have a picaxe servo now, so the motor I used is my own (I believe it can still be connected to Pin B.0 like the servo right ?)

The original intention was to have this picaxe moving until it encounters a piece of furniture (sofa) in a room and stop.
It does not have to stop when it (the bumpers) hit the legs of the sofa because I am not getting it to stop (moving) when it hits another obstacle like the wall. The sofa has legs which are ~23 cm above the ground, and without elevating the ultrasonic sensor as shown on page 42 too much, I am considering an option where I mount the motor on an assembly then have the ultrasonic sensor face upwards (so that when it detects a surface 'on top'), it stops. This top surface it detects is actually the bottom of the sofa.

The original question was related to the fact that I mounted the motor on an assembly (not the one in diagram) and it didn't work. I have changed to a different assembly since then but in parallel, given that I have not used the motor before, I went for a resistance test to see if something was wrong, but in turn it left me with a question why the motor spinned when connected to a multimeter. Hope this clarifies.
 

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inglewoodpete

Senior Member
I wonder if you do not fully understand how a servo "motor" works. Indeed, there is a DC motor inside a servo motor's housing but there is much more than that.

The servo's motor usually drives a chain of gears connected to the output shaft, which usually just changes position rather than rotate like a conventional electric motor.. The rotational position of the output shaft is monitored (usually with a potentiometer) and compared with the position signal being supplied by the controller (in this case, a PICAXE). Circuitry within the servo motor's housing then drives the motor clockwise or anticlockwise so that the output shaft position matches that indicated by the input signal.
 
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pkf

Member
Thanks for pointing out the concern. Yes, I am aware that a servo allows for a more precise positional output not present in a normal motor, but the details you listed have given me insights. I will have to give further thought to the design though. This will include the possibilities of keeping the design to just mounting the ultrasonic sensor to sense the surface when it encounters one, or having a servo motor (for the rotation to be more controlled).
Thanks also to everyone who contributed so that I could better navigate my way.
 

newplumber

Senior Member
Hi yes thanks inglewoodpete good explanation
I thought they just put a little beetle inside the servo and depending how much picaxe juice you feed made the beetle run a few/alot turns JK
no for reals I know nothing but your little explanation makes sense but how it matches the pot is beyond me
like you said tho ...alot more then just a motor spinning
your simple friend mark
 

AllyCat

Senior Member
Hi,

depending how much picaxe juice you feed made the beetle run a few/a lot turns
You might be able to "tell" the beetle what to do, but he also wants some "food". :)

That's why a Servo has three pins / wires. Two wires go to a battery or power supply to "do the work" (or feed the beetle). The third wire (from a PICaxe pin) is only a "control signal" to tell the Servo (beetle) what to do. Not only is the current from a PICaxe pin insufficient to "power" most motors, but the control signal is a short voltage pulse which won't supply any useful energy at all.

Sometimes it's even recommended to use separate batteries for the PICaxe and the Servo (to prevent electrical "noise" from the servo upsetting the PICaxe), but it's a common mistake to overlook that they do both need a "common earth". The first rule of electrical circuits is that they must be a "circuit" (i.e. paths/wires "to" and "from" the separate components).

As for how servos work, they basically use a hardware component called a "Comparator". The hardware can be quite complicated, but much the same can be done with a software loop, something like:

Code:
main:
  IF requested_position[B] >[/B] actual_position THEN motorforwards
  IF requested_position [B]<[/B] actual_position THEN motorbackwards
  IF requested_position [B]=[/B] actual_position THEN motorstop
Lots of extra code is required of course; reading the "positions", controlling the motor, closing the loop, and probably calculating requested_position - actual_position to determine and control how fast the motor is to rotate (the larger the "error", the faster it should rotate).

Cheers, Alan.
 

newplumber

Senior Member
Thanks Alan its funny how I just hook up a servo with some plan/design from a forum here
and it works flawlessly and not even once did I ever think how it works till yours and inglewoods simple explanations
I was always thinking what can I destruct with the little thing and true I did read they can make alot of E noise but I never had a issue
because what i was using it for wasn't important and if it was I would be having issues.

@pkf you could just find a useless copier machine from some second/third hand store recycle the motors out of it
I found a older one and it had 7 stepper motors in it with IFR position dials..of course probably wouldn't work for your project
I guess i am spoiled where I live because no one here knows electronics including me but I have fun testing/burning up rich mans junk for free :)
 
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