Problem measuring servomotor current

Hi,

I’m trying to measure how much current my servo motor pulls in order to setup the circuit properly (I have to reduce 9V to 6V for the servo). The servo is a Futaba S3003.

The problem is, I can only get normal behavior when my multimeter’s plug is in the 10A (unfused) connector. I get readings between 0.1 and 0.4 (therefore 100 and 400 mA), but when I plug the cable into the “mA” (fused) connector the servo behaves very strangely, and stops responding to commands.

I’m not sure I’m checking this correctly so please help if you can.

Also, the manufacturer doesn’t list the operating current of the device. Any way to figure this out easily, or do I just need to check different transistor values in series and see which is the minimum current for proper operation?

Thanks.

Read the two links in my signature for an idea of the current draw and how you need to build your circuit to cope with it.

Duane B

Thank you, both posts are helpful for the beginner that I am, but I still don't understand why my multimeter behaved the way it did. There is also a big difference between the current you measured and the one I did. I'm not saying you're wrong, of course, just trying to understand what's going on here...

Ischia: Hi,

I'm trying to measure how much current my servo motor pulls in order to setup the circuit properly (I have to reduce 9V to 6V for the servo). The servo is a Futaba S3003.

The problem is, I can only get normal behavior when my multimeter's plug is in the 10A (unfused) connector. I get readings between 0.1 and 0.4 (therefore 100 and 400 mA), but when I plug the cable into the "mA" (fused) connector the servo behaves very strangely, and stops responding to commands.

The lower current-measuring range has too much series resistance, so the servo is dropping supply voltage when it tries to draw current. Expect the peak currents to be several amps, the multimeter can't follow the fast spikes.

Thank you very much! Much clearer now.

I assume that you are not putting any load on your servos yet, this will greatly increase the current draw as well.

Duane B

rcarduino.blogspot.com

I will be testing some loads this afternoon. Have you noticed major increases in 60° speed with light loads (weighing as much as a small-to-average screwdriver)? I will be needed very small travels (10°) but they'll need to be fast. I have 5 x S3003 plugged into a 5.2A 6V power supply.

I have 5 x S3003 plugged into a 5.2A 6V power supply.

You also need to measure the voltage drop from your power supply when the servos are under load. Measuring short duration events may not be easy using inexpensive test equipment.

http://rcarduino.blogspot.com/2012/04/servo-problems-with-arduino-part-1.html

One problem is, Duane does not say how he made the servo current measurements. Servo current is not a simple D.C. value, so it's unclear if a DMM will give an accurate value.

When the servo is not loaded, the current tends to have a pulse waveform, with the pulse being about 2-msec long and occurring every 20-msec, in step with the signal input. When the servo is "statically" loaded, ie twisting the servo horn, this pulse gets longer, up to 8-10 msec long. When the servo is moving, the waveform is much more complicated, and has an average value, a lot of ripple, and some noise spikes.

So, it depends upon how you measure it.

Very MUCH so. Consider a meter that 'samples every 1/3 second for an update of 3/sec... If a current pulse occurs during the in between time (non sampled) then the meter will not see it except the amount of current flowing when the meter does sample. In effect you have in the meter a high pass filter and will begin to give accurate readings when Fsample = Fload or direct multiples thereof. Ideally the meter should sample @ 2X Fload and multiples of that. This point is if I remember properly called the Nyquist frequency and is considered the minimum sampling rate. These meters work well with regular recurring ac wave forms and all are specified with a minimum measurement frequency. Your real issue is that you have not met that minimum frequency and your measurements are far too low in frequency to be accurately sampled and reported on the display. There is another more complicated factor and that is the waveform of the load current. Absolute current measurements are best done with a 'sampling' resistor and an O'scope since the O'scope can be set to trigger on the change and report the absolute or peak current which should be taken into account when specifying power sources, remember that wiring, switches, fuses are all series resistances and therefore affect the 'available' current. a 6V 10 A battery @ 10 ohm load will never deliver the whole 10A current, it will over time but the peak current can only be the short circuit current or 6A. It will deliver the whole 10A but over time/resistance.. it will deliver 6 A for 10/6 hours or 1 2/3 hours. a rather big difference... IMO

Doc

With these types of measurements I fall back to my old analog multimeter.

Use a scope and sampling resistor, an analog meter will only give accurate information about RMS (Root Mean Square) voltages and only for periodic waveforms that are sinusoidal in nature. As you depart from a sine wave to a pulse the meter averages the readings and thus doesn't even come close to the required or load current measurement which can lead you in a decidedly wrong direction as I tried to point out in my last post. That was the reason for the battery and load example. Remember that a Fourier transform will show that the faster the rise time of a pulse the more energy there is in it and when time converges to 0 the current becomes infinite that is IF the current can rise in 0 time it is infinite in value. It is the Current pulses that drive the stepper/servo, coil or relay, not the available voltage.

Doc

Docedison: Use a scope and sampling resistor, an analog meter will only give accurate information about RMS (Root Mean Square) voltages and only for periodic waveforms that are sinusoidal in nature. As you depart...

Totally impractical hanger flying for a simple servo setup. :roll_eyes:

Thank you, if I go the scope & RMS route I'll surely be diving into much deeper territory that I need right now, instead of figuring out if what I'm building is doable in the first place... I'll get back to the current issue when I've made some progress (unless I set the house on fire first).

I really brought up all that cr*p to point out how easy it is to be fooled by measurements that from all outward indications look good both in data returned and technique. Frequently there is more than appears on the surface, it helps a little to consider how a device works before attempting to power it up... Saves a world of wasted time and measurement as well...

Doc

I was also pointing out that, if you don't know how the measurements were made, you really don't know how good the data is.

As a general ballpark figure, I've always heard that old-style "standard" analog servos with 44 oz-in torque draw roughly 300-mA. Higher torque servos will be more and digital servos a lot more.

If one really wants to know the stall current of a servo motor, the bottom of the servo can be removed exposing the motor terminals. Connect the ampmeter and power supply to the motor terminals and allow the motor to run until it stalls at the hard stop, then check its current reading.

The problem is that the stall current isn't the same as the pulse current, that's why it's called stall current, All I have tried to say all along in too many words is that the measurement methods you are using won't return good results, if the stall current is equal or better than the pulse current the servo should work well , I do think the pulse might well be somewhat higher as it supplies the energy for the magnet created in the field which requires more energy to 'charge' up or magnetize than it does to maintain it's field which is the stall current. Probably the easiest thing to do is use wires for power and ground that are twice as big as you 'Think' you need. That has always worked for me.

Doc

I do think the pulse might well be somewhat higher as it supplies the energy for the magnet created in the field which requires more energy to 'charge' up or magnetize than it does to maintain it's field which is the stall current.

I would think the current flow would be lower during the building of the motor coil magnetic field, as the current flow is usually impeded while it is doing work to build the field. Once the field is built and not providing an impediment to current flow, the current flow would increase to the stall current value. Just some thoughts.

No because it takes a threshold current to begin to move the armature/rotor which can be several times the stall current or at any rate this is my experience. I designed a solenoid driver for a bi-polar or latching solenoid about 20 years ago. It was a 4700 uF cap "Dumped" into a 3 ohm coil @ 12V dc from a Yuasa NP1.2-12 battery and I measured currents in excess of 10A for 20 - 30 uS... The current was Max until the magnetic field built up that it was that high is perfectly normal. the coil should have drawn 4 A and did in steady state conditions but REQUIRED the large pulse to overcome the hysteresis imposed by the magnetic material (the core or solenoid shuttle) on paper with a 2200 uf cap the device was supposed to work and did only if one charged the cap and connected it directly yo the solenoid 6feet of 16 Ga 2 conductor wire was anough to prevent the operation of the solenoid under 2 ATM. or 30 PSI water pressure on the valve controlled by the soleniod. I also found that it 'required' a Mosfet with under a tenth of an ohm Rdson or it was unreliable with more than 10' of wire. I used a flipflop (7474) and that part has a 40nS rise time it was coupled to the FET gate with a 100 ohm resistor so I wouldn't loose energy charging the gate, I used the edge of the FF output to trigger my scope and typically it was about 5 uS to peak current, reached a plateau 30 us later and was done completely in less than 100uS and the peak was in excess of 10A... Why with a 3 ohm coil (DC resistance). Like the electrostatic field in a capacitor the electromagnet has a magnetic field that must be built up before the rotor/solenoid/armature can begin to work.

Doc