Spectrum Linear servo.

Other Hardware Motors, Mechanics, Power and CNC
1

Hi,

I've worked with a variety of servos in the past (Futaba and Hi-tec) without problems and I've just taken delivery of spectrum linear servo.....
http://www.ebay.co.uk/itm/Spektrum-SPMSH2040L-2-9G-LINEAR-LONG-THROW-SERVO-/380581300862?_trksid=p2054897.l4275

I'm using the standard servo library on a mega2560. But so far I've got no signs of life out of it. I've connected the brown lead to ground, the red lead to 3.3v and the orange lead (which I assume is the control lead) to pin 3.

The spec says the voltage range is 3.2 - 4.2v with a maximum current of 40ma - so the 3.3volt supply on the mega should be fine.

I've no idea what range of pulse widths it's expecting, but I guess it's similar to other servos, I could be wrong.

Here's the full sketch.....

#include <Servo.h>                      // Low level servo library
Servo _servo;                           // servo

void setup() 
{
  Serial.begin(115200); 
  Serial.println("start");
  _servo.attach(3);
}

void loop() 
{
  delay(2000);
  Serial.println("Width 1500");
  _servo.writeMicroseconds(1500);

  delay(2000);
  Serial.println("Width 500");
  _servo.writeMicroseconds(500);

  delay(2000);
  Serial.println("Width 3000");
  _servo.writeMicroseconds(3000);
}

The code compiles and runs (i get the expected output on the screen) but the servo itself doesn't respond. I can't help feeling I'm missing something obvious, but just can't put my finger on it!

thanks

2

"Similar to other servos" would be 1000 to 2000 microseconds, not 500 to 3000.

Where did you get the 40 mA Maximum Current specification? I didn't see that anywhere and other servos can run as high as 1000 mA.

3

I wouldn't power an inductive load like a servo from the 3.3V rail, its not designed for
inductive loads (on old Uno's it could destroy the FTDI chip).

A separate 3V battery pack, or a separate regulator (to limit the risk), might be safer.

That servo also needs at least 150mA to perform to its published specs(*) at 3.3V, just
from the speed/force product - in reality I'd expect more like 0.5A peak. Nowhere do I
see a 40mA figure.

(*) speed 13.5mm/0.13s = 0.1m/s
force = 330gf = 3.3Nm
power = force x speed = 0.33W (more like 0.6 to 1W allowing for friction loss)

4

"Similar to other servos" would be 1000 to 2000 microseconds, not 500 to 3000.

Yes - I started with a range of 1000 to 200ms, but widened the range as I got more desperate.

Where did you get the 40 mA Maximum Current specification?

Good question, I can longer find the link. I think it was from a discussion form somewhere, but I can't find it on any datasheets.

The servo is very small and lightweight so I doubt it draws a massive current. I have checked the voltage of the 3.3 pin and it doesn't alter when I plug in the servo so I think it's ok. I will however try it with a higher current source just in case.

5

40mA sounds like a no-load current.

If in doubt with a small servo assume 1A, more for larger servos.

6

MarkT:
force = 330gf = 3.3Nm

You mean 3.3N not Nm, but clearly typo since you used the figure as force in next step 8) to get power.

And the power of 0.3W would give a current of about 100mA at 3 or so volts. That would be under load, when asked for that force and at that speed, but wouldn't account for start up current which would be significantly more.

OhMyCod:

Where did you get the 40 mA Maximum Current specification?

Good question... I can't find it on any datasheets.

Servo makers do seem strangely silent on the issue of current, I've noticed.

7

OhMyCod:
The servo is very small and lightweight so I doubt it draws a massive current. I have checked the voltage of the 3.3 pin and it doesn't alter when I plug in the servo so I think it's ok.

I'm sure it draws very little current, until you try to spin the motor. :slight_smile:

8

ok, got it work!

Here's what I learnt, firstly, the spec is fibbing, it doesn't start to work at 3.2 volts as claimed, it doesn't show any sign of life until 3.6 volts.
Secondly it has a very tight pulse width range about 1200 to 1800us.
Thirdly, if you give it a pulse width outside of the suggested range it seems to sulk for about 20 seconds before responding to any other commands.

Just a final point, the figure I quoted of 40ma seems to be about right under no-load conditions, although I admit that I've just spotted a calibration certificate dated 1986 on my Avo.

Thanks for your help guys.

9

Hi,
I dont suppose you have your code that worked?
I cant get it going?
Cheers
Tom