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BlueBird BMS-621DMG

Old 08-30-2011, 05:31 AM
  #1  
lockhaty
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Default BlueBird BMS-621DMG

Hi guys, I am new to the world of servos and have just purchased my 1st servo, a Blue Bird BMS-621DMG+HS. When the servo is not powered, if I have to turn te servo arm by hand, it rotates freely just over 180 degrees. However, when I try to send a pulse to the servo, I am only able to control it from just after 0 degrees to around 130 degrees. The servo responds to a PWM on timeof 0.9ms (@ around 10 degrees) to 2.1ms (@ 130 degrees) with a period of 20 ms. Would really appreciate some help!
Old 08-30-2011, 07:26 AM
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fizzwater2
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Default RE: BlueBird BMS-621DMG

what you're describing sounds normal to me.

The following are some specs that I borrowed from www.servocity.com describing the Futaba S3004 (standard) servo:

Control System: +Pulse Width Control 1520usec Neutral
Required Pulse: 3-5 Volt Peak to Peak Square Wave
Operating Voltage: 4.8-6.0 Volts
Operating Temperature Range: -20 to +60 Degree C
Operating Speed (4.8V): 0.23sec/60 degrees at no load
Operating Speed (6.0V): 0.19sec/60 degrees at no load
Stall Torque (4.8V): 44 oz/in. (3.2kg.cm)
Stall Torque (6.0V): 56.8 oz/in. (4.1kg.cm)
Operating Angle: 45 Deg. one side pulse traveling 400usec
360 Modifiable: Yes
Direction: Counter Clockwise/Pulse Traveling 1520-1900usec
Current Drain (4.8V): 7.2mA/idle
Current Drain (6.0V): 8mA/idle
Motor Type: 3 Pole Ferrite
Potentiometer Drive: Indirect Drive
Bearing Type: Single Ball Bearing
Gear Type: All Nylon Gears
Connector Wire Length: 12"
Dimensions: 1.6" x 0.8"x 1.4" (41 x 20 x 36mm)
Weight: 1.3oz. (37.2g)

Note that with a pulse width change of 400 uS (0.4mS) the servo will move 45 degrees. Most servos use a 1500uS neutral pulse width or thereabouts, so moving from 1.5 to 1.9 mS would move the servo 45 degrees one way, moving from 1.5mS to 1.1mS would move it 45 degrees the other way. That's pretty normal for typical servos.

The mechanical ability to travel 180 degrees between stops doesn't mean it's a 180 degree servo.

Old 08-30-2011, 12:06 PM
  #3  
A.T.
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Default RE: BlueBird BMS-621DMG


ORIGINAL: lockhaty Hi guys, I am new to the world of servos and have just purchased my 1st servo, a Blue Bird BMS-621DMG+HS. When the servo is not powered, if I have to turn te servo arm by hand, it rotates freely just over 180 degrees. However, when I try to send a pulse to the servo, I am only able to control it from just after 0 degrees to around 130 degrees. The servo responds to a PWM on timeof 0.9ms (@ around 10 degrees) to 2.1ms (@ 130 degrees) with a period of 20 ms. Would really appreciate some help!
Servos are designed to operate within a fixed digital regime, typically 1500us as neutral, 900us as left extreme and 2100us as the right extreme end.
T
he mechanical "extra" movement is to allow for movement of that centre point when extended sub trims andEPA are used.
Following may also be of interest with that servo:
<dt>Servo - Do not turn by hand
Servo - FAQ : Backlash - Gear Mesh & Deadband Slop, Correct Metal Gear Servo
much more under "Radio Systems, Accessories, Alterations and FAQ." at:
Alan's Hobby, Model & RC FAQ Web Links
</dt><dt>Alan T. </dt>

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