BARNARD MICROSYSTEMS LIMITED

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Servos

from http://www.futaba-rc.com/servos/digitalservos.html

The high speed Futaba S3155 servo unit

FUTM0653
Volts	Torque	Speed
4.8V	21.0 oz-in.	0.11 sec/60°
6.0V	23.6 oz-in.	0.09 sec/60°
Dimensions		Weight
0.86 x 0.4 x 0.79 in.		0.32 oz.

from http://www.futaba-rc.com/servos/digitalservos.html

from http://homepages.which.net/~paul.hills/Circuits/RxDecoder/RxDecoder.html

The signal output by the radio control receiver is a form of PWM. This may be useful for driving servos, but is inconvenient for general electronic applications. Therefore, several of the circuits on this site convert this signal into a voltage first. This is performed by this circuit. It is not a project in its own right, just a building block used by other projects.

First we must see what these signals look like. There is a document here which describes servo signals in some detail, but I’ll reproduce the timing diagram here for simplicity.

A positive-going pulse is sent at regular intervals, at most every 30ms although some manufacturers have a maximum of 20ms between pulses. The width of the pulse is between 1ms and 2ms. A 1ms pulse will cause the servo to turn to its minimum extremity, and a 2ms pulse will cause it to turn to its maximum extremity. The range is linear, which means the centre-point the servo position is achieved by sending 1.5ms, halfway between 1ms and 2ms.

As a point of information, there is a good reason for the space between each pulse. On a multi-channel transmitter, there will be other pulses interspersed between the pulses shown. For example, a four channel system will have pulses for each channel sent in sequence: 1 - 2 - 3 - 4 - 1 - 2 - 3 - 4 etc. There will be a gap larger than 2ms between pulse 4 and pulse 1 to tell the receiver that the sequence is about to restart and the next pulse is pulse 1. The gap between the other pulses is less than 2ms. The receiver therefore knows which pulse comes next, and can direct it to the appropriate servo output. Therefore the waveform shown above is for just one servo channel.

Above one can see the waveforms from the radio receivers to three servos, showing the synchronisation between the pulses. Each pulse length varies from a minimum of 1.2 mSec to a maximum of 2.09 mSec. The servo supply voltage was 5.0 volts, although the servo did work with a voltage down to 2.2 volts, but became erratic when the voltage dropped to 2.1 volts. When the supply voltage was 5.0 volts, the output pulse height was measured to be 3.344 volts.

Notice that the falling edge of the pulse to servo #1 coincides with the rising edge of the pulse to servo #2. As the length of a pulse is increased, so one notices all the pulses to the right moving to the right to maintain the synchronisation between the falling and rising pulse edges, as shown above.

The time difference between the leading edge of the pulses to any servo is measured here to be 18.4 mSec. This falls within the spec of the value being between 10 and 30 mSec. Thanks to Steve Borley for the above oscilloscope images.

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