RC Servo

[Atomic_Sheep]

Hi guys, I have a question in regards to RC servos. The battery pack for them is 4.8V 600mAh and each servo has three wires sticking out of it. (+ve), (-ve) and signal.

1.) Why is there no ground terminal? and what is the ground terminal designed to do?

2.) Given the battery pack specs, does that mean that each servo is running at 4.8 volts and 600mAh? Or does that amount decrease by a factor of the number of servos that get connected up?

3.) From what I read, the servo expects a signal every 20ms or so and the signal usually varies from 0-0.2ms, based on this and the fact that the battery is 4.8V and 600mAh what should be the amplitude of the VDC? According to the same source it should be around 3-6VDC (VDC - voltage direct current?) but how is this figure derived?

4.) Also how do you generate such a signal? Lets say the radio control joystick from a range of 0-100 is currently at 50% which means the servo should have an angle of 45 degrees (assuming a servo turns 90 degrees) then the potentiometer behind the joysticks controller is set at around 50% of its total motion also which means that... how does that 50% potentiometer setting translate into varying lengths of pulses?

Here's the thread where I got most of my info from:

https://www.rc-cam.com/servotst.htm

Please also note that just one pulse of info doesn't make the servo move the whole way, it only makes the servo move part of the way. You need a series of the same pulse in order for the servo to move to the desired location.

 

[Nigel Goodwin]

Hi guys, I have a question in regards to RC servos. The battery pack for them is 4.8V 600mAh and each servo has three wires sticking out of it. (+ve), (-ve) and signal.

1.) Why is there no ground terminal? and what is the ground terminal designed to do?

'Ground' is really only a reference, not anything specific - if you were to conenct a ground to a servo (for whatever reason), you would 'probably' choose the -ve - purely because that's the comon convention, back from the valve only days.

2.) Given the battery pack specs, does that mean that each servo is running at 4.8 volts and 600mAh? Or does that amount decrease by a factor of the number of servos that get connected up?

They run off that voltage, and take whatever current they require - imagine it's a bucket of water and a number of chickens are drinking from it, each takes what they want - the first chicken doesn't have to drink the entire bucket full.

3.) From what I read, the servo expects a signal every 20ms or so and the signal usually varies from 0-0.2ms, based on this and the fact that the battery is 4.8V and 600mAh what should be the amplitude of the VDC? According to the same source it should be around 3-6VDC (VDC - voltage direct current?) but how is this figure derived?

It's simply derived from the power supply, in this case 4.8V, minus a few losses.

4.) Also how do you generate such a signal? Lets say the radio control joystick from a range of 0-100 is currently at 50% which means the servo should have an angle of 45 degrees (assuming a servo turns 90 degrees) then the potentiometer behind the joysticks controller is set at around 50% of its total motion also which means that... how does that 50% potentiometer setting translate into varying lengths of pulses?

It simply alters the timing on a monostable or astable oscillator.

Here's the thread where I got most of my info from:

RC-CAM Projects: R/C Servo Tester

Please also note that just one pulse of info doesn't make the servo move the whole way, it only makes the servo move part of the way. You need a series of the same pulse in order for the servo to move to the desired location.

Servo's require continuous pulses, they can't move instantly, it takes them time to move - the position is compared to the pulses width, and moved until they match.

 

[DamoRC]

(1) The -ve is analogous to a "ground" terminal (low side)
(2) Each servo is running nominally at 4.8V and each will typically draw a couple of hundred mA. The 600mAh refers to the battery capacity - this pack can deliver 600 mA for an hour (ish). As the servo moves the voltage will probably drop a little as the servo draws power from the battery but it should be running at approx 4.8V. If you are running multiple servos together, the voltage drop will be greater but, unless you are using a lot of servos and a really small battery pack, this should not cause any problems
(3) I am not sure that the 3 - 6 V DC is derived per se. Rather the logic in the servo will work provided the voltage is above 3 V and can tolerate up to 6 volts before smoking something (so these are recommended operating voltages)
(4) There are many ways I have seen this done - google RC Servo and 555 timers - lots of DIY servo tester circuits are based on the 555. The following link might be useful.

https://personal.denison.edu/~kretchmar/391/Lab1Kretch.pdf

Edit - Sorry Nigel - I hit "Post Reply" before I saw that your response was posted

 

[Atomic_Sheep]

It's simply derived from the power supply, in this case 4.8V, minus a few losses.

So the servos most likely pick up pretty much any voltage as a signal as long as its greater than 0?

(4) There are many ways I have seen this done - google RC Servo and 555 timers - lots of DIY servo tester circuits are based on the 555. The following link might be useful.

https://personal.denison.edu/~kretchmar/391/Lab1Kretch.pdf

Excellent Suggestion... didn't even think to use google for this one, then again I didn't really know what to search for in the first place.

 

[audioguru]

imagine it's a bucket of water and a number of chickens are drinking from it, each takes what they want - the first chicken doesn't have to drink the entire bucket full.

Hee, hee, ha, ha, ho, ho!
That is a good one.