Author Topic: a few servo questions.  (Read 3284 times)

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Offline kwanziTopic starter

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a few servo questions.
« on: August 18, 2007, 10:19:57 PM »
If the servo is connected to power but the signal wire is not connected, is it supposed to move?  When I connect the servo to power, it twitches for a second. what does this mean? Also, if a servo is rated at 6 V max and the voltage I measured from the power source was found to be 6.22, is this significant enough to completely burn out the servo? 

One more thing.  Can you see anything wrong with this:  Using two relays connected to a microcontroller to act as signal switches, and connecting the signal wire of each servo to the output of each relay?  And both servos are connected to the same power source. 
thanks....

Offline Robotboy86

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Re: a few servo questions.
« Reply #1 on: August 19, 2007, 02:05:37 AM »
It shouldn't burn it out, if you are SUPER worried, you can build a very simply circuit..  power in, 6v power convertor, and power out.  Will drop that probaly a SMALL amount

Offline Admin

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Re: a few servo questions.
« Reply #2 on: August 19, 2007, 09:51:22 AM »
Quote
If the servo is connected to power but the signal wire is not connected, is it supposed to move?  When I connect the servo to power, it twitches for a second. what does this mean?
Sometimes electronic noise that appears like a normal signal can get to the signal line, causing random twitches. Dont worry about it.

Quote
Can you see anything wrong with this:  Using two relays connected to a microcontroller to act as signal switches, and connecting the signal wire of each servo to the output of each relay?  And both servos are connected to the same power source. 
Unneccessary. The servo signal line is low power, so you can hook that up directly to the microcontroller. For a typical DC motor however, then you would want to use a MOSFET, not a relay. A microcontroller cannot power a relay :P

Offline kwanziTopic starter

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Re: a few servo questions.
« Reply #3 on: August 19, 2007, 11:34:00 AM »
thanks for the feedback.  But I had a problem when I connected the servos directly to the microcontroller.  The servos were not giving the right amount of torque, so I measured the voltage as the servos were moving.  The voltage dropped to about 3.5 V when they moved and then went back up to about 5 V when they weren't moving.  I guess they weren't getting enough current...What's the best way to fix this?

Offline kwanziTopic starter

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Re: a few servo questions.
« Reply #4 on: August 19, 2007, 02:44:49 PM »
So if the MOSFET is the best solution to this problem, I was wondering why a relay doesn't work?

Offline Admin

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Re: a few servo questions.
« Reply #5 on: August 19, 2007, 06:14:07 PM »
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The servos were not giving the right amount of torque, so I measured the voltage as the servos were moving.  The voltage dropped to about 3.5 V when they moved and then went back up to about 5 V when they weren't moving.  I guess they weren't getting enough current...What's the best way to fix this?
Quote
Also, if a servo is rated at 6 V max and the voltage I measured from the power source was found to be 6.22, is this significant enough to completely burn out the servo?
Im confused what your power supply is. What type of battery are you using? What is it's rated voltage? The problem you are having is that your power supply is not supplying power fast enough, causing drops during power drains.

I recommend using either a NiMH or NiCad type battery, rated at 6V, and at least 1200mAh. Do not use a voltage regulator for this, it will fry.

Quote
So if the MOSFET is the best solution to this problem, I was wondering why a relay doesn't work?
A relay is a high power switch. A MOSFET is a low power switch. A relay will require 100's of mA while a MOSFET requires .01's of mA. This is a very common confusion people have, so a quick search of the forum will give you more explanation.

Offline kwanziTopic starter

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Re: a few servo questions.
« Reply #6 on: August 19, 2007, 06:25:14 PM »
Thank you for the reply.  I'm new to this, so i'm using a Basic Stamp Homework Board (BS2 microcontroller).  It has a 9 V battery power supply.  The servos directly connected to the board didn't work, so I attached two relays in place of the servos to get the signals.   I made a second 6 V power supply with 4 D batteries.  I used this to power the servos, and connected the signal wire of each servo to the output of each relay.  Unfortunately, the servos got fried...do you think that using a MOSFET in place of each relay would work (with new servos!) and a 6 V NiMH or NiCad type battery?  I'm still a bit confused about why the servos completely broke down.
Thanks!

Offline Joe

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Re: a few servo questions.
« Reply #7 on: August 19, 2007, 07:14:44 PM »
2 major reasons you do not want to use a relay. 1) Relays are not designed for high speed switching. They require much current and create harmful electrical noise. Could be what fried your servo. You're lucky it didn't fry your BS. They can be used to drive a DC motor that just gets turned on and spins continuously at 1 speed, but that's not what you're trying to do. 2) You are dealing with a Servo not a plain DC motor. Servos are easy because you don't need any special hardware to operate it. You just send it pulses from your microcontroller (through the signal wire) by setting the pins high and low with the program. They require a specific frequency and duty cycle but you can just find some code on the Web and it should be easy enough to modify. A plain DC motor requires something to help your microcontroller put current through the motor (transistors). Servos basically have those transistors, along with logic circuits and more, all inside that black case. They move partway around and stop. DC Motors spin continuously. The little DC Motor inside the servo is spinning continuously while the servo arm is moving to a new position but with the gear train and and feedback knob, the arm can't and doesn't want to spin continuously around (without physical modification at least).

And you never want to hook up any kind of high current device to the +5 and ground from that strip on the board. The separate battery for the power and ground (black and red) sounds like your doing that part right. You do need to make sure those batteries are grounded together.
« Last Edit: August 19, 2007, 07:15:59 PM by Joe »

Offline Admin

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Re: a few servo questions.
« Reply #8 on: August 19, 2007, 07:44:28 PM »
As a summary of the above posts:
- Connect servo signal line directly to the BS output pins
- DO NOT use relays or mosfets :P
- use a 6V NiMH battery pack to power the servos

Good reading that will really help you understand:
http://www.societyofrobots.com/batteries.shtml
http://www.societyofrobots.com/actuators_servos.shtml

 


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