Controlling a 4 wire servo

Thread Starter

DQQpy

Joined Aug 31, 2017
17
Hi,

I'm trying to control a 4 wire servo that was used in an RC car for the steering. It has two power connections, but I can't make sense of the other ones. If there is power on the red and black wires, the motor turns into one position and stays there, no matter what I put on the other wires. I also tried to vary the voltage of the power, no difference. How can I control its position?

The motor: https://drive.google.com/file/d/0B8Ox1li7YRxoa3ZBY2RNakxCT0k/view?usp=sharing
The board it was connected to: https://drive.google.com/file/d/0B8Ox1li7YRxoREt1Z3RNcXp4OU0/view?usp=sharing
 

ebeowulf17

Joined Aug 12, 2014
3,274
I found some descriptions of this issue in rc sites, like this one:

https://www.rcgroups.com/forums/showthread.php?1461600-Want-to-use-a-3-wire-servo-where-there-was-a-4-wire-on-my-Boat

Sounds like it's not a servo in the normal sense at all. Two wires are tied directly to the motor and two more are tied to a pot which can be used for position sensing. The active electronics that drive the motor one direction or another in order to achieve a certain position are external...

In other words, you'd need something like an H-bridge driver to control the motor, as well as a circuit to read the servo position and determine what signal to send to the h-bridge driver. Doesn't sound like fun!
 

LesJones

Joined Jan 8, 2017
2,696
It looks to me like the grey and white wires to the white two pin connector are just in series with the motor. Trace out the shematic of the small board in the servo and tell us the numbers on the IC. I suspect the motor only rotates in one direction and is started by a short pulse (Closing the connection between the white and grey wires) The motor will then contunue to rotate and stop in sequence in one of four positions. These positions will corespond to (1) straight, (2) right, (3) straight, (4) left and then back to the first straight position. I suspect the IC is a diode array used to supress voltage spikes from switching the motor. I suspect the plastic disc on the end of the shaft closest to the IC is a four position rotary switch.

Les.
 

Thread Starter

DQQpy

Joined Aug 31, 2017
17
That sounds really interesting.

In other words, you'd need something like an H-bridge driver to control the motor, as well as a circuit to read the servo position and determine what signal to send to the h-bridge driver. Doesn't sound like fun!
But what would these signals look like? PWM? Could I reuse something of the old controller board?

It looks to me like the grey and white wires to the white two pin connector are just in series with the motor. Trace out the shematic of the small board in the servo and tell us the numbers on the IC. I suspect the motor only rotates in one direction and is started by a short pulse (Closing the connection between the white and grey wires) The motor will then contunue to rotate and stop in sequence in one of four positions. These positions will corespond to (1) straight, (2) right, (3) straight, (4) left and then back to the first straight position. I suspect the IC is a diode array used to supress voltage spikes from switching the motor. I suspect the plastic disc on the end of the shaft closest to the IC is a four position rotary switch.
Les.
On the servo IC it says NK2903H, 0104-DK, but what do you mean by trace out the schematic?
Also, I just tried to bridge the gray and white wires, but nothing happened.
The plastic disc I assume you're talking of looks like it detects the position via 3 pins and some conducting material, here is a photo of it:
https://drive.google.com/file/d/0B8Ox1li7YRxoZlZJNnB2T21qOVU/view?usp=sharing
 

ebeowulf17

Joined Aug 12, 2014
3,274
That sounds really interesting.


But what would these signals look like? PWM? Could I reuse something of the old controller board?


On the servo IC it says NK2903H, 0104-DK, but what do you mean by trace out the schematic?
Also, I just tried to bridge the gray and white wires, but nothing happened.
The plastic disc I assume you're talking of looks like it detects the position via 3 pins and some conducting material, here is a photo of it:
https://drive.google.com/file/d/0B8Ox1li7YRxoZlZJNnB2T21qOVU/view?usp=sharing
Hmmm, I should've taken a closer look at your pictures before I commented earlier. I don't see any way that the wiring arrangement in your pictures would match up with what I was reading about. I think what I described is a totally different 4 wire setup. Sorry for leading you astray!
 

ebeowulf17

Joined Aug 12, 2014
3,274
That diagram looks promising!

As I look more closely at the pics, it seems like neither of the external white and grey wires goes directly from the outside world to the motor. Each goes to to the board, potentially straight to the chip pins marked L and R, which would make sense since the external board that originally drive this thing says RT and LF near the outputs for those two wires.

Two separate white and grey wires go from the board to the motor and they seem to line up with the chip pins on the diagram that go to the motor.

It still doesn't make a lot of sense to me how this would work in practice. Does it only go extreme left, extreme right, and maybe center? Maybe voltage on the LF wire makes it go left to a certain point, and voltage on the RT wire makes it go right to a certain point? Or could it be looking for a servo style pwm signal on either left or right input (but not both at the same time) to move it a specific proportional amount left or right, and in the absence of such signals it seeks center?

Sorry, I should stop guessing now. I love a good mystery, but obviously I have no idea what I'm talking about here!
 

Thread Starter

DQQpy

Joined Aug 31, 2017
17

jayanthd

Joined Jul 4, 2015
904
Why do you think it is a servo motor ? It only rotates a certain angle for steering ?

May be it is a dc motor with feedback. Maybe dc motor while running is generating dc voltage as generator and providing it as output. Open the motor and see its commutator. maybe it has two commutators. One to feed dc voltage and one to take out dc voltage.
 

Thread Starter

DQQpy

Joined Aug 31, 2017
17
Why do you think it is a servo motor ? It only rotates a certain angle for steering ?
May be it is a dc motor with feedback. Maybe dc motor while running is generating dc voltage as generator and providing it as output. Open the motor and see its commutator. maybe it has two commutators. One to feed dc voltage and one to take out dc voltage.
I'm absolutely not sure if that's a servo, it just behaves similiar. The angle is limited mechanically by its casing. The motor doesn't have screws, and I don't think I wouldn't destroy anything.
Does the original circuit work?If so, look at the signals on the wires while driving it.
It may work but I don't have the remote to control it anymore.
 

jayanthd

Joined Jul 4, 2015
904
I'm absolutely not sure if that's a servo, it just behaves similiar. The angle is limited mechanically by its casing. The motor doesn't have screws, and I don't think I wouldn't destroy anything.

It may work but I don't have the remote to control it anymore.
What is your needs ?
 

jayanthd

Joined Jul 4, 2015
904
I dont have any datasheets. Yes I thought so too.
Also, I just found the remote of it, but little me made sure it won't be of any use :(
It is an RF remote control.

I can design a new 433 MHz RF RC controlled car for you using L293D if you are interested in one. It will have speed control and direction control (forward, reverse, left, right).
 

Thread Starter

DQQpy

Joined Aug 31, 2017
17
Thanks for the offer, but I already found the solution, and I feel really stupid.
All you have to do is put grey and white into 5V and GND, depending on which you put where it steers in each direction.
But the important part is to use two different power sources for red/black and grey/white, or nothing happens.
So it does indeed look like I need a H bridge, but I wonder how I should organize 2 power sources, and which voltages they need...
 

jayanthd

Joined Jul 4, 2015
904
Thanks for the offer, but I already found the solution, and I feel really stupid.
All you have to do is put grey and white into 5V and GND, depending on which you put where it steers in each direction.
But the important part is to use two different power sources for red/black and grey/white, or nothing happens.
So it does indeed look like I need a H bridge, but I wonder how I should organize 2 power sources, and which voltages they need...
Motor probably need 6V. You can use a 9V or 12V power source. You can use a LM7806 regulator for motor. If you need higher voltage like 12V or 9V you can directly get them from battery.
 
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