Create a device for providing constant tension to string

[zerostar]

Hi all,

I am think of a project related to the tension and the effect shown below:

An object attached to a string roller:

1.When the object go right (forward), the strings will be retracted.


2. When the object go left(backward), the strings will be released from roller
When Stop, roller can spin anti clockwise direction for keeping the tension


Case 1

Case 2

For doing this, a motor attached with a roller head will be used as roller and using arduino for programing.
However, which types of sensor should be used to detect the tension or the movement of object?

Thanks.

 

[konradIC13]

I think you can monitor if the string is in tension with the motor current. When its gonna rise too high the you will know the motor have difficulty rolling the string back which means its already on high tension. If the current is low it means the motor can roll it more and tension is not high enough.

So, case one, the object is in position, roller not working, it might periodicaly try to roll the string back to, if the object is anchored somewhere the current will rise too high and the motor will be shut down . When you go to right hand side the periodic check will see that the current when starting motor is not rising too high so it will start rolling it untill the string is tensed and current again is too high which means the motor has to be stopped.

In case two same mechanism can be used to roll the string back (if current to high, the motor is blocked, no more rolling back) and the MCU will check periodicaly with motor if the string can be rolled back more. During the peroids where there is no check there will be moment when there will be no voltage on motor allowing for free rotation, its a moment for possible moving the object to left.

The motor circuit will need some control and monitoring to prevent shortcircuits and stuff when motor will be driven by MCU and when you will unroll the string forcing motor movement (generating current).

 

[zerostar]

I see, thank a lot, you are my saver!

 

[zerostar]

however, how to monitor motor current?

 

[konradIC13]

You can either use external component like those:
**broken link removed**
that convert measured current into appropriate voltage signal
or use build in Arduino ADC by using very small value resistor in series with circuit and then measure voltage drop on it and from that calculate current that flows in there.

The resistor should have low resistance to not affect voltage goint to your device yet big enough to make the voltage drop redeable by ADC. If the voltage is too small to read effectively you can use op-amp to amplify it.

So, if you have a 0.1 ohm resistor and there is 2Amps current the voltage across it is U=IR = 2*0.1 = 0.2V, also while picking resistor watch out for power dissipation in resistor P = I ^2 *R (may need to pick resistors less than 1 ohm if high currents flow).

 

[zerostar]

got it, thinks.

 

[zerostar]

Is there any examples using such principle?

 

[misterT]

If the system is not very dynamic then I would build a wire tension meter using three pulleys, and then control the motor to regulate the tension. This way you can have a brake, or use a worm gear, so that you do not need to constantly power the motor.

Driving the motor with constant current source automatically controls the tension, but it does heat up the motor and waste power. Lab power supply with current limit, and voltage set to "high", could be the easiest solution. If the system is very dynamic, then just use a constant current source.

You can use google to find examples of "wire tension meter" or a "constant current source".

 

[zerostar]

Thanks