Joystick control from Analog Input

[dbh6]

Hello All,

PLC is a SLC504

I have an application through which i'am to control a extruder forward and reverse (0-100%) using a joystick. Now when the operator uses the joystick the joystick will trasmit a 0-10V signal to a 1746 Ni8 analog input card module, count ranging from -32768 to 32767, which is scaled to 0-10V then based on that analog value received from that joystick we will control the Extruder using a analog output module ( type not decided yet). So basically get a 0-10V signal from joystick then use that 0-10V value to control the analog output module 0-100%.

Now their are four positions for the joystick. For example for simplicity when the joystick is free meaning when no one is using it, lets say the voltage reading 4.9 volts. That means at 4.9V the extruder should not move. Then lets say anything from 0V to 4.8 V the extruder moves backwards, and finally voltage value from 5 all the way to 10V the extruders moves forward.

How can i make it so based on that example control the analog output card 0-100% value? to handle the joysticks forward, backward and neutral positions??

 

[PLC]

Just curious
Why would an extruder want to go backwards?

 

[dbh6]

@ Plc good question that's one of my questions for the customer but for now I'm just trying to figure out how to do the control

 

[dbh6]

Anyone?

 

[TimWilborne]

There are analog joysticks out there that are pretty much potentiometers. But usually there would be a separate one for each direction. I think trying to keep it set at rest to 4.9 volts is going to be tough. You are going to find over time you will have problems with the extruder drifting due to the fact that the voltage is not 4.8 or 5V at rest.

Do you have two analog inputs and two discrete inputs available? I would get an analog joystick that also has a discrete switch for each direction. That way you know whether the operator has the joystick in the forward or reverse direction or at rest from the discrete, then you have a forward analog speed and a reverse analog speed

I've never used these guys but they came up on a quick Google
http://www.p3america.com/industrial_joystick_full_size_selection_guide.htm

 

[bernie_carlton]

Provide for a generous 'deadzone' in the middle. Let's say 4.5 to 5.5 means no movement. Check for noise and at least double or triple it.

Perhaps an additional momentary which would also have to be closed to permit movement regardless of where the pot is. Don't start movement if the voltage is outside the deadzone when the button is first pressed.

Of course the external circuitry should have all appropriate safetys.

 

[dbh6]

I just got information that their is another selector switch that controls the directions for forward reverse and but none for at rest so if it the selector switches are not In The forward and reverse directions then they must be at rest or neutral . As TW controls pointed out the joystick is the potentiometer to control the speed. @ Bernie thanks for the heads up I will definitely incorporate some kind of deadband to compensate for noise etc.

The only point I'm trying to verify is once we know what the rest analog value is and have setup our appropriate dead bands through which we can start movement from is that what's used to control our output %???

For example lets we have the selector switch In the forward direction and say we define at 5.1 v we start movement so to control our output % our scaling would be 5.1V is 0% output while 10V would be 100% output and do the same in the reverse direction? The only thing different for reverse would be the voltage range that we specify

So let me know if this is correct?

 

[shooter]

forward ramp from 5.5 to 10 volt going from 0 to 100%
backward (and yes normal for releasing pressure) from 4.5 to 0 volt gives 0 to 100% watch it is negative slope

 

[dbh6]

@ shooter you are absolutely right. Only gotcha is when your are in the reverse direction if you use the scale instruction to scale your 4.5 to 0 volts to 0 to 100% the scale instruction only allows the raw input min to be less than your raw input max, so instead I used a CPT instruction so the scaling can work correctly

 

[shooter]

that is depending of the scaling and plc type, with codesys this is no problem as the numbers are all real.