PLC questions

[daught]

Hello. First I would like to say sorry, I am really new to PLCs. Just yesterday I was on wikipedia reading what they really are. Truth is I am not very knowledgeable at electronics beyond basics. I am however on a crash course right now. The company I work for just got a huge project and the weird stuff got handed to me.

My company requires from me control a vehicle gauge cluster for two minutes. What I mean by that, is that the speed and tach gauge needs to be at any position I want it to be during those two minutes. I also need to control the speed of the transition between those two points. Basically I need to make the gauges seem like they are working on the road for two minutes. I figure I can do that with the PWM output. Is that correct? Can I also do it with the AO? What is the difference between the AO and the PWM?

Also I need to control the steering wheel with a stepper motor for two minutes like the car would be driven. Can someone recommend me a stepper motor and a stepper motor driver for such an application to be controlled by your PLC?

Also is there enough programming space on the device to program all these controls?

Thank you very much and again sorry for my "newbiness"

[support]

It will depend on the full scale voltage and current consumption of the gauges that you are using to determine if the PLC has output that can drive them directly. How many gauges do you need to drive? The AO output from T100MD888+ PLCs are 0-5V high impedance type (sources only micro-ampere current) so not suitable if the gauges need mA of current to drive them.

PWM will output voltage proportional to the duty cycle but for T100MD888+ it using inverted logic due to use of N-channel MOSFET output drivers(0% = 22V, 100% = 0V). But if you use the T100MD2424+ you can get two channel of PWM outputs that are positive logic (0% = 0V, 100% = 24V) so it may be useful for driving analog gauges.

You need to choose a stepper motor with enough torque that can turn the steering wheel at ease. You can consult the stepper motor suppliers and they should have matching stepper drivers. Our PLC requires driver that can accept 24V NPN inputs with pulse and direction input types.

[daught]

What do yo u mean by "could be in inverted states"? What will that depend on?
What does the T100MD2424+ have to prevent such thing from happening? I will be buying the T100MD2424+ anyways because there might be other stuff I want to control, but still please explain the above.
Also concerning the AO can I use a MOSFET transistor so drive the gauges?
In the end what would you recommend I use for driving the gauges? PWM or the AO with a transistor?
Also again the most important concern is if there is enough space on the device to program all these controls to run for two minutes.

Thank you.

[support]

I think I need to rephrase my response. Will follow up with a new response shortly.

[support]

The two PWM outputs on T100MD888+ are N-channel MOSFET type and they sink current when turned ON, but each is internally pulled up to power supply voltage (either 12 or 24VDC) via a red  LED and a 5.6K pull up resistor. To control power supplied to a load, you need to connect the "+" terminal of the load to the positive of the DC power supply (e.g. 24V or 12VDC) and the "-" terminal of the load to the PLC's output and the load will be turned ON and off rapidly within the PWM period but the average power applied to the load will be proportional to the duty cycle.  Whether this will work for your application will depend on the what's needed to drive your gauge.

The two PWM outputs on T100MD2424+ are P-channel MOSFET and  at 0% duty cycle they are turned OFF but at 100% duty cycle they are connected to the +positive power supply voltage (24V or 12V DC).  So at 50% the output is about 50% of the power supply voltage (in earlier post I made a mistake to say that it is 22V at 100%,  which I have amended that post subsequently). This is more likely to work with a gauge that will display a reading proportional to an average input voltage between 0V and 24V (or 12V) DC.

To increase the current drive capability of the AO, you can use an operational amplifier wired as a voltage follower which is probably easier than using a MOSFET in linear mode. There are plenty of text book example of voltage follower OpAmp circuit . E.g:

http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm

[daught]

Thank you very much. Your explanation is very clear. This project is starting to take shape. Things are going well and after this pilot project we might have 10 more with Nissan and even more with other car manufacturers. Cant wait to get my hands on these PLCs they really seem wonderful hardware.


Again, I ask. Is there enough space to program two minutes of the described control in these units?

[support]

How many time will you be changing the data sent to the PWM within that two minutes? Since you are dealing with automobile hardware I can't imagine you will be doing more than a few hundred set points during two minutes? The PLC has 6K words (12K bytes) of program memory which is sufficient quite sophisticated control due to the very efficient programming language.  But the program space can be increased and the data space can also be used to supply parameters to the program without taking up program space. With extended EEPROM you can program 7.7K words of set point data to be used by the PLC.

[daught]

If for some reason I run out of space on the PLC, from what I understand I can just use the PLC as a Serial I/O Board connected to a laptop where all the programming can be stored. Is that correct? Will all the PLC functions (I/O) be available to be programmed on the laptop?

[support]

Yes, all PLC I/Os and internal variables are available to be controlled from a host computer. You can see the following link for a sample visual basic program:

http://www.tri-plc.com/applications/VBsample.htm

You can change a parameter to be used by SETPWM command from a PC and then command the PLC to set the PWM output to the new data anytime.