Internet PLC Forum

Neal,

Glad that you found a simple solution.

I find that most of the documentation (if any) for devices is wrong. I reverse engineer the I/O circuitry of most devices. With Modbus devices, I have to run test software to figure out how stuff works.

I  have run into a few rare exceptions.  I have worked with Modbus devices from both a Finnish and Swiss company that were actually document correctly.  And they had technical support that could help with application issues!

Best regards,

Gary d

Neal,

The video answers a lot of questions that I was going to ask.  The dial test indicator answered the first 4 or 5 questions.

Your indicator moved a couple of marks. I am guessing about 0.002". I see a lead screw driving a carriage and assume that it is directly coupled to a stepper motor (no intermediate gear box). The most common steppers move 1.8 degrees per full step.  I don't know the pitch of your lead screw, but 5 mm per turn is common. Each full step for a 1.8 degree stepper would move the carriage about 0.025 mm (or about 0.001").  So I am guessing that your stepper is not getting too many step pulses.

My initial guess is that it is not noise on the output of the FMD8810. If you have a stray bit of wire that bridges the PLC outputs, then you can create a "wired-or" condition. If either adjacent output goes active then both will.  I'm done this both with intent and by mistake.  This might get you the behavior shown in the video.

You could put a red LED in series with the step signal heading to the stepper driver.  This is the poor man's alternative to the scope. If it flashes when the stepper shouldn't be moving then you have learned something without having to find the scope probes.

I would instrument the PLC code to generate an output that goes active when the PLC firmware issues a stepper movement command and goes inactive when the command completes. If this new OUTPUT goes active when the carriage jerks, then your firmware issued a stepper movement command. If the new OUTPUT is inactive when the carriage jerks then it is probably not firmware. Sync the scope to that signal and you can go hunting.

What are you using for a stepper driver? I have been using Chinese stepper drivers based on the TB6600 chip. They were inexpensive and I could find schematics to verify the input circuitry.  Most of these drivers were designed to be connected to small computers with 0..5V logic. I added an external resistor in series with each PLC OUTPUT to limit the current to 10 mA as I was working with 0..24V logic (PLC).

Gary

Neal,

I am running 2 stepper motors from an Fx series PLC.  I have not run into the problem that you are seeing.  The attached photo is my stepper controlled valve test bed. You can see the steppers, the belt driven cams, the limit switches, drivers and big 24 VDC supply for the steppers.  The rest of the test system is a couple meters distant.

Stepper systems are not as easy as they appear.  The bis issues are torque and inertia.  If your stepper doesn’t have enough torque to accelerate the load, you will lose steps on acceleration.  You may gain steps on the de-cel if the inertia of your system is so great that it overrides the holding torque of the stepper. I had a client that when the stepper coasted past the last step that the high voltage generated by the stepper (acting as an alternator) would destroy the driver circuitry.  I destroyed both of his prototypes in less than one minute.  I was unable to convince him that his prototypes would need a bit more development before they were ready to ship..

Most SSRs are optically isolated.  The PLC's view of an SSR is an LED with a series resistor (to limit current) connected to +24 VDC.  There is nothing inductive about the SSR's input structure. There is no need for additional circuitry.  I don’t think this is your problem.

If you have access to a modern oscilloscope, it is probably time to get it out.  If you can modify your PLC firmware to set an OUTPUT bit when your firmware issues a stepper move command and then clears this OUTPUT on completion of the move command. Look carefully at what this output does.  It is very possible that your firmware is buggy and actually responsible for the unwanted stepper movement.

Now that you have a way to trigger the 'scope you can study the PLC stepper signals on the 2nd channel.

Stepper systems are not as easy as they appear.  The bis issues are torque and inertia.  If your stepper doesn’t have enough torque to accelerate the load, you will lose steps on acceleration.  You may gain steps on the de-cel if the inertia of your system is so great that it overrides the holding torque of the stepper. I had a client that when the stepper coasted past the last step that the high voltage generated by the stepper (acting as an alternator) would destroy the driver circuitry.  I destroyed both of his prototypes in less than one minute.  I was unable to convince him that his prototypes would need a bit more development before they were ready to ship.

If the problem is not firmware and not physics (torque and inertia) then I would look carefully at how you have handled power and ground wiring.  I would worry about how you have wired both the SSRs and the stepper driver to ensure that they are all correctly referenced to the PLC +24VDC supply.

I am happy to help you sort this stuff.  You can contact me via my email address in my profile.  I am sure that your problem is simple.  I am sure that I can can help you stare at it just differently enough to figure out the problem.

Gary d

If DIP switch #4 is ON then the PAUSE LED should stay ON and should not be flashing. If PAUSE LED keeps flashing then it looks like the PLC is being rebooted consecutively and could be a problem with the power supply.

What type of power supply? How many volts and how much current can it supply.

Try changing the power supply if you have a different power supply unit.

You are right that I2C bus is usually meant for short distance (typically on board or on an adjacent board due to the fact that it only has active low output while high level output depends on pull up resistors. It should work over 20 to 30cm away but make sure that there is no noisy signal that can be coupled onto the I2C signal path.

The I2C-FRTC module that are sold with our FMD and Nano-10 PLCs however employs an I2C buffer chip which allows it to interface to other I2C buffer chip using higher voltage (up to 15VDC) and load capacitance of 4000pF. That can help to extend the I2C range to longer range (more than 10m) provided you design everything correctly, using higher I2c voltage level and lower value pull up resistors.

If the remote module is far away it is better to use RS485 or Ethernet to communicate with a remote I/O board that support common communication protocol. For example another FMD88-10 can be connected up to 1000meters away via RS485, or on the other side of the planet if communicate via the Internet.