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Old Sun 21 October 2018, 11:35
Just call me: Mike
South Jordan, UT
United States of America
Inexpensive way to extend control box functions

With the advance in electronics, don't overlook the inexpensive ways there are to greatly extend the capability of a control box. For instance, when using a water cooled spindle, it is very important to interlock the coil to the contactor to the spindle through a pump switch relay to insure that the pump is running before the spindle turns on. But, what if the pump switch relay is on but the pump is not pumping water through the spindle? How do you know that you have a problem? That's where a microcontroller (Arduino, Raspberry Pi, or Beaglebone) can help.

All three of those microcontrollers are available world-wide. All three cost less than $75 ready to run. All three have lots of online tutorials that show how to easily add input devices and output devices to monitor and control devices. The Raspberry Pi and the Beaglebone can run NodeRed which allows you to see the status of anything monitored on your computer or on your smart phone. (I've been using an old - original model - Raspberry Pi to monitor computer temperatures at a customer site. I can see the temperatures any time and any where via my iPhone. The Pi sends a text message when temperatures reach a preset threshold.)

One drawback of the Pi and the Beaglebone is that they work only with 3.3v devices. That can be changed by using transistors to control devices connected to the output lines and opto-couplers for devices connected to the input lines. Another "drawback" is that output circuits can be the wrong polarity until your program starts to run. That may take a few seconds while the Pi or Beaglebone boots. The way around that problem is to monitor the outputs when the Pi or Beaglebone is booting to see if the signal is High or Low and then to design your output circuit to have the opposite polarity. For instance, if the output is in the High state until your program takes control, an NPN transistor connected to that output would be On while the Pi or Beaglebone was booting, so you design your circuit to require a Low signal (NPN transistor OFF). If you're not that familiar with connecting transistors to a microcontroller, do a little reading on the Internet. Pay particular attention to a circuit called "Sziklai pair" or "complementary feedback pair". That circuit requires both an NPN and a PNP transistor but it retains the original polarity of the circuit. (Transistors are cheap. You can buy 100 NPN or PNP transistors from Mouser or Digikey for about $7.00 per 100.) Another thing to keep in mind is that a PNP transistor turns on when the voltage is 0 and turns off when the voltage is about 0.7V above ground. That means that the High voltage to a PNP transistor must be the same 3.3V as the Pi or Beaglebone. That's where the Sziklai Pair design works. I use it to control 24VDC circuits when I want a Low signal from the Pi or Bone to be the active state. The Sziklai Pair costs $0.15 for the transistors.

It may sound complicated (and it is for a little while), but the beauty of using a computer is that the computer becomes a very low-cost PLC. A PLC is used in every factory to control complex machinery. It uses the same types of I/O devices that you can connect to an Arduino, Pi or Bone. It uses the same logic to control things, i.e.; if the pump is On and the water temperature is in the safe range and the flow meter shows that water is flowing through the spindle, then, send a "water okay" signal to the controller, else send a "water fault" signal to the controller, or, (if none of the limit sensor are active, then, send a "no limit" signal to the controller, else, send a "limit" signal to the controlle, or, if the vacuum switch is on and the vacuum sensor is in range, then, send a vacuum good signal to the controller, else, send a vacuum fault signal to the controller. You get the idea.

Now, IF you decide to use microcontrollers to enhance your controller box, you should take a close look at the Centroid Acorn board and CNC software. That board has 8 inputs and 8 outputs build in. It replaces the customary breakout board and it also replaces Mach3. It connects to your Windows 10 computer with an ethernet cable instead of the obsolete printer parallel port (which is hard to find on new computers) and it is less expensive than buying a license for Mach3, a breakout board and an ESS board.

IF you find yourself saying, "I would buy a $400 water cooled spindle and VFD if I only had a way to ensure that water was flowing and that the water was sucking the heat away from the spindle" or "I would add pneumatic pop-up pins to quickly aline sheets of MDF if I had a way to ensure that every pin had retracted" or anything else that you think of in the middle of the night, then it might be time to try an Arduino, a Raspberry Pi or a Beaglebone.

My FIRST choice would be the BeagleBone Green (not the wifi model). It is $45 from Mouser. It is fully ready-to run when you take if from the box if you have a Windows computer that has a USB port. It has 96 connections for easy expansion. The Raspberry Pi costs about $40, but it requires you to buy a MicroSD card and that you download and install an operating system to that card - it's easy to do, but it can be frustrating if you have had little experience with microcontrollers. The actual cost of a Pi, ready to use, is about $75. The Arduino is about $30 ready to run. You write programs for it with a Windows, Apple or Linux computer using the free software available online. It has fewer I/O lines and no ethernet built-in; but, it's easy to use for less complex monitoring.

Spend a little time on YouTube watching videos that show what you can do with an Arduino, Raspberry Pi or Beaglebone and then start writing down the things that you would do if only it were possible. Then do it!
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