The two relays can be used to enable and disable a spindle and a secondary function, such as a vacuum cleaner or a coolant pump. The spindle usually is a router or a plasma torch. In the case of a router, the relay is switching 120/240V AC, so a 250V varistor is provided to protect the relay contacts. To drive the relay coil, a solenoid/relay power stage is employed. The CNC controller enables these tasks when a job is submitted to the machine, and it disables them once said job is complete.
The charge pump is a block put into place to ensure operator safety. Since it is not possible to guarantee that the computer's parallel port will send the right control signals at all times, there is a chance for the tool (the router blade or the plasma torch) to go live at any given point in time. This problem becomes apparent when the computer is rebooting and the parallel port's state is unknown.
A CNC motherboard revolves around a backplane that accepts motor driver modules to drive the CNC machine axes. (Source: Texas Instruments)
To have these tools spring to life at any time other than while running a job can result in serious operator injury, so it is crucial that we limit their operation. The CNC controller offers a heartbeat signal in the form of a 12kHz pulse, which is only available when the CNC controller is running properly. We are using a simple MSP430 microcontroller to sample the 12kHz signal and allow for relay enablement only when said heartbeat is present.
The input functions are mostly optional except the emergency stop (E-stop), a resource I would not skip. The E-stop is an important safeguard, as pressing this button commands the CNC controller to halt all operations immediately. The other four inputs, however, are optional. In the CNC motherboard, we have allocated the remaining four inputs to home sensors, which can be used to bring the four axes to a known starting position. I like having this function on CNC milling machines, but I find they are not as necessary on CNC plasma cutters.
Last, but not least, the CNC motherboard includes a series of regulators, such as a buck converter to generate 3.3V to power up all the logic and a linear regulator to generate 12V to power up the relays.
Jose Quinones is an applications engineer for the analog motor control group at Texas Instruments.
Jose, this is an interesting set up. The power and low cost of microcontrollers has allowed a wide range of very interesting hobbyist projects that would be very difficult in the past.
Is there more detail about this controller on the TI site. I know some guys who would be interested.
I couldn't find a link about where I can get one. I want to use that CNC setup for some home projects (CNC mill and lathe).
On another note, I see it uses a parallel port, serial communication. That is becoming a problem with CNC setups these days. Most new desktop computers, laptops, micro-computers, and the like, do not have parallel ports onboard. In many cases, a parallel port card would need to be installed. And hopefully, the system has an expansion slot available.
What most people do is buy old, or surplus, computers from somewhere. My current CNC mill runs off of a PC I found in the garbage many years ago (Celeron 700MHz Compaq desktop). I think it is time to change how these CNC drive systems work. USB is the only way to go. So, built in support for USB to serial in a good stop-gap solution.
A friend who's an engineer & runs a small prototype shop tells me that none of the serial options can function quickly enough to handle the 'feedback loop' in starting, stopping, & moving the tool. (He's rebuilding a German 5 axis mill & currently dealing with that issue.)
I'd like a source for this board too. I've considered doing somthing like this myself so I can let my lathe do the work accurately while I drink coffee. My lathe did not come with change gears to enable it to cut threads. After spending lote of hundreds of dollars, I still can't cut metric! On the internet there is a website concerned with "electronic lead screw" kits, a single stepper controller to cut any pitch (coordinated lead screw movement). So many things to do!
Please note that parallel is not serial, but 8 bits wide, and bidirectional. I use old machines running under DOS and one would be amazed how smoking hot these machines really run as far as pure processing horsepower.
USB, while a high through put, are bogged down by multitasking. For example you can buy a USB to parallel converter to run an older printer. But to run high speed on this bus, you can't. Connect a scope and repeatedly toggle one bit. Very poor performance overall.
Ahh, yes. About 20 years ago my son and I built a robot using the stepper motors, power supply, and motor driver board from big industrial-grade daisywheel printer. We discarded the logic board and designed and built a simple interface board to interface the motor drivers to..a parallel port.
We used an eight-bit control word, where three bits allowed us to control up to eight motors (two implemented), four bits were used to control individual motor phases, and one bit indicated whether the driver should be at full power (to step) or half-power (to hold position). This design, of course, permitted forward and reverse motion, and half-stepping by energizing two phases at once.
Thanks for your comments and taking the time to read this article! In the upcoming weeks I will be finalizing the application note which will contain details such as schematics and source files. I am also studying different venues to having these boards available for purchasing on some sort of an eStore.
With regards to the USB, I will soon be posting a completely different CNC solution which revolves around USB. It is the one I am using on my 3D printer. However, the truth is as perfect as this solution is for a 3D printer, I would not use it with a router, mill, lathe, plasma CNC table. Any of the later, operate much better when using a CNC controller such as MACH3 and USB solutions have not been able to pair up with their parallel counterpart. In my experience, adding a PCI parallel port to any PC out there works perfectly fine. I have done it with 3 completely different computers (the ones I use for my CNC Mill, CNC Plasma and CNC Router) and as of today have had no problems.
Let me know if there is anything else I can help you with!
When you get your design done, Hackaday.com would probably post it to a very wide audience. You imply this is your design and not a TI product.
It is beefier than most of the open-source units out there (mainly for 3d printers).
You may find a good demand for this. One possible eStore is adafruit.com or the Maker Store (make magazine), esp if you open this up as an open-source or offer it in kit form.
Goto hackerspaces.org and find a hacker space near you - take it by for an intro and show&tell. You might even sell the demo unit on the spot!
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