PLC for CNC toolchanger

Hello,
I'm new to PLC's and into hobby level automation of CNC/manual machine tools.
I've got a small CNC lathe (bad controls) with an 8 position rotary tool turret. It's rotated by a small 24VDC gear motor and uses some type of sensors (not an encoder, but this might be a better way of doing it?) to 'read' the turrets' 8 positions.

I'm using CNC control software that runs off a standard PC and uses the 25pin parallel port for very limited digital IO. This toolchanger problem would require more IO than I have available. It seems like this would be a great little project (learning experience) for a small/inexpensive PLC.

I would need to communicate with the PLC via the 0 or 5V digital output of my parallel port. Hopefully, I could set it up so that when I programmed an 'M06 T02' in the CNC software the PLC would rotate the turret to tool position #2 etc... Maybe I could do shortest route logic so that the PLC would compare the current and commanded postion and pick whether to rotate CW or CCW.

I've been looking at the DL05 or DL06 fixed IO PLC's at Automation Direct. They seem to be pretty affordable and possibly a good learning platform.

Any recomendations for what PLC, PLC software etc. might be a good learning platform (inexpensive) for this type of application? Any other feedback on what PLC features like analog/discrete IO, programming functions, memory, counting capability would be needed for a application like this would be great. Thanks.

snaggletto,

If you use a DL-05, or DL-06, your first problem will be an Input voltage mis-match. The Direct Logic DC PLC's have an Input range of 12 to 24 volts DC, with 10 volts DC being the minimum ON digital input level, and 2 vdc being max OFF. I don't think that a 5 volt input from your PC computer will trigger an ON level... You will have to do a voltage conversion, using solid-state relays or something else (DC to DC signal converter?)...

Please explain what you have coming out of the 25 pin parallel port. Are you saying that you will have the +5 vdc available on only one pin, or can it be switched to different pins for diffferent functions? If different pins for different functions, then you can make this work, and an Automation Direct PLC would be suitable for this project (if you can come up with a way to translate the voltages).

One analog input module and one anlalog output module would be nice, if you have the bucks. Also, get plenty of digital Inputs, as you can use these for bringing in leads from BCD Thumbwheel switches. Your sensor may be a BCD (Binary Coded Decimal) input device. Thumbwheel switches can be purchased cheaply from an electronics supplier (Allied, Newark), and are useful for inputing number values into a PLC, such as tool positions, timer values, and other switch settings. Typically for many PLC projects, you need 3 to 4 times as many Digital Inputs as you do Digital Outputs.

PC to PLC

IMO it is more efficient to use serial communications for PC to PLC communications. Most all PLCs have a serial port that comes free, you pay for the I/O. If you need the higher capacity Outputs use SSRs. If you need programmability, what is the PC doing? If the PC is just an HMI, again use Serial Comms.

Hey,
The PC runs the CNC software which outputs step/direction pulses (2 bit output per motor). I have a total of 12 bits of output, giving me control of up to 6 motors. Generally you have up to 4 motors and 4 general outputs which are usually tied to SSR's. There is a total of 5 bits of input which is used for limit and home switches for each machine axis and usually a fault input for the motor drives to put the control into software E-stop.

Auxillary functions have M codes like M03 (spindle CW), M08 (coolant pump on), M06 (toolchange). These can be configured for output to the relays or to a step/dir pin if using less than 4 motors. The CNC software can address another parallel port for more IO. I would prefer using a PLC though, for the learning experience.

I see what you mean about having to have 12-24V for input on the DL05 - DL06. It looks like the DL205 series has a module that will take 5V input.