Hi,
I am about to start a CNC 5 Axes retrofit
with 5 Analog drives and incremental encoders
I have some very basic questions:
1.What are the benefits of using motion modules (1756-M02AE) vs High-speed-counter and an Analog output?
There are many. The main thing is that you must re-invent the wheel if you use a high speed counter and analog output card. This 'wheel' consists of a motion profile generator, error handling, diagnostics, limits switches, the PID, feedfowards, anti-windup and much more.
The SLC is slow compared to the DSP on the M02AE ( encoder ) or M02AS ( absolute SSI )
The SLC is not deterministic. Neither is the Control Logix but the motion cards are. The motion cards can be set up to have a loop time of 250 microseconds if necessary. It usually isn't but at least the cards will always update at time you specify. You can't say that for a PLC. PLC interrupts are not that deterministic. There is a lot of jitter because interrupts must be turned off during many instructions or rungs. The interrupts in the motion controller are off for only a few DSP instructions at most which is a small fraction of a microsecond. The motion conroller also uses a FPGA to get the position feedback and latch it at the beginning of the scan instead of relying on software to read the positions at uncertain periods.
The Control Logix has motion blocks that make programming easier that writing the motion commands processor in ladder. The motion control blocks are documented.
The Control Logix and its motion cards use 32 bit floating point DSPs.
Something to think about. How fast are the analog outputs of PLC cards? By fast I mean what is their time constant? The analog output of motion controllers is usually very fast so the output will reach the desired value in one scan.
I am not sure that Rockwell even has a counter card that works properly for a motion control application. I keep hearing about reseting counters at the roll over when the counter cards should NEVER be reset, except for some cases during homing .
Do the PLCs even have a PID suitable for motion control? PIDs with Ti and Td in minutes is not good. A motion controller should be able to handle systems with time constants in the millisecond range not minutes. The control logix has many PIDs. Do you know which PID is best for motion control?
2.What are the benefits of using CLX compared to SLC (in motion)?
It depends on how much of the motion control is done in the PLC. Some motion controllers can connect directly to either a CLX or SLC5/05 over Ethernet and then the PLC does little but store parameters and handle some errors. In this case the PLC makes little difference. If all the motion control is done in ladder in the PLC then I would want all the power I can get.
3.The Drives are 20 years old, should I consider replacing them as well?
Can't say. Not enough info.
4.Are there any benefits in moving to absolute encoders?
Yes, you don't have to home the axes since the motion controller will always be able to read the absolute position. This can save time. It can also be safer. It is also possible to start where you left off if the power goes out but this takes some programming to determine the current state on start up. Sometimes the PLCs retentive memory makes this easy IF no one has moved anything while the power was off.
I read a lot of posts about doing motion control in the PLC ladder instead of using motion controllers. I just shake my head and wonder when people will stop abusing themselves.
