Electrical Solution: PCBs and Cables
Overview
We had a challenge: design the electrical system of a new automated metrology machine to be quicker to assemble, easier to troubleshoot yet still be cost effective. The machine was expected to be low volume but would be in production for years.
The machine required a PC for data acquisition and analysis, four axes of servo motion, and a PLC for the I/O such as safety and pneumatics.
The Past
Previous machines had used a motion controller with break out boards and discrete wiring to analog servo drives, and then discrete wiring to DSub break out boards, and finally DSub cables to the motors. The I/O was a mixed, with some I/O provided by the motion controller, and some by a PLC; in both cases, the I/O used discrete wires and terminal blocks to connect to the pneumatic solenoids and other I/O.
Crimping, labeling, routing, connecting, and testing all those discrete wires took a lot of time. It was too easy to make a mistake, which often resulted in hours of troubleshooting.
Typical Solutions - High Level
There are a number of possible approaches. At a high level, options included moving to network motion controller and network I/O, using a PLC for everything including motion control, or using a hybrid approach using networked drives and a PLC for I/O.
Since the motion profiles were not complicated, we decided a motion controller was not needed. However, using network drives made sense, because the cost was about the same as the previous analog servo drives, and the wiring was much easier (network cable instead of discrete wiring). Since the commands were coming from the PC, using a PLC for motion control would have only added complications.
For I/O, we considered using networked I/O such as the Wago 750 system and field bus interfaces to pneumatic manifolds, but given that our machine was small, the large added cost was not worth it. We also looked at PC-based I/O, but for 24VDC I/O, it was more expensive than a PLC and did not provide the PLC's instant on programmability. Therefore, we chose to use network drives with a PLC.
Typical Solutions - Detailed
Given our choice of network drives and PLC, the most common choice would be EtherCAT drives with an Allen Bradley, Siemens, or maybe Automation Direct PLC. However, we choose to use CANOpen drives from Copley Controls combined with a Panasonic PLC. Why? Because this solution was cost effective and simplified the wiring.
CANOpen drives had sufficient performance, and better real time performance on standard Windows; moving to EtherCAT drives would have roughly doubled the cost with performance benefits for this machine. The Copley drives used standard DSub cables, so we designed several different break-out boards that connected to the Copley cables and provided the appropriate connectors for the motor and stage that drive used. The result was that we could connect a drive to a motorized stage in minutes instead of hours.
We use the IEC61131 standard when programming PLCs and there were no other reasons for us to use a specific brand. The problem with most PLCs is that they use terminal blocks, so you have to use discrete wiring and external terminal blocks (for power, common, etc). The Panasonic FP series PLCs are compact, affordable, and use IDC ribbon cables for I/O. The ribbon cables made it easy for us to design a break-out board to connect to the PLCs and then have the appropriate connectors for everything else.
We carefully chose connectors to optimize assembly time while being reliable and easy to source. To give one example, we used a pneumatic manifold with a 26-pin IDC ribbon cable connector, which connected directly to the PLC break out board. So instead of having to individually wire all the solenoids, all the technician had to do was connect one ribbon cable. This gave us most of the advantages of a field bus interface without the huge cost.