Simple PLC Positioning Control
Recently we had a request from a customer who wanted to expand and improve his CNC tending operation. For more than a year the plant had been using a UR10e collaborative robot from Universal Robots to load and unload parts from one of his CNC machines. He was very pleased with the initial ROI on the project, and with the ongoing value of that installation that he decided to implement the UR Cobot on a second CNC machine. The idea was to use a single Universal Robot to simultaneously tend both CNC machines. Unfortunately, the second machine was out of the cobots reach.
Together with HTE's TECHTEAM a low-cost solution was developed that added a horizontal seventh axis to the cobot. This additional axis would move the robot between the two CNC machines as needed. A simple extrusion frame with some slider rails was used, and a mount for the robot was made to slide back and forth on these rails. A stepper motor was used to drive a belt to move the robot mount, all of which was pretty straight forward. You can see the application in action in this brief video.
The question remaining though was how to drive the stepper motor? The initial installation included an HTE specified Mitsubishi iQ-F PLC to control hand shaking with the robot and the CNC. The iQ-F PLC has highly functional positioning control built right in! This made setting up the stepper motor control very simple. Simple tables to setup the parameters are included which allow for up to four axis of positioning control. HTE's TECHTEAM was able to specify the optimal units of measurement, the number of pulses of resolution the stepper motor had, and also to define the gearing ratio of the belt drive.
On this application, we only had 3 positions we wanted the robot to go to. To make things even easier, we used the built in Position Tables to pre-define these 3 positions. Now the PLC program is about as simple as it gets! We just tell it which of the 3 table entries to use, and the iQ-F takes care of everything else for us. Very simple!
Here is what the Axis setup looks like:
Here is what the Position Table looks like:
It does not get much easier than that! With these configuration tables we were able to use two of the Transistor outputs on the iQ-F PLC to drive the Stepper Motor. These could also be used to drive a servo if required.
- No need to count Stepper pulses in your PLC program!
- No need to figure out if you should turn on the direction changing output or not!
- Just pick a Table number and tell it to go!
This was a great low cost solution that was easy to implement and allowed our customer to get even more out of his existing Universal Robot.
Regarding positioning, the FX5U comes with the built-in capacity for 200kHz high speed inputs and outputs. This provides the capability to control up to 4 axes of positioning with stepper or servo motors.
The FX5U is a cost effective solution for:
- interrupt operation,
- multistage speed operation,
- simple linear interpolation, and
- simultaneous start of 2 axes.
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