Q & A for PLC system integrators
Q. In one of my projects, I am using a lower end PLC. I have only digital outputs. It’s difficult to provide frequency output from this model but I need measured traverse. Do you have a solution?
Yes sir. There are in fact two options. In first option you can use our CTL (cut-to-length) controller. Give a start (run) signal from PLC and the CTL will manage everything else. CTL has a built in HMI and relay outputs and many other features. But this one, sure is more expensive of the two options.
If the end use does not justify the cost, there is one more way. We will generate the clock when a run input is received and give you a clock output. This clock out can be counted by your PLC and decision about stoppage, direction change or slow down can be taken by the PLC accordingly. Thus in this case 3 digital inputs will be required from PLC – start, direction and slow down. Slow down may not be required if the speeds are lower than 60 RPM. This is extremely economic solution, which allows a use of lower end PLC.
Q. And if analog signal of 4 to 20 mA is available, can you give proportional speed?
Of course!! Only thing is that you will have to specify the speeds at 4 and 20 mA input. If direction reversal is essential, momentary slow down and re-acceleration to original speed can be incorporated. It is advisable to use a start signal too, so that frequency output at 4 mA input will not cause any motion at the lowest speed.
Q. We are using a advance model of PLC and have PWM outputs.
In that case you can directly interface the drive with PLC. Sourcing or sinking outputs can be interfaced. But the most important thing to remember is that if you wish to achieve high speeds, a proper acceleration and retardation must be provided, which takes care of the properties and parameters of motor and mechanical system. Starting and stopping at base speed of 60 RPM is important to achieve desired performance.