Good day all

I currently have an application with a custom-written positioning block that is used to position a scale that runs on a linear track below various hoppers. The position is detected by proximity switches positioned along the track. The scale movement is controlled by hard-wired VSD with fixed speed setpoint and the block simply runs the VSD along the track until a proxy is made (or until extreme limit is made). As you can imagine, it is not very good at detecting proximity switch failures until the scale is well past the intended position. There is no encoder for position feedback on the scale. 

We are upgrading the PLC from S7-300 to S7-1500 and wonder if I can use the positioning TO to implement this or if it will be better to just add a few watchdog timers to the existing positioning block to help with proximity switch failures.

Seeing as there is no position feedback, I would imagine I can only implement a virtual axis but cannot see any way in which to manually override the axis position using the proximity switches. I have used positioning TO's before but only with machines that had encoders and G120 drives via PROFINET so the TO worked nicely. 

Hello,

Pos TO need always encoder.

There is an application "Encoderless Positioning".

SIMATIC S7-1200 / 1500: Encoderless Positioning with SINAMICS G

https://support.industry.siemens.com/cs/de/en/view/109767951