power dropped down

Hello,

We were wondering how you guys avoid dataloss when the power drops down? Are there possibilities to store data (fast) in some memoryplaces in the plc (siemens S7-200). With our plc we drive motors to certain predetermened positions stored in our PC (visual basic).
When the power drops down during a drive to a certain position, will we be able to know the exact position of the motor when the system reboots?

Thanks,

Juan

There are several ways to accomplish this. An absolute encoder maintains positioning data, many drives can retain positioning data, plc's can retain data. Personally I would want the devices to update the PC software as to position.

State some details on what drive, what is being used to provide position info..ie encoder or ? What PLC? Do you have a program already established that works but on power loss you lose the info?

Provide some details and there are many here that are very experienced with motion control.

Don't know about the S7-200 series, but some PLCs have a 'Last scan' bit that is on during the last program scan before dropping out of run mode. If you've got one, use it to enable moving data to a bank of storage registers. When power is restored, use the PLC's 'First scan' bit to retrieve the values.

Note that the above method could only tell you the position of your motor when the PLC stopped. If the motor coasted further after the PLC shut down, the only way to know its position would be some form of absolute encoder, LDT, or some means of measurement that's independent of the PLC.

Rsdoran:

For the moment we use a visual basic program to instruct the plc to which position he has to drive the motors, by using a frequency variator. All these connections are established by the RS232 port of the PC through a PPI cable, and from the plc to the micromaster (freq var) through a serial connection.

We use OMRON incremental encoders with a resolution of 360ppr. Each motor contains a encoder and a brake, which drives his own axis.

In our system we use a Siemens S7-224 plc. And yes, we have already a program that works, but we want to optimize the system so that we know the exact position when the power drops down.

Juan

PS We attach a part of our system in a zipfile to this message.



Steve Bailey:

We are considiring your solution about the first-last scan bit. Thanks for the information, but do you have any existing configurations or examples available.