zero speed switch

[jay7880]

I'm doing a sand silo system at a foundry..an outside company has provided the control cabinet and programming...the conveyors normally have zero speed switches on the tail end but most are missing or broken...I do have proximity switches that are the same style as what the foundry season all their other conveyors...what can I do to replace with the proximity switches? Have the programming changed to make them a time delay switch and to close a contact in the plc to sound an alarm it the set time expires or can I simply use them in the same way as a zero speed switch

 

[Doug-P]

Lots of components look identical in physical appearance. You have to look up part numbers and compare the descriptions on each switch to see how alike they are or if there are any special features built in. They're probably both just simple ON/OFF sensors. It's a good bet the zero speed function is being done in the PLC.

 

[dginbuffalo]

Hello- your details are a slight bit fuzzy. Is the control system new or was it existing? If the switches are missing or broken, how does it still work now? Can you get into the programming and post what they have now?

A true zero speed switch that i have seen has a built in timer so it closes it's contact on the first pass of the target, and as long as it pulses on the target in a set amount of time, it's contact remains closed. The programming is probably done around this concept. This means you most likely won't be able to drop a prox in as a direct replacement without some reprogramming.

That being said, yes- you can use a prox and a programmed timing circuit to get the same results. There are was to do it with both a time on or time off delay. Let us know if you need any help with that.

 

[danw]

A lot of that kind of severe service used zero speed switches built to survive that service.

If replacing stuff with zero speed switches is of interest, the specs for Siemens (formerly Milltronics) are here:
http://www.lesman.com/unleashd/catalog/belt/bsfm_mfa4.html


http://www.lesman.com/unleashd/catalog/belt/bsfm_mfa4.html

 

[Lancie1]

What can I do to replace with the proximity switches?

The attached picture is probably the method mentioned by DGin Buffalo in Post #3. You have to carefully set the ON and OFF timer Preset values so that the alarm does not give false alarms. The times are dependent on the tail pulley maximum RPM and also the width of the target mounted on the pulley.

 

[dginbuffalo]

Exactly Lancie. Good point of dual timers in case the conveyor target stopped directly on the prox.

 

[Aabeck]

I've been using a method almost identical to Lancie1's for years, just have been calling it a "Stall Detector" & been using it on both conveyors and motorized agitation systems.

 

[Lare]

Exactly Lancie. Good point of dual timers in case the conveyor target stopped directly on the prox.

If you need to save timers on program or want use only one timer for this, then you can do this with creating first one shot of proximity switch. On 2nd rung use run indicator and negated one shot before timer.

 

[Doug-P]

A couple of refinements would be to latch the timer done bits to know if the switch failed ON or OFF and to inhibit timing until the conveyor has reached speed.

 

[dondon_alba]

Please see the attached file. You can program your zero speed sensor same as rung 2 & 3.Use telemecanique XS7-C40FP260 proximity switch.