relay logic question

[Gustavus]

Not a computer question -- but rather a relay logic question. I am building a controller for a bridge crane. The drive motor for the crane is DC, so reversing polarity on the motor reverses the direction of rotation of the motor and hence the direction of movement of the crane. In the past I have used a DPDT latching relay for this purpose -- where applying voltage to the relay coil caused it to change state (closed to open or open to closed). There was only one coil and a mechanical latch. The Allen-Bradley latching relays I purchased for this job have two coils. One causes the relay to close and latch and the other causes it to release the latch and open. One leg of each coil is steered by a contact which is closed only when the relay is in the opposite state. That way voltage can only be applied to the coil that will open the relay when the relay is latched in the closed position -- and viceversa. When the limit switch on the crane (one at either end of the I-beams it runs on) close, one of the things you need to do is reverse the polarity of the voltage on the motor so that when it starts up again, it will be moving in the opposite direction. If I apply a voltage from the limit switch to the switched inputs to the coils on the latching relay, since the voltage is steered to the appropriate coil as I described above, the polarity switching DPDT changes state, but that connects the other coil and it then switches state again -- chattering back and forth between open and closed for as long as the voltage from the limit switch is applied.

I have tried all sorts of relay schemes to solve this problem, with no success so far. Since these relays handle significant current, they cost quite a bit so I am loathe to junk them and try to find the single coil latching relays I am accustomed to.

Suggestions will be appreciated.

 

[Roguestar]

http://forums.anandtech.com/ca...tid=50&flcache=2765357

Perhaps better suited in here? :/

 

[Mark R]

Connect one limit switch so that it sets the latch, and the other so that it resets the latch. No need to steer the signal.

So, if the crane moves 'forward' with the directional relay open - when it hits the maximum forward limit switch, have that limit switch close the relay. When it hits the maximum reverse limit switch, have that switch open the relay.

 

[Gustavus]

Thanks Mark for the suggestion. I had wired the two limit switches in parallel so that either one closing activated the stop/run relay which controlled power to the motor. My experience had been you can burn out a motor by reversing it when under load, so my logic design called for stopping the motor first with the limit switches and then iniiating the polarity reversal from a voltage obtained from a stop contact on the stop/run relay. That guaranteed power had been removed from the motor before the polarity was reversed. I will try to figure out how to use your suggestion to use the limit switches to directly reverse polarity while removing power from the motor first. I don't have any delay relays in the shop, but maybe I can find a couple if necessary.

Again, thanks for the suggestion. I was fixed on thinking of the limit switches in parallel since I knew they had to stop the motor. Now I will think of them as independent and see where that goes.