RoboteQ AX500 manual Adding Safety Limit Switches

Adding Safety Limit Switches

in an attempt to reach a fictitious position. In many applications, this may lead to serious mechanical damage.

To limit the risk of such breakage, it is recommended to add limit switches that will cause the motors to stop if unsafe positions have been reached independent of the potentiome- ter reading.

If the controller is equipped with and Encoder module, the simplest solution is to imple- ment limit switches as shown in “Wiring Optional Limit Switches” on page 78. This wiring can be used whether or not Encoders are used for feedback.

If no Encoder module is present, an alternate method is shown in Figure 40. This circuit uses Normally Closed limit switches in series on each of the motor terminals. As the motor reaches one of the switches, the lever is pressed, cutting the power to the motor. The diode in parallel with the switch allows the current to flow in the reverse position so that the motor may be restarted and moved away from that limit.

The diode polarity depends on the particular wiring and motor orientation used in the appli- cation. If the diode is mounted backwards, the motor will not stop once the limit switch lever is pressed. If this is the case, reverse the diode polarity.

The diodes may be eliminated, but then it will not be possible for the controller to move the motor once either of the limit switches has been triggered.

The main benefit of this technique is its total independence on the controller’s electronics and its ability to work in practically all circumstances. Its main limitation is that the switch and diode must be capable of handling the current that flows through the motor. Note that the current will flow though the diode only for the short time needed for the motor to move away from the limit switches.

SW1	SW2
	Motor

Controller

FIGURE 40. Safety limit switches interrupting power to motors

Another method uses the AX500’s Emergency Stop input to shut down the controller if any of the limit switches is tripped. Figure 41 shows the wiring diagram used in this case. Each of the limit switches is a Normally Open switch. Two of these switches are typically required for each motor. Additional switches may be added as needed for the second motor and/or for a manual Emergency Stop. Since very low current flows through the switches, these can be small, low cost switches.

AX500 Motor Controller User’s Manual 69