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kku

Vertical Servo Axis (using G5 servo)

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I am not really clear how the braking works for a G5 series motor. It looks to me there is dynamic braking. Does this mean actively using the motor to slow down and stop the motor? Then there is a holding brake which is supposed to hold the axis (to prevent it from falling) but not for e-stop braking. Is this correct? So there must be an independent external brake to hold the axis to prevent it from falling (after the motor is turned off). Is this correct? If there is an external brake holding the axis then what is normally done when someone needs to manually raise / or lower the Z-axis? Is there normally an over-ride switch to release the brake to move the axis with the servo motor power off?

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A vertical axis requires a brake (or some other mechanical means) to hold it when power is off. The drive should have a means of releasing the brake whenever it wants to run. Many drives have the ability to program an output specifically as a brake release.

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What's a common way to release the external brake to move the axis when the servo motor power is off?

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A standard way to do this is to use a brake that must be energized to release. You would then create a series circuit for the power for that brake. The series circuit would include your safety circuit (MCR or safety relay), the fault output for that axis and an output from the servo drive that is active when the servo is enabled. You may need to use relays on the fault output and enabled output, if they are not already contact outputs on the servo drive. This way, if the safety circuit is not enabled or there is a fault on the drive or if the servo is not enabled, the brake will clamp the motor shaft and not allow it to move. Depending on the application, you may need to use a safety relay with a time-delayed output for control of the brake.

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Vertical axes should always have a mechanical brake and not rely on the servo to maintain torque to hold a load.

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Thanks for your repsonses. What about having to move the vertical axis while the servo motor is off? How is the external mechanical brake released in order to move the axis? For example if the station was e-stopped while the vertical axis is at the down position, then the servo motor turns off and the mechanical brake activates (de-energized). How would you release the mechanical brake to allow the vertical axis to move up (out of the way) to remove an obstruction?

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A station being "e-stopped" implies that the power has been disconnected. That complicates things a bit. One way to do it is to connect the brake to the drive through a normally closed relay contact and use a normally open of the same relay as a manual release. You can the use a pushbutton to energize the relay and release the brake. Larger brakes often have a mechanical release but smaller servo size brakes usually don't have one.

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We have systems in-house that utilize a "brake override" to allow maintenance of the axis. It utilizes a normally open relay contact (as described above) in parallel with the contacts for the safety and monitoring circuit. The coil leads are run to a connection point where an external power source is connected. The external power source has a momentary pushbutton in it to allow power to flow to the coil of the oveerride relay when pressed. The connection point utilizes a connector to make connections easy. We store the brake override power supply in a separate location from the machine where only our Maintenance or Engineering departments can access it. BE CAREFUL DOING THIS ON A VERTICAL AXIS, THOUGH! Doing this on a vertical axis requires a good procedure and proper devices to control the descent of the axis when the brake is released. You may want to think about how you would do that on your machine (blocks, slings, crane, come-along, etc).

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