Mixer door motor

I have a problem with my cement mixer. It is a pan mixer and the door is on a shaft that rotates 180 degrees to open and then 180 degrees in the other direction to close. I have a motor close coupled to the shaft for movement. There are limit switches to set end of travel in each direction. My problem is this: lately, the door coasts irregularly to a stop. What I mean is that after hitting the limit switch, the motor is de-energized, but sometimes the door closes short or it overshoots. I have checked for the obvious faults(sheared keys, broken coupling, bad mechanical linkage) and all seems to be well. This leads me to suspect something in the gearbox. We cannot shut down long enough to evaluate the gearbox or investigate and do the proper fix. So......I am wondering if a VFD could run the door slowly enough and decel to the same stop every time. And I neglected to mention that the action of the mixer blades will occasionally work on the door and cause it to drift enough to open it in overshoot. Is there a VFD that can brake the motor after it is stopped to prevent this? I am reluctant to use a brakemotor because the door is quite heavy and I am concerned that a sudden stop would shear something. I' sure someone in PLCSland knows something that can help me. Thanks in advance.

We have exactly such doors in our mixers. Only they are for foundry sand, not cement (which means that our application is probably heavier than yours on the door/gear).
We use a gearmotor with a mechanical brake on the shaft to achieve exactly the same as you want - to stop quickly and keep the position after the movement has stopped.
Dont worry about the impact of the brake on the gear/door.
The "kick" from when the motor starts is much harder than when the brake activates.

You could use a VFD and program it to slow at a point and then stop where you want. Then do DC injection braking to hold it a couple of seconds. With the weight of the door, you will probably need the dynamic braking enabled on the drive when decelerating to the slow speed. It would allow a shorter slow speed time. Also with a VFD, you may be able to run the motor up to 2X base speed to close the door faster.

I do believe you would be better served using a mechanical brake that sets when the drive stops. The drive run contact can be used to trigger the brake.

Is there a VFD that can brake the motor after it is stopped to prevent this? I am reluctant to use a brakemotor because the door is quite heavy and I am concerned that a sudden stop would shear something.

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If you use a drive say a Danfoss you have a programable relay contact and you would use this to control your motor break. In the drive the are many ways to do this one is to use the AT SPEED and then set your speed to say 3 HZ in this perimeter. Now when you tell the drive to run the mechanical brake holds until it sees 3 hz. Then it releases and your motor comes up to speed. It works the same in stopping if you need at braking torque you`ll need a braking resistor also, but it doesn`t sound like the resistor is neeeded. Now let the drive decel the motor down to 3 hz and then the mechanical brake will engauge. This way your brake only holds the load after it is stopped. You will also have alot more control when stopping.