How to Issue a Stop Command in MagneMotion MagneMover Lite

Background

A client is currently using an overhead track of MagneMover Lite (MML) motors and vehicles to store and transport their product. When the product is ready for operations at the ground level, the MagneMotion vehicles carrying the product are sent to elevators that bring them in groups of three to the factory floor, where operators can unload from two racks: an upper and a lower rack.

Since the elevator needs to interface with both the ground racks and the overhead track, we must split the elevator and each ground rack into separate paths with their path beginnings defined by terminus nodes. A Terminus node defines the start or end of a path where vehicles move to or from their current path. When a vehicle is moving from the elevator to a ground rack, it must exit from the elevator terminus node and enter the ground rack terminus node, which accepts the vehicle onto the path. This process is repeated for all three vehicles in either direction, both boarding and departing the elevator.

While the elevator is lowering to the ground racks, a stack light flashes and emits an audible warning to alert operators that vehicles are arriving. Still, the client requested adding two photoeye (PE) sensors and a stop to prevent vehicles from colliding with any operators who are in the way of the arriving vehicles. We worked on a similar MagneMover Lite solution with an elevator system during the pandemic, helping Ginkgo Bioworks optimize their manufacturing process for developing RNA.

Path Suspension

To help our client with this specific motion control request, we initially implemented logic to issue a path suspend command when the PE sensors were tripped. A suspend command causes any vehicle in motion on the path to decelerate and stop based on its current velocity and commanded acceleration.

It is not recommended to issue a stop command to a path while a vehicle is either entering or exiting terminus nodes. Due to this, if a PE sensor was tripped after the first vehicle had departed the elevator and while the second vehicle was in the process of exiting the elevator, there would be a delay before a suspension command could be issued to the ground rack path, causing the first vehicle to continue moving even after the PE was tripped.

Implementing the Stop Command

MML does not contain an explicit “stop” command; instead, you can issue a move command with a velocity of zero, keeping all other parameters (acceleration, path number, position, move direction, PID set) the same. Once you are ready for the vehicle to move again, you issue another move command with a nonzero velocity.

To implement the stop command, we cached the velocity, targeted position, and MoverID (unique ID assigned to MM vehicle) of each vehicle that was on a ground rack when the PE sensor was triggered. The cached velocity was used to ensure that any vehicle with a non-zero velocity was issued the “stop” command when a PE sensor was triggered. The targeted position was then used to reissue the same move before the vehicle was ordered to stop. The MoverID of each vehicle was cached after it was commanded to stop, allowing us to confirm all carriers on the path have been commanded to stop if the PE sensor has been triggered.

This solution was implemented across four elevators, each with two ground racks, and our Automation team has received only positive feedback from the client about the vehicles’ improved stopping times.