MES in vehicle manufacturing - Position Calculation

     In vehicle Final Assembly Shop, conveyor lines are used to move vehicles, which bring an issue of vehicle identification.

We know that normally RFID Reader or fixed Barcode Label Scanner are installed at line head or line end stations, because when vehicle comes into or ready to leave line, carrier of vehicle will be standing still for a while, and we can use that period of time for vehicle identification.

But in Final Assembly Shop, most assembly stations especially Error Proofing stations(such as Torque Gun, Part Picking, Marking, Gluing, Filling) are in the center of line, assembly operations are made when vehicle is moving.

In vehicle industry, we normally use Encoder and Sensor together with RFID Reader to calculate vehicle’s real time position.

 

1.    Encoder

The so-called Encoder is a special device which can output its speed and rotated angles in real time.

After connecting to conveyor line, Encoder will be running with same speed as conveyor motor, and the data of speed and angle will be integrated into conveyor PLC, and PLC will convert them into speed and distance of conveyor line movement.

Encoder is a device with high accuracy, it also offers state(active/not-active) and direct(clockwise/anti-clockwise), and it’s very convenient to integrate with PLC.

 

2.    Sensor

In vehicle industry, paired through-beam Photoelectric sensors are used quite a lot. It consists a sender and a receiver, the sender sends out a beam of light, then receiver will check if it can receive the light in a period of time, if not, that means the light is blocked by some object, then PLC will set a Blocked signal.

A vehicle in plant normally is loaded into a special carrier, and moving in specific direction, when vehicle with carrier comes to specific position, it’s supposed to block sensor’s light, we can regard sensor’s Blocked signal as vehicle in position.

 

3.    Application example

As showed in diagram 2.8-1, conveyor PLC integrated with 1 Encoder and 4 Sensors in this case:

Diagram 2.8-1: Conveyor and Encoder, Sensors

 

The functions of 4 Sensors:

1)       Sensor #1 collects signal when vehicle is loaded into line. This sensor is installed at line head station, so when vehicle comes to station, the carrier will block the light of sender.

2)       Sensor #2 check whether it’s an empty carrier. This sensor is also installed at line head station, but it has different height than sensor #1. If it’s not empty, then the body of vehicle should block the light of sender.

3)       Sensor #3 check whether carrier comes to RFID Reader, if yes then triggers RFID reading.

4)       Sensor #4 check whether carrier comes to 70% position. Because in Final Assembly Shop, all stations of same line have same station length, so when vehicle of 1^st station comes to 70% position, that means the vehicles in other stations should come to 70% position as well. And in theory all assembly and Error Proofing operations should be done by 70% position, and we need to use the left 30% of time to reset tools and get vehicle ready to leave current station.

And then inside PLC we set a dataset to maintain VIN number, and conveyor position of each vehicle:

VIN sequence	VIN number	Conveyor position
1	VIN1	1000
2	VIN2	7000
3	VIN3	13000
4	VIN4	19000

 

After a period of time, as conveyor line moves forward, its position number will increase accordingly, so as vehicles’ position.

So by reading Encoder, we can get real time position of conveyor line, and from which we can convert it to actual position range of each station:

Station	Position range
0	0~6000
1	6000~12000
2	12000~18000
3	18000~24000

 

By comparing vehicle position and station position, PLC can calculate vehicle’s current station.

For example vehicle #3 has position of 13000, which is at range of 12000~18000, which refers to station #2.