Web cut machines are used where a material such as fabric is fed continuously at high speed and needs to be cut to a specific length.  The material is cut on the fly without stopping, allowing higher throughput to be achieved than could be realized if the material was stopped every time a cut is made.  In the application described here, a carriage carrying the cutting tool must be accelerated to meet the speed of the feed material before the cut occurs.  The solution to this type of application is to use an electronic cam to synchronize the motion of the shear with the motion of the conveyor.

In a flying shear web-cut application, the material to be cut is fed on a continuous conveyor that is driven by an open-loop motor (figure 1).  The shear is mounted on a carriage under servo control that runs parallel to the conveyor.  The shear accelerates to meet the velocity of the material to perform the cut at the correct location.  When the cut is complete, the shear rapidly decelerates and moves back to the starting position to begin the next cutting cycle.  This results in equal length pieces of material being fed to the next machine process. 

There are three major electro-mechanical systems which make up this flying shear application:

(1)   In-feed conveyor (master) - This is run by an open-loop motor, which is not commanded by the motion controller.  An encoder is attached to the motor, which feeds position information to the controller. 

(2)   Flying shear mechanism (slave) - The flying shear axis is driven by a closed-loop servo motor, which is commanded by the motion controller.  The position of this slave is determined by the position of the master as well as the defined cam profile.  This servo motor is connected to a lead screw, which drives the knife to match the speed of the conveyor.

(3)   Knife - This application uses an output bit to fire a pneumatically driven knife downward into the fabric at the proper position.  Other applications may use rotating knives or blades cutting across the material.

This section summarizes the requirements for the machine described above:

(1)   One axis to move the shear

(2)   Forward and reverse limit switches for the shear axis

(3)   An additional encoder input for the master encoder

(4)   Electronic Cam to simulate the motion of a mechanical cam

(5)   Consistent material cut length.  A registration eye is not required

(6)   A digital output bit to activate the knife

(7)   Stand-alone operation (no operator interface or host computer)

This section describes the Galil hardware and software products chosen to implement the machine’s control system.  Below is a complete bill of materials followed by a description of major components.

Controller: DMC-1416

Since the flying shear application does not require a host computer to be connected to the controller, we choose a stand-alone controller.  The DMC-1416 controller supports both Ethernet and RS-232 communications as well as non-volatile program memory, making it ideal for stand-alone applications.

The DMC-1416 is also packaged with an integrated PWM amplifier.  The DMC-1416-BRUSHLESS is capable of driving a brushless motor at up to 6 Amps continuous at 60 volts.  The DMC-1416 can handle the most demanding applications with such features as an extra encoder input for electronic cam and uncommitted I/O.

Motor:  BLM-N23-50-1000

For maintenance-free operation, we choose a brushless motor. Galil’s NEMA 23 #BLM-N23-50-1000 brushless motor, or equivalent, is appropriate because the axis requires less than 0.3 Nm of continuous torque.  An incremental encoder with 1000 cycles per revolution is installed on the motor resulting in 4000 quadrature counts per revolution. Hall sensors are not required on the motor as the incremental encoder provides commutation tracks for input to the amplifiers.

Electronic Cam

The key to this application is the use of the ECAM (electronic cam) feature of the Galil controller.  With ECAM, any slave axis or set of slave axes can be linked to a master axis to simulate the motion of a mechanical cam.  This enables periodic synchronization of one or more axes of motion to one master.  The master axis can be any motor-driven axis or encoder. 

Galil motion controllers treat the ECAM function as a table-based relationship of slave positions versus master positions over one cycle.  The flying shear application defines one cycle as the distance the master encoder moves for one complete cycle of the slave axis (the cut length).  The slave axis cycle consists of a rapid acceleration to match the conveyor speed, slewing at speed during the cut, rapid deceleration, and finally a return to the start position. 

Advantages of an electronic cam over a mechanical cam include the ability to programmatically change the cut length and the profile.  Also, there's no need to change expensive mechanical parts like mechanical cams to change the profile.

Program Structure

It is assumed that the entire cam cycle has been defined at this point.  Please refer to the appendix for a detailed derivation of the cam table.  The final step in the design process is to put all of these pieces together into a program for the motion controller.  The pseudo code for the program is as follows:

(1)   Program Start - #AUTO (runs automatically on power up).

(2)   Home Axes – Send the flying shear axis to its reverse limit switch and then an index pulse.  This insures the shear carriage is properly aligned on the lead screw before beginning a cycle.

(3)   Define Parameters:

(a) Define X-Axis auxiliary encoder as ECAM master and X-Axis main encoder as ECAM slave (Galil EA command).

(b) Define slave and master modulus for a single cycle (Galil EM command).  This would be 0 mm for the slave (because it advances and returns to 0 within each cycle) and the length of the cut for the master.

(c) Define master interval length (Galil EP command). 

(4)   Enter ECAM table in encoder counts.  This is based on the distances calculated in the appendix:

5      Appendix: Derivation of ECAM Profile

This section details the derivation of the ECAM table used for the flying shear.

Specifications

The first step is to define the performance specifications for the application.  The following numbers define the flying shear application:

cut length:                              250 mm

linear speed of material:             500 mm/s

minimum duration of cut:            100 ms

To convert from millimeters to counts, we need to know the following:

Conveyor roll diameter:                         12.73 cm

Conveyor encoder resolution:                4000 counts/revolution

(p * 12.73 cm / rev) * (1 rev / 4000 counts) * (10 mm / cm) = 0.1 mm/count or 10 counts/mm

Flying shear encoder resolution:             2000 counts/revolution

Flying shear lead screw pitch:                2.5 cm/revolution

(2.5 cm / rev) * (1 rev / 2000 counts) * (10 mm / cm) = 0.0125 mm/count or 80 counts/mm

Calculation

                        Slave Velocity vs. Time Graph

To begin the application design, it is helpful to calculate a slave profile by graphing its velocity as a function of time (figure 4).  Values needed for this graph may be calculated as follows. 

(1)   A cut length of 250 mm at 500 mm/s gives a single cycle time of 500 ms.

(2)   The speed of the slave during cutting must be equal to the speed of the fabric.  This is given as 500 mm/s.

(3)   The minimum duration of the actual cut (knife in contact with material) is 100 ms.  This is the minimum time during which the slave speed must equal the fabric speed.