Sensor Function for Torch Height Control (Additive Manufacturing)

Subject: Sensor Function vs COMARC for torch height control

Controller: YRC1000 (applicable to older controllers)

Product: Sensor Function and COMARC

Application: Welding

Robot: ALL

Summary:

Additive Manufacturing is a new application area for welding which can benefit from a way to maintain torch height control while welding to account for variation in height control.  Yaskawa has two methods that can be used for height control; COMARC through arc sensing or Sensor Function using a current sensor.  Both use the same hardware, but Sensor Function is recommended for Additive Manufacturing.  The sensor board (ACP02) can be configured for either COMARC or Sensor Function in Maintenance Mode (Users can change it).  Sensor Parameters can be configured for either Function, but it is not routine to switch between them due to files being initialized during set-up.

COMARC is a standard product for through-arc-seamtracking based on current feedback from the GMAW arc.  It requires weaving motion and corrects Up/Down to maintain stick out and Left/Right to follow the joint.  The COMARC file allows the selection of correction directions and just the UP//DOWN directions can be specified for torch height control.  Weaving is required and the COMARCON instruction will initiate weaving as well as tracking.  The weave can be a very minimal amplitude of a wire diameter to simulate stringer beads or a much wider weave of several millimeters.

However, weaving motion is directional and based on the vector the robot is moving.  It is determined at the time the COMARCON instruction is executed and assumes trajectory based on the approach point and Z+ in Robot Frame.  The weaving plane can be established using Reference Points (REFP1, REFP2) to follow other directions, but the reference points have to be in the program ahead of the COMARCON instruction.  Additionally, changes in robot travel direction without corresponding changes in travel angle will maintain the original weaving plane which may be out of phase with the travel direction.  Because Additive Manufacturing is performed in the XY plane with the torch perpendicular and moving multiple directions, COMARC is not recommended for the above reasons.

Sensor Function is a generic function that uses analog channel feedback to control the direction or speed of the robot from its programmed path.  It can be used for any application, such as grinding, with a force sensor or contour following sensor with analog output.  Yaskawa has used it for AVC torch height control for TIG welding with a voltage divider circuit for input.  For GMAW the current feedback from a Hall Effect sensor can be used for torch height control similar to COMARC.  The ComArc Option can be purchased to get current sensing hardware and then Users can reconfigure it use Sensor Function for torch height control.  Sensor Function option includes a wiring pigtail for connecting analog sensors (not provided) into the desired channel for direction/speed control.

The use of Sensor Function for torch height control during Additive Manufacturing-GMAW project can be reviewed in the video “Universal Sensor for Additive GMAW.mp4”.  The remainder of this document will cover the details of converting a system with ComArc option to utilizing the Sensor Function with the ComArc sensor box.

Background:

Sensor Function is designed to support different analog channels to provide a proportional analog to “steer” the robot in different X, Y, Z directions while it follows a programmed path.  The robot will correct in the plus/minus in the direction to maintain a Threshold voltage value from the sensor.  For torch height control we use the Z direction (CH3) to correct in the torch up/down direction. 

Sensor Function Tool Correction from YRC1000 Options Instructions for Sensor Function, 179329-1CD

For the Sensor Function to provide torch height control it is necessary to align the Tool Z axis with the welding torch.  Most torches have an angled torch barrel, so it is necessary to adjust the Orientation angles in the Tool File to align the Tool Z axis with the angled torch barrel.  This normally is achieved by setting a -45 degree angle in the Ry tool angle setting.

ComArc hardware uses a Hall Effect sensor with a 5v/500A analog output.  This results in a scale that is close between voltage and amperage; i.e. 2.00 volts is approximately 200 amps.  Sensor Function will correct in the specified direction to maintain a Threshold Voltage value while it is turned ON and stop correcting the path when turned OFF.  Parameters are used to enter the resolution of the sensor analog which controls the “gain” or response of the robot correction.  This can be adjusted to make sure the robot height control responds quickly, but not so much that it continually hunts up and down.  ComArc hardware is wired to connect the current sensor into Ch 1 (AN0) for ComArc current sensing.  Parameters for Sensor Function allow the hardware Ch 1 to be assigned to Ch 3 for the Z height control.

Operation:

The use of Sensor Function for torch height control is conducive to Additive Manufacturing because it is a single instruction after the ARCON instruction and an instruction to turn it OFF just before the ARCOF command. It will correct in torch height direction regardless of the direction of travel. WEAVE commands can be used with Sensor Function, but it is not required as it is with ComArc. Weave instructions are directional depending on the robots forward travel while Sensor Function is not. Therefore, if a wide weld bead is desired it is best to have the path generator create a zig-zag path to generate an oscillation with the desired bead width.

Following would be a typical start/stop sequence for a weld with height control:

This example has an optional SCF#(2) tag which will use a Sensor Condition/Monitor File setting. This allows a min/max value to be set for the analog voltage and control how the robot should respond in case of an abnormal reading.

This example has three linear positions (without speed tags), but there is no limit to the number of points that can be between the ACORON/OF instructions. The INFORM job has a limit of about 9000 lines. Our offline post processor was set to limit jobs to about 6000 points which allowed some memory for instructions. Larger builds had JOB CALLs for multiple layers.

The Sensor Function will work to maintain a Threshold Voltage value specified in the instruction.  Parameters specified in the Set-Up section will control the gain of the correction.  Make a test weld with desired tip to work distance and display the Sensor Input Status to note the average value.  This can then be entered as the TV= setting in the ACORON instruction.

Press ROBOT>SENS OUT STATUS to display the realtime input of the sensor analog.

Setup:

The ACP02 sensor board is used for both COMARC and SENSOR Functions. In MAINTENANCE mode the status is shown under SETUP-->OPTION BOARD-->ACP02 with MANAGEMENT security. You should make a memory backup (CMOS.bin) before making changes and you can download the SE parameter settings in ONLINE mode if you want to save your COMARC settings for later use.

If installing a new ACP02 board it will be necessary to use the first Pull Down Menu to specify USED for the sensor board.  If already installed for COMARC use the second Pull Down Menu to change the selection to UNIVERSAL SENSOR.

The DETAIL setting is not necessary on a single robot system.  Universal Sensor Function is not available on multiple robots; i.e. two robots with sensors, it is possible to have multiple robots with one robot equipped with Sensor Function (R1).

Pressing ENTER will generate a prompt asking to CONFIRM and SAVE the change.  Pressing ENTER again will initialize the Sensor (SE) parameters.  Cycle power to return to ONLINE mode or press SYSTEM>CPU RESET> and Confirm to restart in ONLINE mode.

You can verify that Sensor Function is successfully installed by pressing the INFORM LIST button on the pendant with a JOB displayed.  It should show the SENSOR menu with ACOR instructions (previously had COMARC instructions).  If it does not show ACOR instructions, select the SETUP>TEACHING Cond.>LANGUAGE LEVEL and toggle menu between SUBSET and EXPANDED.  This will cause the INORM list to refresh and it should show the ACOR instructions.

Parameters:

The following values were found to provide basic torch height control welding with the COMARC hardware box. 

Setting SE000 is not necessary.  It is set according to the configuration of the sensor board selected during the set up in Maintenance mode.  Do not change it.

Setting SE022=1 allocates the current feedback signal physically wired into AN0 (CH1) to the correction channel 3 used for Tool Z correction.  If the sensor was wired into AN2 normally used for channel 3, then SE022=3 would be used.

Setting SE028=-800 establishes a ratio of 8 mm/volt or 100 amps on the hall effect sensor.  This was found empirically by adjusting the value until the torch height remained stable at a desired height.  The negative value accounts for the inverse relationship of current and stick out; an increase in stick out will cause a decrease in current.

Setting SE034=0 was left at default value. It is a signal level that the sensor must see to respond, otherwise it will not correct to minor variations less than the Dead zone level.  Leaving this set to 0 will allow the sensor to be more responsive.

These parameters may need to be adjusted for the application.   Also, the Threshold voltage value is approximate and will not necessarily match the ammeter on the power source.  Experiments should be used to determine a Threshold voltage value to maintain the desired torch stand off and it could vary from the corresponding current value by 50-100 amps.

The Sensor Function can respond to other signals for side to side (Y direction) or an analog for speed correction. These were not used in Yaskawa’s experimental additive builds. The use of the Sensor Function cable (186572-1) provides a pigtail with wiring for the different channels if desired.

Touch Sensing:

The use of the ComArc hardware includes a circuit for Touch Sensing.  This applies a voltage to the wire while the robot approaches the part and detects it when the wire touches and the circuit is closed.  There should be Macro jobs in the INFORM list for different search patterns.  This document does not cover Touch Sensing because it operates in the standard fashion as other ComArc systems.  Touch Sensing can provide a means to search for the part height prior to starting the arc for Sensor Function to regulate the torch height.