Basics of servo weld gun
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Basics of servo weld gun with encoder system, robotic systems, motors with feedback, Ball screw, resistance welding, trans gun, compact robot welding gun
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Basics of servo weld gun
- 1. Resistance WeldingResistance Welding Servogun BasicsServogun Basics
- 2. What Is A Servo?What Is A Servo? 1. Motor 2. Feedback Device 3. Controller
- 3. Servo MotorServo Motor Stator Windings Feedback DeviceNeodymium Iron- Boron magnets in Rotor Feedback ConnectorPower Connector
- 4. Servo Motor ConfigurationsServo Motor Configurations Frameless Direct Drive Rotary
- 5. Feedback DeviceFeedback Device Used for motor commutation and position feedback. • Hall Effect Devices • Encoders • Resolvers
- 8. Motion ControllerMotion Controller Motion Generator Encoder/ Resolver Commutation and Current Loop Power Stage Position Loop Velocity Loop Motor Controller Drive / Amplifier
- 9. Applying Servos to RWApplying Servos to RW 1. Measurements 2. Integration With Equipment 3. Controlling The Actuator 4. Drive Configurations 5. Equalizing 6. Additional Considerations
- 10. Position MeasurementsPosition Measurements • Gun Open • Retract Position(s) • Tip Touch • Pre-weld position • Post-weld position • Electrode wear
- 11. Force MeasurementsForce Measurements • Approach force • Weld force • Tip dress force • Return force Motor current feedback (motor torque) can be used to determine:
- 13. Typical Robot IntegrationTypical Robot Integration PLC Robot Weld Control PLC Robot Controller Amplifier Servo Motor Teach Pendant Electronic Operator Interface
- 14. Typical Robot IntegrationTypical Robot Integration
- 15. Robot Remote AmplifierRobot Remote Amplifier PLC Robot Controller Amplifier Weld Control Servo Motor Teach Pendant Electronic Operator Interface
- 16. Robot Retrofit IntegrationRobot Retrofit Integration PLC Robot Controller Motion Controller Amplifier Weld Control Servo Motor Teach Pendant Electronic Operator Interface
- 17. Machine IntegrationMachine Integration PLC Motion Controller Amplifier Weld Control Servo Motor Electronic Operator Interface Amplifier Servo Motor Amplifier Servo Motor Amplifier Servo Motor
- 18. Controller Integral to Weld ControlController Integral to Weld Control PLC Motion Controller Amplifier Weld Control Servo Motor Electronic Operator Interface
- 19. Controller Integral to MotorController Integral to Motor PLC Weld Control Electronic Operator Interface Servo Motor
- 20. Servo Controller Integral to MotorServo Controller Integral to Motor
- 21. Controlling the ActuatorControlling the Actuator Trapezoidal Move Profile
- 22. Controlling the ActuatorControlling the Actuator P-Curve Move Profile
- 23. Controlling the ActuatorControlling the Actuator S-Curve Move Profile
- 24. Controlling the ActuatorControlling the Actuator Chart courtesy of Savair/ARO
- 25. Controlling the ActuatorControlling the Actuator
- 26. Controlling the ActuatorControlling the Actuator
- 30. Tubular Linear MotorTubular Linear Motor
- 31. Matching the Actuator to the GunMatching the Actuator to the Gun Gun Ratio = A B A B
- 32. Why is Equalizing Necessary?Why is Equalizing Necessary? Part Position or Gun Position Error Expected Actual
- 33. Why is Equalizing Necessary?Why is Equalizing Necessary? Part Fit-up Gap
- 34. Why is Equalizing Necessary?Why is Equalizing Necessary? Cap Wear
- 35. Why is Equalizing Necessary?Why is Equalizing Necessary? • Electrode Deflection • Weld Indentation • Metal Thickness Variation
- 36. Equalizing ServogunsEqualizing Servoguns 2. Robot Equalization 1. Pneumatic Equalization 3. Servo Equalization Photo courtesy of Matuschek GmbH
- 37. Additional ApplicationAdditional Application ConsiderationsConsiderations 1. Training 2. Maintenance 3. Power Consumption
- 38. Packaging of Servo Vs PneumaticPackaging of Servo Vs Pneumatic
- 39. Resistance WeldingResistance Welding Servogun BasicsServogun Basics Thanks to the following for providing material to be used inThanks to the following for providing material to be used in this presentation:this presentation: CenterLineCenterLine Savair/AROSavair/ARO Milco ManufacturingMilco Manufacturing California Linear DevicesCalifornia Linear Devices Matuschek GmbHMatuschek GmbH Nachi RoboticsNachi Robotics Other Internet SourcesOther Internet Sources
Editor's Notes
- Index Page
- An AC Brushless Servomotor is illustrated. This motor is very reliable since: the shaft is well supported by sealed bearings the motor stator windings are close to surface of the motor for good heat dissipation rare-earth magnets embedded in rotor are dimensionally stable so the rotors air gap can be minimized. The servomotor also includes: A feedback device - required for commutating the motor (energizing the stator windings at the right time to make the motor turn) and for positioning. There are several different types of feedback (encoder, resolver, hall-effect devices) Electrical connectors there might be one or two (motor power and feedback) or in some cases pigtails are also available. fixed or repositionable (pointing straight out or parallel to motor axis). It is important to ensure there is adequate room provided for the mating connectors and cable. Electric brake – might be provided to hold the motor position. This is not common in North America. It usually adds a few inches and several pounds of weight. There are IEC Standards for motor frame sizes and shaft dimensions. The length of the motor can be changed to add more or less stator windings so the motor torque can be matched to the load requirements. Ratings:- RPM (3000 – 5000 common) - torque varies with motor design and magnet permeability - temperature rating will determine what duty cycle the motor can operate at.
- The same basic elements (rotor and stator) are manufactured as frameless or direct-drive motor components to be incorporated into proprietary inline (direct-drive) servo actuators.
- Index Page Commutation – selective excitation of the motor coils to generate rotor motion. The rotor will turn so the imbedded magnets can line up with the magnetic field in the energized stator coils.
- Encoders utilized masked discs to create electric pulses as the beam of light is interrupted during shaft rotation. In the most common version, the quadrature encoder, there are two pulse outputs that are 90 degrees out of phase. Position is determined by counting pulses and the direction is known by the sequence of the two channels. There is usually a third marker pulse output that occurs once per rotation. Because it is only possible to count quadrature encoder pulses to determine how much rotation has occurred from the starting point, they are called incremental encoders. Sometimes battery backup will be incorporated to power the encoder so the count is retained. Absolute encoders are also available which output a code that corresponds to every unique shaft position. It would be common for there to be 500 to perhaps 1,000 counts per millimeter of tip movement.
- Basic Resolver Has the same function as the encoder but it generates an amplitude modulated signal based on shaft position. It is essentially a rotary transformer with one primary and two secondary windings. As the shaft rotates the transformer ratio changes and the voltage of the two output signals vary. Since the two outputs are 90 degrees out of phase the resolver is absolute within rotation (no repetition of signal) Why use Resolver or Encoder? designers choice – not much difference in our application resolver has slightly better high temperature performance Resolver might have a higher reliability because there are fewer components resolver does not rely on logic detection so it has a better performance at higher speeds (above 10,000 rpm)
- “Loop” in this illustration refers to a PID control loop (proportional/integral/differential) Example of a Position loop -Inputs are:- Position increment - Position error - Proportional gain - Integral gain - Feed forward gain Output is:- Command position The gains are required so the control can look ahead and predict what command is required to make the motor end up at the desired place at the desired time. These PID loops might be very easy to setup in a control with an Auto-tuning function, or take a very long time (perhaps a couple of hours) with trial and error. The Controller calculates what motion is required and the Drive or Amplifier translates the commanded signal into a high power signal that can be used to move the motor. The weld gun is inherently a difficult device to control because the mechanical system is not rigid. The gun components allow deflection and can act like heavy spring that is compressed when closing and then released when the tips go to open.
- INDEX PAGE
- The feedback device allows the User to gather considerable actuator position information from the servogun. Note: The relationship of the actuator position to tip position will be affected by the rigidity of the gun. Gun Open position This position does not have to be fully open to a mechanical stop gun open can be defined by a window (position greater than a minimum “open” position). Retract position may be discrete position value(s) or infinitely adjustable position (i.e. robot) Tip Touch position if the initial touch is done at a reduced force it is possible to reduce the initial tip impact (noise, bounce) can be used to calculate the fit-up gap once pre-weld position is known Pre-weld position can be used to measure or verify the material thickness. It will likely be necessary to touch the tips occasionally to learn the tip wear and get a zero measurement. Note: the tip wear would be the total wear (top and bottom) which might not be the same for both tips. can be used to not allow the electrode force to increase from a reduced initial value when the gun is not at the proper closed position. can be compared with tip-touch position to determine a value for fit-up gap (sheet spacing) for process monitoring. Post-weld Position can be compared with pre-weld position to determine a value for weld indentation (or set-down) Electrode wear must be directly measured by closing the gun at some interval might be possible to close on a standard to determine the wear on each tip. This may be affected by rigidity of equipment components. Full stroke the control can watch for electrode breakage or excessive cap wear.
- Accurate motor current detection is required by the motion controller to control the motor. It is possible to use the current (or torque value) to very reliably control the tip force. Approach force can be at a value lower than the weld force to determine if there is interference with tip motion. Motion can be stopped if the tips run into something they are not supposed to (e.g. a clamp or part that is out of position ) Weld force can be varied between welds or within a weld sequence (for forging). A definitely Not recommended (but Customer requested feature) is to increase the electrode force if the scheduled weld force is not enough to pull parts together. Return force can be reduced to detect tip stuck or interference An auxiliary measurement device such as a load cell or transducer can be integrated for improved accuracy but of course there are reliability issues created with placing these devices in the welding environment.
- Typical Parts in Control systems. The structure of the control system and the communication hierarchy can change to best suit customer or project requirements.
- In a typical robot integration, the amplifier is generally an auxiliary axis. This is the preferred method for robot integration because a lot of position control is necessary. The control is required not for the resistance welding process, but for situations where gun might be partially closed inside a part when power goes out (automated fault recovery). Teach pendant – can program/operate robot weld and gun The motor must be compatible with the robot manufacturer’s choice of voltage and feedback device. If the robot manufacturer’s motor is not used, this might result in a number of variations of motors/actuators to address a number of gun sources.
- The ARO table shows different voltages and feedback for various robots. The key note in the table is that the feedback devices are not compatible amongst robot brands.
- A remote amplifier is used in some applications where the robot controller is not reconfigureable or of adequate output capability to match a motor. The motion control is still commanded by the robot, but motor drive output can be more universal (to match a large, new, or different motor). Various communication options are available such as: RS232/RS485, SERCOS, Firewire, etc.. In the future, more advanced robot options should be available to make these interfaces easier for the Integrator or User.
- In a retrofit situation a separate motion controller can be used to drive the servomotor. This motion controller can use the existing control inputs (Enable, SV1, RV1, Schedule Selects) and Outputs (In position, not faulted). This provides the ability to have multiple retract positions and forces without any other system or programming changes.
- Hardware currently exists to integrate the entire control package for fixture applications. A multi-axis motion controller can control several servoguns within a tool such as a geo (geometry setting) station. Example: Allen Bradley SLC programmable controller, 1394 Motion controller and Medar’s SLC card control.
- Some feel motion control should be done in the weld control since it has responsibility for values and timing. A new generation of welding controls will likely be developed to control servoguns, depending on Customer demand.
- In the future – the control may be integrated into the motor so these devices are independent of what they are connected to.
- Example of integral SmartMotor (Animatics Corporation) that has a complete built-in motion controller.
- The motion controller drive signals have three basic profiles. The trapezoidal profile is used in general purpose applications because it has the fastest acceleration and deceleration. Of course, this high rate of acceleration also translates into high motor stresses and high mechanical stress. Note: Jerk is rate of change of acceleration.
- The parabolic profile acceleration has a softer transition to and from the maximum velocity. This profile reduces the level of mechanical stress to get to speed.
- The S-curve has the lowest level of mechanical stress because acceleration increases to start load moving and backs off as the desired velocity is achieved. This profile works well for welding guns since the heavy gun bodies can be started and stopped with lower currents and much less stress.
- The green line is the position of the tips when they are approaching the part. The black line is during the weld cycle. Note the trapezoid waveform to close the gun to the part (purple line under the green line segment). In the lower current profile, the high current pulses correspond with starting and stopping the motor.
- Current (torque) and velocity data from a CenterLine servogun. The S-curve velocity profile is calculated by the motion controller based on the travel distance. The command and feedback velocities are superimposed on one another except during the squeeze. The command velocity has to remain positive although there is no actual motion or there will be no force developed. There is some minor difference between the command torque and the feedback torque because of system response to loads. The step on the current square wave (the highest part of the curve) is a reduced closing force value which quickly transitions to full force if the position criteria are satisfied in the program.
- The force output using the motor current (torque) is very accurate. The data points are actual readings taken with a force gage. It is possible to take the motor conversion factor (in N-m/amp) and perform a conversion to to pounds-force (lbf), kilograms-force (kgf) or decaNewtons (dN). The data points have a slight ripple about the line because of the efficiency of the motor pole alignment. In this particular case is an 8-pole motor was used. If fewer poles are designed into the motor the ripple will likely be larger. In this case the ripple value is not significant and it is not necessary to apply any scaling correction.
- Rotary motion must be converted into linear motion to operate standard welding guns. Not all servoguns incorporate linear motion (some use rotary) but these are the most common actuator forms. Top center: ARO pinch gun Bottom left: CenterLine C-gun Bottom right: Milco scissor gun
- Roller screws are the best technology for converting rotary to linear motion because they have: a very high efficiency rating very good high speed performance a lot of contact area for excellent life Most importantly, for the plant environment, they can be taken apart and put back together without effect. In the case of a ball screw, if it is disassembled there is no way to put it back together again, even if all of the balls can be found (without affecting the operation and life rating). Of course, there is a penalty for using the roller screw; they are currently not manufactured in the U.S. and when this is combined with their complexity there is a pretty significant cost premium.
- Parallel drives generally utilize a drive belt to transmit torque between two pullies. They are: Short - not much longer than the required length of screw. Compact – in photograph of example, the distance from the centerline of the rod to the mounting face is very small. Standard motor frames can be incorporated Repositionable to move the motor out of the way. A lot of common components are preserved when stroke lengths changed. Inverse Parallel Another version of Parallel configuration In-Line Two versions: with hollow shaft (shown) or solid shaft (not shown) motor. Solid shaft motors (with standard motor bolted to the end of the in-line actuator) tend to make a long actuator. Custom actuators with hollow rod or frameless motors are light weight and compact.
- A good way to generate linear motion is to not require conversion. This patented tubular linear motor made by California Linear Devices has one moving part (the shaft). Like the rotary AC servomotor, this motor design uses windings on the outside to facilitate cooling. Also, the length of the winding stack can be increased to increase force output.
- When matching the actuator to the gun it is necessary to consider the actuator ratio and the gun ratio. The actuator ratio is determined by selecting a proper combination of motor speed, motor torque, drive reduction and screw pitch. There is a tradeoff to be made between speed and force. The for the motor to run faster, it will generally have a lower torque rating. The gun ratio is based on the mechanical configuration. It is possible to use a lower force actuator by making “A” longer but this sacrifices gun size and weight. The correct way to apply an actuator is to use the gun arm stress (properties of the material the arm is made up of) to drive the actuator output force. The objective is always to keep the gun as small as possible. This might require the actuator to produce forces exceeding 4,000 lbf.
- One of the big pushes for servoguns is to eliminate the use of air. The feature that is questioned most is equalizing because most gun manufacturers are still using pneumatic equalizing. Equalizing is provided to take care of unpredictable gun position errors. Examples of sources of these errors: Part not repeatable Servogun position error Servogun setup error ** Could actually be all three causes If a solid gun arm is used to pull the part to a predetermined position it can: Introduce unpredictable stress (and distortion). Pull parts off of nest blocks Place stress on clamps and fixture
- The unpredictable error could also relate to the fit-up of the parts. It might not be the weaker part that is in the wrong place. If you pull on the assembly to pull a particular location to the right position, who knows what is happening to the entire assembly?
- Cap wear is usually consistent but it may not be equal on both caps.
- Other sources of position errors include: Deflection – Consistent but might not be the same on both electrodes Indentation – Usually consistent once learned. Does the rate remain the same for both caps as cap wear occurs? Metal thickness – not predictable but variation might not be significant
- Equalizing methods. Pneumatic – inexpensive and many times standard. Robot- soft servos (in multi-axis) - this is difficult to manage when the reaction of the system varies as the mass of the gun is affected by gravity. Servo – Matuschek system illustrated - additional servo axis controlled by the welding control
- Training- big shift in skills - good to avoid big shift in philosophy - will traditional welder setup staff be able to use equipment Maintenance- Tuning - How to close gun for maintenance - How easy to change motor/actuator Power consumption- enough power on the buss? - current during weld may be 5 to 30 amps depending on the size of gun/actuator