Dynamic Compensation in Motion Control: Fine-Tuning Trajectories

ByHANI_AUTOMATION HANI

In high-precision manufacturing, even minor mechanical imperfections can lead to significant positioning errors. Whether it’s uneven pitch distribution on a guide rail, backlash during direction reversal, inaccurate triggering on conveyor belts, or fixed mechanical offsets, these issues demand real-time compensation. Modern motion controllers offer powerful instructions to handle such scenarios without stopping production. This article dives into four practical compensation methods that keep your motion trajectories accurate and your processes running smoothly.

Scenario 1: Real-Time Trajectory Fine-Tuning with Handwheel Trigger

In many applications, operators need to make small adjustments to a moving axis without halting the machine. This can be achieved by coupling a master axis to a slave axis and adding a virtual compensation axis. The CONNECT instruction links the master and slave with a 1:1 ratio, ensuring synchronized position, speed, and acceleration. A separate virtual axis is then superimposed onto the slave using ADDAX. An external signal, such as a handwheel or sensor, triggers incremental moves on the virtual axis, effectively adding or subtracting pulses from the slave’s trajectory in real time.

For example, a rising edge on input IN0 might add 5 pulses, while a falling edge subtracts 5 pulses. This method is ideal for manual correction during continuous motion, such as aligning a cutting head or adjusting a dispensing needle on the fly.

BASE(0,1)
ATYPE=1,1
UNITS=100,100
DPOS=0,0
SPEED=20,20
ACCEL=200,200
DECEL=200,200
TRIGGER
CONNECT(1,0)  AXIS(1)        ' Link axis 0 to axis 1, ratio 1:1
MOVE(100)  AXIS(0)           ' Axis 1 moves 100, axis 0 follows
BASE(2)
ATYPE=0
UNITS=100
ADDAX(2)  AXIS(1)            ' Superimpose virtual axis 2 onto axis 1
WHILE 1
  IF SCAN_EVENT(IN(0)) > 0 THEN     ' Rising edge on IN0
    MOVE(5)  AXIS(2)                ' Add 5 pulses to trajectory
  ELSEIF SCAN_EVENT(IN(0)) < 0 THEN ' Falling edge on IN0
    MOVE(-5) AXIS(2)                ' Subtract 5 pulses
  ENDIF
WEND

The oscilloscope trace would show the slave axis following the master with small step changes at each trigger event, confirming the compensation is applied without interrupting the main motion.

Scenario 2: Laser Height Compensation for Z-Axis Alignment

In laser measurement systems, maintaining a constant focal plane is critical. A laser sensor mounted ahead of the camera measures the height deviation from a reference plane and sends the data to the controller via serial communication. The controller uses a virtual axis to superimpose the correction onto the physical Z-axis. When a trigger signal (e.g., IN1) indicates the camera is about to capture, the MOVEABS command instantly adjusts the Z position by the measured offset, ensuring the camera is perfectly focused.

This technique is widely used in automated optical inspection (AOI), PCB drilling, and laser marking, where surface height variations must be compensated at high speed.

DIM HighData  ' Height deviation data from laser
BASE(3,4)
ATYPE=1,0      ' Axis 3: physical Z, Axis 4: virtual
UNITS=100,100
DPOS=0,0
SPEED=20,20
ACCEL=200,200
DECEL=200,200
ADDAX(4) AXIS(3)  ' Superimpose axis 4 onto axis 3
WHILE 1
  IF SCAN_EVENT(IN(1)) > 0 THEN     ' Rising edge trigger
    MOVEABS(HighData) AXIS(4)       ' Compensate Z by deviation
  ELSEIF SCAN_EVENT(IN(1)) < 0 THEN ' Falling edge trigger
    MOVEABS(HighData) AXIS(4)
  ENDIF
WEND

By setting HighData = 1.5 in the command line, the oscilloscope shows the Z-axis moving exactly 1.5 units upon trigger, demonstrating precise compensation.

Scenario 3: Flying Shot Latch Correction for High-Speed Vision

In high-speed inspection systems, capturing images "on the fly" without stopping requires precise position triggering. However, servo drive delays can cause the actual trigger position to lag behind the target. By using a high-speed probe input on the drive and the controller's latch function (REGIST), the exact position at the trigger moment is captured. The difference between the latched position and the target is then used to correct the next move, effectively eliminating the lag in subsequent cycles.

This method is essential for applications like chip mounting, label inspection, and high-speed sorting, where even a few microns of error can cause rejects.

BASE(1)
ATYPE = 1
SPEED = 10000
ACCEL = 100000
DECEL = 100000
DPOS = 0
OP(0,0)
TABLE(0)=100
HW_PSWITCH2(1, 0, 1, 0, 0, 1)   
MOVEABS(TABLE(0)+50)
DIM regist_imode
regist_imode = 4   
BASE(1)                                
REGIST(regist_imode)             
IF regist_imode = 4 THEN
   WAIT UNTIL MARK
   ?"Mode",regist_imode ,"Latch position REG_POS",REG_POS       
   OP(0,0)       
   TABLE(0)= TABLE(0)+(TABLE(0) - REG_POS)   
   WAIT IDLE       
   DPOS = 0   
ENDIF
REGIST(regist_imode)  
HW_PSWITCH2(1, 0, 1, 0, 0, 1)     
MOVEABS(TABLE(0)+50)   
WAIT UNTIL MARK     
?"Mode",regist_imode ,"Corrected latch position REG_POS",REG_POS

In a typical test, the preset flying shot position was 100, but the actual latch captured 100.02. After correction, the second shot latched exactly at 100, proving the compensation works.

Scenario 4: Pitch Error and Backlash Compensation

Mechanical inaccuracies like lead screw pitch errors and backlash can degrade positioning accuracy. Pitch error compensation involves storing correction values in a TABLE array, where each entry represents the pulse offset at a specific position interval. The PITCHSET command defines the compensation points and interval. Backlash compensation, on the other hand, adds extra pulses when reversing direction to take up mechanical play.

These features are crucial in CNC machining, semiconductor manufacturing, and metrology systems where sub-micron accuracy is required.

ATYPE(1)=6
UNITS(1)=100
DPOS(1)=0
BASE(0)
ATYPE=1
UNITS=100
SPEED=100
ACCEL=500
DECEL=500
TABLE(0,0*UNITS(0),-30*UNITS(0),-50*UNITS(0),30*UNITS(0),50*UNITS(0),0*UNITS(0))
DPOS=0
MPOS=0
PITCHSET(1,0,100,6,0)  ' Start at MPOS=0, 6 points, interval 100
TRIGGER
MOVE(700)
MOVE(-700)
WAIT IDLE
PITCHSET(0,100,100,6,0)

The oscilloscope reveals six distinct compensation segments along the encoder feedback, confirming that the position error is corrected at predefined intervals.

Key Takeaways for Precision Motion Control

Dynamic compensation techniques are indispensable in modern automation. By leveraging virtual axes, high-speed latching, and table-based corrections, motion controllers can overcome mechanical limitations and deliver consistent accuracy. Whether you're fine-tuning a trajectory with a handwheel, aligning a laser sensor, correcting flying shot positions, or compensating for pitch errors, these methods ensure your system performs at its best. Implementing these strategies not only improves product quality but also reduces downtime and waste, making them a smart investment for any precision-driven industry.

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