This article extracts and summarizes the most common causes of misalignment and corresponding countermeasures from numerous application cases. It aims to help equipment manufacturers' commissioning personnel quickly locate problems, take appropriate measures to improve equipment anti-interference capabilities, and ensure proper grounding of equipment to guarantee normal operation.
The phenomenon, causes, and solutions of displacement (both regular and irregular displacement).
01
Regular deviation
Deviation phenomenon 1: When making reciprocating movements, the deviation increases as it moves forward (less).
Possible cause ①: Incorrect pulse equivalent
Cause Analysis: Both synchronous pulley structures and rack and pinion structures have machining accuracy errors. The motion control card (PLC) is not set with accurate pulse equivalents. For example, if the previous batch of synchronous pulley motors advanced the machine by 10mm per revolution, while this batch of synchronous pulley motors, which have larger synchronous pulleys, advance by 10.1mm per revolution, this will cause the machines to travel 1% further per run than the previous batch.
Solution: Before shipping, use the machine to draw a square that is as large as possible, then use a ruler to measure the actual size, compare the ratio between the actual size and the size set on the control card, and then add it to the control card calculation. After repeating this process three times, a more accurate value will be obtained.
Possible cause ②: Timing conflict between the trigger edge of the pulse command and the level transition of the direction command.
Cause Analysis: The driver requires a specific timing sequence for the edge and direction command level transitions of the pulse commands issued by the host computer. However, some PLCs or motion control cards are not programmed to meet this requirement (or their own rules do not conform to the driver's requirements), resulting in the pulse and direction timing not meeting the requirements and thus being misaligned.
Solution: The PLC software engineer advances the direction signal. Alternatively, the driver application technician changes the pulse edge counting method.
Misalignment Phenomenon 2: During operation, the motor vibrates at a fixed point, and after passing that point, it can run normally, but travels a shorter distance.
Possible cause: Mechanical assembly problem
Cause Analysis: The mechanical structure experiences significant resistance at a certain point. Due to issues with the parallelism, perpendicularity, or design of the mechanical installation, the equipment experiences high resistance at a particular point. The torque of the stepper motor decreases with increasing speed, making it prone to jamming at high speeds, while it can move forward when the speed decreases.
Solution:
1) Inspect the mechanical structure to determine the cause of jamming, such as high frictional resistance or misaligned slide rails.
2) Insufficient stepper motor torque. Due to end-customer demands for increased speed or load, the motor, which was originally capable of meeting the requirements, may lack sufficient torque at high speeds, leading to stalling. Solutions include setting a higher output current for the driver, increasing the supply voltage within the driver's allowable voltage range, or replacing the motor with one that offers higher torque.
Misalignment phenomenon 3: The motor's reciprocating motion does not reach the correct position in either direction and the offset is constant.
Possible cause: Belt clearance
Cause analysis: There is a reverse clearance between the belt and the timing pulley, which causes a certain amount of idle travel when the belt moves back.
Solutions: If the motion control card has a belt backlash compensation function, it can be used; or the belt can be tightened.
Misalignment Phenomenon 4: Non-overlapping Cutting Trajectories
Possible cause ①: Excessive inertia
Cause analysis: The inkjet process of the flatbed printer is controlled by a grating and has a scanning motion. During cutting, it follows an interpolation motion. The two trajectories do not overlap because the X-axis carriage of similar equipment has a small inertia and is positioned by a grating, resulting in accurate printing position. However, the Y-axis gantry structure has a large inertia and poor motor response. During the interpolation motion, the Y-axis tracking is poor, causing partial deviation of the trajectory.
Solution: Increase the Y-axis reduction ratio and use notch filtering to improve the rigidity of the servo drive to solve the problem.
Possible reason 2: The overlap between the blade and the nozzle is not properly adjusted.
Cause analysis: Because the cutter and nozzle of the plotter are both mounted on the X-axis carriage, but there is a coordinate difference between them. The plotter's host computer software can adjust this coordinate difference to make the trajectories of the cutter and nozzle coincide. If it is not adjusted properly, the plotting trajectory will be separated as a whole.
Solution: Modify the blade and nozzle position compensation parameters.
Misalignment Phenomenon 5: Drawing a circle as an ellipse
Possible cause: The X and Y axes of the platform are not perpendicular.
Cause analysis: The XY axis structure is misaligned; for example, a circle is drawn as an ellipse, or a square is drawn as a parallelogram. This problem occurs when the X and Y axes of the gantry structure are not perpendicular.
Solution: Adjusting the perpendicularity of the X-axis and Y-axis of the gantry crane can solve this problem.
02
Irregular deviation
Deviation phenomenon: Deviation occurs intermittently during operation. The deviation is random and the magnitude of the deviation is uncertain.
Possible cause ①: Interference causing motor misalignment
Problem Analysis: Most non-periodic deviations are caused by interference, while a small number are caused by narrow pulses emitted by the motion control card or loose mechanical structures.
Solution:
If interference occurs frequently, an oscilloscope can be used to monitor the pulse frequency to determine the timing of the interference and thus identify the source. Removing or moving the pulse signal away from the source can resolve some of the interference. If interference occurs only occasionally, or if the location of the interference source is difficult to determine, or if the electrical cabinet is fixed and difficult to move, the following measures can be considered to solve the problem:
① Ground the driver; ② Replace the pulse line with a twisted-pair shielded cable; ③ Connect a 103 ceramic capacitor in parallel at the positive and negative ends of the pulse for filtering (pulse frequency less than 54kHz); ④ Add a ferrite ring to the pulse signal; ⑤ Add a filter to the front end of the driver and controller power supply.
Note: Common sources of interference include transformers, coil relays, frequency converters, solenoid valves, and high-voltage power lines. When planning electrical cabinets, signal lines should be kept away from these sources of interference, and signal lines and high-voltage power lines should preferably be routed in separate cable trays.
Possible cause ②: Narrow pulses appear in the pulse train
Problem Analysis: The customer's motion control card sends pulse trains with a duty cycle that is too small or too large, resulting in narrow pulses that the driver cannot recognize, leading to misalignment.
Solution: Investigate the cause of this problem in the controller; is it a pulse interface issue or a software algorithm issue?
Possible cause ③: Loose mechanical structure
Problem Analysis: Connectors such as couplings, timing pulleys, and reducers, which are fixed with set screws or clamps, may loosen after a period of operation under high-impact conditions, leading to misalignment. For timing pulleys fixed with keys and keyways, check for gaps between the key and keyway; for gear and rack structures, check for the clearance between the two.
Solutions: For critical components and high-stress structural screws, use spring washers, and apply screw glue to the screws or set screws. Use keyways to connect the motor shaft and coupling whenever possible.
Possible cause 4: The filter capacitor is too large.
Problem Analysis: The filter capacitor is too large. The cutoff frequency of a normal RC filter is 1/2πRC. The larger the capacitor, the smaller the cutoff frequency. Generally, the pulse terminal resistor of the driver is 270 ohms. The cutoff frequency of the RC filter circuit composed of 103 ceramic capacitors is 54kHz. If the frequency is higher than this, the amplitude attenuation will be too large, which will cause some effective signals to be unable to be detected correctly by the driver, eventually leading to the deviation.
Solution: When adding a filter capacitor, the pulse frequency needs to be calculated, and it is essential to ensure that the maximum passing pulse frequency meets the requirements.
Possible cause 5: The maximum pulse frequency of the PLC or motion control card is not high enough.
Cause analysis: Generally, the maximum allowable output pulse frequency of a PLC is 100kHz. The motion control card varies greatly depending on its pulse-generating chip. In particular, motion control cards developed with ordinary microcontrollers may cause misalignment due to insufficient pulse frequency.
Solution: If the maximum pulse frequency of the host computer is limited, the microstepping of the driver can be appropriately reduced to ensure the motor speed.
