Motion Control Offset Problem With Stepper And Servo Using

- Sep 13, 2017-

Analysis and Countermeasure for Motion Control Offset Problem

The bit  problem is one of the common problem which equipment factory faces during the equipment commissioning, as well as 

equipment using process,with stepper motor and servo motor using, one of the common problems facing. The deviation may be 

caused by improper mechanical assembly, may be unmatched control system and the drive,perhaps electromagnetic interference

within the device, the workshop equipment to interfere with each other or improper ground wiring during device installation

and so on.

This article from a number of application cases, refining and sorting out the most common causes and countermeasures to help equipment manufacturers to quickly locate, take appropriate measures to improve the equipment anti-jamming,proper grounding to ensure device normal operation.

Regularity deviation phenomenon, reason and solution

Q: do reciprocating movement, the more forward the more (or less).

Possible reason ①: Impulse equivalent is wrong

Reasons: whether it is synchronous wheel structure or rack and pinion structure, there are processing accuracy error. The motion control card (PLC) does not set exact pulse equivalent. For example,last a group of synchronous wheel motor rotation of a circle,equipment forward 10mm,current synchronous wheels a little bigger, motor rotate one lap,device forward 10.1mm, it will lead to the batch of machines each time running more 1% distance than the previous equipments..

Solution: before the machine finished,make machine draw a square as large as possible,then use the ruler to measure the actual size,calculate the actual size and control card setting size the ratio,and add it to the control card operation, repeated three times, will get a more accurate value.

Possible cause ②: The trigger of the pulse command conflicts with the level change timing of the direction command

Reasons: the driver requires the host computer issued pulse command edge along with the direction command level conversion has a certain timing requirements.But some PLC or motion control card programming does not meet this requirement (or its own rules do not meet the requirements of the drive), resulting in pulse and direction timing can not meet the requirements,then offset.

Solution: control card (PLC) software engineers put ahead direction signal. Or the driver application technician changes counting mode of pulse edge.


Q: motor shake at the fixed point when moves, after this point,it is normal operation, but moving distance is less

Reason: Mechanical assembly problem

Analysis of reason: mechanical structure has big resistance at a point.Due to the parallelism or vertical of the mechanical installation,

or unreasonable design of the equipment,lead to a point in a larger resistance, stepper motor torque changing law is the faster speed, the smaller torque, it is easy to stuck in the high-speed section,if speed down,it can go across.

Solution: 1, check the mechanical structure for stuck,the friction is large or the slide is not parallel and so on. 

              2, stepper motor torque is not enough. As the end users require to speed up or increase the load, resulting in the original

motor can meet the requirements of high-speed torque but is not enough now, resulting in high-speed stall phenomenon. You can set a larger output current through the drive or increase the supply voltage within the allowable voltage range of the driver, or replace the motor with greater torque to solve the this problem.

Q: motor reciprocating movement,back and forth,do not go in place and offset fixed

Cause: Belt gap

Analysis of the reasons: there is a backlash between the belt and the synchronous wheel, there will be a certain amount of empty course.

Solution: If the motion control card has a belt backlash compensation function, it can be used; or tighten the belt.

Q:cut trajectory does not coincide

Possible reasons ①: inertia is too large

Analysis of the reasons: flat-panel cutting machine inkjet process is controled by the raster control, scanning movement, cutting with interpolation movement,the two trajectories do not coincide, because, similar equipment X-axis car inertia smaller and positioned by raster,inkjet location accurate, while the Y-axis gantry structure inertia is large, the motor response is poor, the Y-axis does not follow well when interpolation movement

Solution: increase the Y-axis reduction ratio,use notch function to improve the servo drive rigidity to solve the problem.

Possible reasons ②: knife and nozzle coincidence does not tune well

Analysis of reasons: because the cutter and the nozzle are installed on the X-axis car, but the two have coordinates error,the machine can adjust the coordinates of the machine to do the knife and nozzle track coincidence, if not tune, cut trajectory will be separated as a whole.

Solution: modify the knife and nozzle position compensation parameters.

Q:draws a circle but comes an ellipse

Possible reasons: XY axis platform,two axes are not vertical

Cause Analysis: XY axis structure, graphics offset, such as drawing a circle into an ellipse,the square offset into a parallelogram. Gantry structure X-axis and Y-axis is not perpendicular leading to the problem.

Solution: Adjust the gantry X-axis and Y-axis perpendicularity to solve the problem.

Ir-regular deviation phenomenon, cause and solution

Q:offset from time to time during running period, offset with a chance,uncertainty deviation

Possible reason ①:disturbance causes the motor offset

Cause Analysis: Most of the aperiodic offset due to interference,small part caused by motion control card output narrow pulse or mechanical structure loose.

Solution: If the interference occurs frequently,use the oscilloscope to monitor the pulse frequency to determine the time of interference occurred,then determine the interference source,remove or make the pulse signal away from the interference source can solve part of the interference.If the interference occurs by chance, or it is difficult to determine the location of the interference source or difficult to move the fixed electrical cabinet,consider to take the following measures to solve the problem:

① drive connect to ground,

pulse cable change to twisted shielded cable,

③ pulse positive and negative side parallel 103 ceramic capacitor filter (pulse frequency is less than 54kHz),

④ pulse signal holded by magnetic ring,

⑤ Front of driver and the controller power supply add filter.

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Note: Common sources of interference include transformers, coil relays, frequency converters, solenoid valves, high voltage wires and so on. When plan the electrical cabinet,should avoid the signal cable close to these sources of interference, signal cable and high-voltage power supply cable should be divided into different trunking wiring.

Possible cause ②: Pulse bunch appears narrow pulse

Problem Analysis: motion control card output pulse bunch duty cycle is small or too large, there is a narrow pulse, the drive can not identify, resulting offset.

Solution: find reason for this problem from controller, is the pulse connection ports problem, or software algorithm problem


Possible reasons ③: mechanical structure loose

Problem Analysis: Couplings, synchronous wheels, reducers, etc. with jackcrews fixed or the screw clamps connecting piece may be loosened after a period time under rapid shock applications,cause offset. Use the key and keyway with the fixed synchronous wheel,notice there is a gap between the key and the keyway or not;the rack and pinion structure,notice that the gap between the two.

Solution:key part/strong force bearing structure screws must use with a cushion, and screws or jackscrew should be coated with screw glue. The motor shaft and coupling are connected with keyway as much as possible.

Possible reason ④:The filter capacitor is too large

Analysis of cause: the filter capacitor is too large, ordinary RC filter cut-off frequency is 1 / 2πRC, the greater capacitor the smaller cut-off frequency,the general driver pulse resistance is 270 ohms, 103 ceramic capacitor RC filter circuit cutoff frequency is 54khz, frequency is higher than this,the amplitude will large reduction,and cause some of the effective signal can not be correctly detected by the driver, eventually leading to offset.

Solution: adding filter capacitor need to calculate the pulse frequency,make sure that the maximum pulse frequency meet requirements

Possible cause ⑤:PLC or motion control card maximum pulse frequency is not enough high

Analysis of the reasons: the general PLC allow the maximum output pulse frequency 100kHz, motion control card according to its pulse chip difference frequency, especially the general microcontroller development of the motion control card may be due to the low pulse frequency ,then lead to offset.

Solution: If the host computer maximum pulse frequency is limited, in order to ensure the speed,can appropriately reduce the drive subdivision to ensure the motor speed.

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