What are the factors that affect the accuracy of picking trajectory of injection molding manipulator?

The factors that affect the trajectory accuracy of the injection manipulator can be divided into four categories: mechanical structure, control system, fixture and environmental conditions. These factors interact with each other and directly determine the positioning repeatability and trajectory smoothness of the end effector, as follows:

A, mechanical structure factors (basic factors)

Wear and clearance of mechanical transmission parts

Ball screw, linear guide rail and gear reducer of manipulator are the core transmission components. Long-term use will lead to screw pitch error, guide rail clearance and gear meshing clearance, which will lead to "idle stroke" and trajectory deviation when the shaft moves. For example, when the back clearance of the lead screw exceeds 0.03mm, the trajectory deviation of the reverse motion will increase obviously; Wear of the slide block of the guide rail will cause the end flange to swing and the coordinates of the picking position to drift.

Mechanical installation and insufficient rigidity

Parallelism and verticality errors will accumulate during the installation of each axis, which will lead to trajectory distortion during multi-axis linkage; If there is a gap between the end flange and the fixture, the positioning error will be enlarged.

The cantilever of manipulator is too long and the load of fixture is too heavy, which will cause the elastic deformation of the structure, the end will shake at high speed and the trajectory accuracy will decrease. Especially when the light manipulator is equipped with a heavy fixture, the deformation will increase significantly.

Assembly accuracy of parts

If the coupling of servo motor and lead screw has a large deviation in coaxiality, it will produce periodic vibration; The wear of joint bearings will lead to the decrease of the positioning accuracy of rotating shaft, and these assembly errors will be reflected in the end trajectory.

Second, the control system factors (core control factors)

Servo parameters and control algorithm

Unreasonable setting of position loop gain, speed loop gain and acceleration feedforward parameters of servo driver will lead to positioning overshoot, response lag or motion jitter. For example, if the gain is too high, it will easily lead to vibration, while if the gain is too low, the positioning will be slow and the deviation will be large.

The choice of trajectory interpolation algorithms (such as trapezoidal acceleration and deceleration, S-curve acceleration and deceleration) directly affects smoothness: trapezoidal acceleration and deceleration have impact in the start-stop stage, which easily leads to trajectory fluctuation; S-curve acceleration and deceleration is softer, but the parameter setting is complex and needs to match the load characteristics.

Coordinate calibration error

Incorrect calibration of tool coordinate system (TCP) is a common problem. If the fixture clamping center is not taken as the origin, there will be a deviation between the actual picking position and the teaching position. When the multi-pose calibration is insufficient, the TCP error will be amplified with the change of manipulator attitude.

The workpiece coordinate system is set incorrectly, or it is not linked with the mold opening stroke and ejector pin position of the injection molding machine, which will lead to the interference between the picking trajectory and the mold cavity or the deviation of the picking position.

Encoder and feedback accuracy

The encoder resolution (such as 17-bit and 23-bit) of servo motor determines the position feedback accuracy, and the low-resolution encoder cannot capture the tiny displacement. Loose encoder wiring and signal interference will lead to the loss of feedback data, which will lead to sudden change of trajectory. In addition, the semi-closed loop control manipulator can not compensate the transmission chain gap, and its accuracy is lower than that of the full closed loop control (with grating ruler/magnetic grating ruler feedback).

Third, the fixture factors (direct execution factors)

Weight and center of gravity deviation of fixture

The weight of the fixture exceeds the rated load of the manipulator, or the center of gravity deviates from the center of the end flange, which will increase the load inertia of each axis, lead to the slow response of the servo system and the decline of the trajectory tracking accuracy; Especially when moving at high speed, the centrifugal force caused by the shift of the center of gravity will aggravate the end jitter.

Adaptability of fixture and product

If the clamping force of the fixture is too large/too small, the product will be deformed or slipped, which will indirectly affect the stability of the picking trajectory (for example, if the product slips, it will trigger the manipulator to retry and the trajectory repetition deviation will increase).

The insufficient number of suction cups and uneven adsorption surface of vacuum fixture will cause the product to be skewed and the trajectory will be slightly offset during the transfer process; The cylinder of pneumatic gripper is worn and the piston rod is loose, which will also cause clamping position deviation.

Fixture installation accuracy

The parallelism and verticality errors between the fixture and the end flange will make the actual coordinates of the picking position inconsistent with the teaching coordinates. For example, the deflection of the fixture will cause the mold cavity to be scraped when the product is picked up, and the trajectory will be forced to be adjusted.

Four, the environment and working conditions (external interference factors)

Environmental condition influence

The change of workshop temperature will lead to thermal expansion and cold contraction of manipulator metal parts, and the size change of lead screw and guide rail will lead to trajectory deviation; High temperature environment (such as around the injection molding machine) will also accelerate the wear of parts and reduce the lubrication effect.

The vibration of the workshop floor and the impact of injection molding machine closing will be transmitted to the manipulator, which will lead to extra jitter during the movement and destroy the smoothness of the trajectory. Dust and oil pollution enter the guide rail and lead screw, which will increase the friction resistance and affect the movement accuracy.

Operating conditions and load changes

When the manipulator runs too fast and the acceleration is too large, it will produce a large inertial force, which exceeds the tracking ability of the servo system and leads to the track lag; Especially in the complex trajectory of multi-axis linkage, high speed is easy to cause trajectory distortion.

Unstable pick-up load (such as fluctuation of product weight and change of pick-up resistance caused by die sticking) will make the load inertia ratio of servo system mismatch and the trajectory accuracy decrease; For example, when the product sticks to the mold, the manipulator needs extra force, which leads to the deviation of the picking position.

Signal linkage and interference

The delay or interference of the linkage signal between the manipulator and the injection molding machine (such as mold opening in place and ejector pin returning) will lead to the wrong timing of taking parts and the disharmony between the trajectory and the mold action; In addition, workshop electromagnetic interference (such as frequency converter and welding machine) will affect the signal transmission of servo system and encoder, causing abnormal trajectory.