In the robot manufacturing process, after final assembly, its horizontal and vertical alignment must be corrected to determine the reference points for the program. What methods do various companies currently use, and which is more reliable?
In most industrial robot applications, the teach-and-playback programming method still dominates, which requires the robot to have good pose repeatability, but not high requirements for its absolute positioning accuracy;
As the application scope of robots increases, more and more applications require machines to have high absolute positioning accuracy in the operating space. For example, in vision systems, robots need to accurately reach the target point based on the position of the object determined by the vision system, which tests the robot's absolute positioning accuracy.
Calibrling the mechanical zero point is the first step in improving the robot's position accuracy and linear path accuracy. Its purpose is to ensure that the theoretical zero point in the control algorithm coincides with the actual mechanical zero point, allowing the mechanical linkage system to correctly respond to the control system's position commands. (The mechanical zero point is only one fundamental factor in improving robot positioning accuracy; other factors can be found in the Evaluation Methods (CPA) of Industrial Robot Control Algorithms.)
When the zero point is lost, the robot cannot correctly perform Cartesian space motion.
The zero point generally needs to be recalibrated in the following situations:
After replacing transmission components or mechanical parts such as motors/reducers;
Collision with the workpiece or environment;
Manually moving the robot joints without controller control;
The main methods for calibrating industrial robots currently include the following:
Inclinometer (Representative manufacturer: ABB)
When replacing drive and transmission components such as motors and reducers, ABB requires the robot to be recalibrated using the Calibration Pendulum and Levelmeter 2000 kit (Figure 1). This kit is primarily used by ABB's after-sales department and integrators. Ordinary customers typically only need to perform the Updating revolution counters operation on the teach pendant. The internal data required for this operation is generated by the tilt meter calibration. The tilt meter is essentially two tilt sensors (manufactured by WYLER, Switzerland, http://www.wylerag.com/en/products/inclination-measuring-sensors/digital-inclination-sensors/inclination-sensor-zerotronic/). ABB has created a bracket to integrate the two tilt sensors into a single instrument, enabling simultaneous measurement of tilt angles in two directions. A smooth mounting surface (except for small-load robots like the IRB120 and IRB1200) is typically found on the upper or lower arm of the ABB robot; this is used to mount the tilt meter. Those interested in the specific usage of the tilt meter can refer to ABB's "Operating manual – Calibration Pendulum".
Figure 1 ABB Zero Calibration Kit
Figure 2. Tilt meter used on the base.
Figure 3. Tilt meter used on the boom
Figure 4. Inclinometer used on the output flange
advantage:
High accuracy is required because the gravitational acceleration varies at different latitudes, so WYLER's tilt sensors require a zero-return operation during use.
It is easy to operate. ABB's control system has a built-in program (Service Routine) to work with it. Users only need to install the tilt meter in place and call the built-in program to automatically complete the zero calibration.
shortcoming:
Expensive. A single WYLER tilt sensor plus LevelMeter2000 costs around 60,000-80,000 RMB, while a complete ABB kit costs 200,000+ RMB (prices from a few years ago, for reference only).
It's cumbersome; zero-point calibration is required whenever the motor or reducer is replaced.
EMD (Representative Manufacturer: KUKA)
KUKA's zero-point calibration device is called EMD (Electronic Mastering Device, formerly known as EMT, an upgrade from KRC4), which is essentially a high-precision displacement sensor (Figure 5).
KUKA's mechanical body features a pair of large grooves and a round hole with a corresponding pointed groove on each axis. During calibration, the large grooves are used for coarse positioning. Then, the EMD is installed into the round hole, with the other end connected to the KUKA control cabinet. The controller then automatically controls the robot to move at a very slow speed to find the lowest point of the movement, i.e., the mechanical zero point. For detailed operation instructions, please refer to the video: https://www.youtube.com/watch?v=6TVe965SPPA.
Figure 5 KUKA EMD
Figure 6. Schematic diagram of KUKA EMD zero-point calibration
advantage:
It is simple to operate, reliable, and the zero-point information is stored on the joint. Even if the motor/reducer is replaced, it can still be calibrated using EMD.
The cost is relatively low, and ordinary users can also keep one of their own and perform calibration at any time.
If an EMD is not purchased, a dial indicator can be used instead, in which case the zero point needs to be determined manually.
shortcoming:
Zero-point information is stored on the mechanical parts, which requires extremely high precision in machining.
If a dial indicator is used instead of an EMD, the function of automatically finding the zero point cannot be achieved.
Positioning pins, keys, grooves (Japanese manufacturers, domestic manufacturers)
In mechanical assembly, locating pins and keys are often used to determine the relative positions of different components. These two methods can also be used to define the initial positions of different components in simple robot zero-point calibration. As shown in Figure 7, each of the two components that rotate relative to each other has a pin hole. When the joint is rotated until the axes of the two pin holes coincide, the pin can be inserted; this position is the zero-point position of the corresponding joint. For key positioning, refer to Figure 8. Each of the two components that rotate relative to each other has a keyway. When the joint is rotated until the corresponding surfaces of the two keyways coincide, the key can be inserted into both keyways simultaneously; this position is the zero-point position of the corresponding joint.
When using the groove method for zero-point calibration, simply aligning the two grooves is sufficient to determine that the zero-point position has been reached (Figure 9). This method is coarse positioning and is only suitable for situations where the absolute positioning accuracy of the robot is not critical.
Figure 7 Zero point calibration - positioning pin
Figure 8 Zero point calibration - key
Figure 9 Zero point calibration—groove
advantage:
Simple processing, low cost, and easy operation
shortcoming:
Poor accuracy
