In traditional machinery, shafts are connected by a mechanism for transmission. For example, as shown in the diagram, the master and slave shafts are connected by a flat belt. When the master shaft starts rotating, the slave shaft rotates as well. Assuming the master and slave shafts have the same pulley diameter and a ∇ mark is made on each pulley, initially positioned directly above the center, after a period of operation, due to belt slippage, pulley diameter errors, and other factors, the ∇ mark on the master and slave shafts will be in different positions! This indicates a phase shift between the master and slave shafts.
▲ Figure 1 shows a phase shift in a flat belt drive.
If it's simply used for power transmission (like a generator belt in an engine), phase shift is fine; however, if it's used for synchronization control (like a belt controlling valves, crankshaft, and ignition timing in an engine), problems will occur! Mechanistically, to avoid phase shift, a regular belt can be replaced with a timing belt and gears! As shown in the diagram, even after long-term operation, the phases of the master and slave shafts remain consistent, meaning they are synchronized!
▲ Figure 2 shows that the timing belt can maintain phase synchronization.
Having said all that, we can finally get to the main point! What would happen if we replaced the aforementioned mechanical transmission with a servo-driven electronic cam? As shown in Figure 3, we remove the belt, use an encoder to acquire the position of the spindle, and transmit it to the servo in the form of A/B phase pulses. The servo then uses a linear electronic cam to drive the slave shaft to follow the direction of the transmission.
▲ Figure 3 shows the replacement of belt drive with servo electronic cam - resulting in offset.
The actual test results showed that the phase shift occurred because the encoder pulse was interfered with, and the shift became more and more obvious over time! Pulse interference is difficult to completely suppress. In the variable environment of the factory, no matter how well the wiring is done, pulse deviation will always occur, it's just a matter of time! Therefore, simply using the encoder pulse to drive the electronic cam cannot achieve the synchronization effect of the timing belt, at best it can only achieve the effect of a flat belt!
So how can we improve this?
We can actually mimic a timing belt because it has teeth, so it won't slip and accumulate errors! We can use an imaginary tooth to imitate it, and clearly define the width of the tooth (the distance between the teeth). This allows us to create a virtual timing belt, the so-called "synchronous shaft"! This "tooth" can be represented by any periodically occurring signal on the spindle (or the encoder's Z signal), as shown in the diagram below:
▲ Figure 4 uses a synchronous shaft to avoid phase shift
A marker is installed on the spindle as a "tooth," and a sensor reads the signal into the servo's DI. Based on the encoder model, the number of pulses (R) should be present per spindle revolution. Since there's only one tooth per revolution, the tooth width is R (in spindle pulses). Thus, whenever the servo senses a "tooth," it knows it should receive R pulses. If the number is incorrect, compensation is made to ensure the total pulse count maintains the correct relationship with the number of teeth, ensuring the master/slave axis phases never shift and remain synchronized! This function is already present in Delta's ASD-A2 and ASD-MR servos. In the cam's spindle source P5-88.Y, selecting a physical pulse is equivalent to using a flat belt; selecting a synchronous axis is equivalent to using a timing belt—very convenient! For setting instructions, please refer to the A2 cam synchronous axis setting method!
Note: There are other methods to overcome cam spindle pulse leakage, such as:
1. Using a virtual spindle will not leak pulses, but the spindle must also use a servo motor, and cannot use a general motor with an encoder!
2. Although cam alignment can be achieved, it is usually reserved for correction of the slave shaft, because it is used for error compensation of the slave shaft!
▲ Tracking and editing function
▲ Flying shear function
