Diagnosing drives

It is a well known fact that mechanical drive systems can be effectively replaced by electronic drives. The problem is, once your new electronic drive system has been built, how do you validate that it is working correctly, and if it is not, how do you re

At Quin Systems, the answer comes in the form of a software package called PTS `Scope. Recently, the software was put through its paces in the development of an electronic system that was designed to replace a mechanical indexing system to provide a means of accurately filling sachets.

As this was a retrofit, the PTS `Scope proved invaluable in tuning the machine performance as the machine ran, and isolating potential failings. Indeed, without these diagnostic tools, the required result would not have been possible.

The mechanical machine indexing system (Figure 1a) had proved unreliable. Quin Systems were commissioned to replace the original system with a servo driven system. In the original machine, a main drive shaft provided position and speed information, while a cam and linkage provided indexing of the sachets.

Unfortunately, the cam design was progressive, each index was slightly too big and after 20 or so movements, the photocell would detect the misalignment and brake the system, using the clutch and motion compensator to absorb the next index. Worse, the original design provided no means to change the packet size or tolerances on the fly, without considerable engineering work.

The replacement of the mechanical system with an electronic system was reasonably simple. In the redesigned system, an encoder was used to measure the main machine speed and position, while a servo motor provided a belt drive directly to the nip rollers. The original photocell was retained (Figure 1b).

A Quin systems Q-Drive MAP was installed and connected directly to the encoder, photocell and motor. Additional equipment included a power supply unit for the encoder and photocell, mains isolators for the Q-Drive and an operator interface panel.

Non-linear profiling

The non-linear cam profiling was performed by the Quin Q-Drive MAP. The servo motor was controlled to follow the master encoding using user selected sachet sizing. In PTS terminology, this is called `mapping’. Not only can this provide flexibility, since the size of the map can be recalculated within the PTs, it can also be fast, since the non-linear relationship between the master encoder and the slave motor is performed as part of the motor control calculations.

Once the Quin Q-Drive MAP had been installed, the machine was immediately run. The PTS `Scope was used to monitor the initial results, as shown in Figure 3a. Immediately, a number of results were determined. First, it was clear that the machine worked and that all installation work had been successful. Second, the motor control system needed tuning to the mechanics – previous tuning had been performed with the motor not attached to the mechanics. Finally, it could be determined that the photocell was positioned in such a manner that it detected the index mark while the index move was at peak speed.

Performing motor tuning using PTS tuning `Scopes resulted in the machine performance shown in Figure 3b. Again, this was monitored as the machine ran. Here, the white trace has been changed to show reference error, which is the position difference between the detected index position and the desired index position. At times, the reference error was very large, meaning that a large position correction was applied by the PTS system, as part of the non-linear mapping between the machine lineshaft and the servo motor.

This was unexpected. The mechanical system had seemed to be performing but the PTS `Scope highlighted a serious problem, resulting in some sachets being positioned outside the machine tolerance.

What was wrong? Was the product slipping, or the index mark poorly detected? Investigation revealed that the photocell (retained from the original machine) was, in fact, of a low specification with a slow response time. Coupled with the mechanical positioning (at maximum index speed) this led to the multiplication of any error and the resultant large reference correction applied by the PTS.

An alternative to replacing the photocell was simply to move it, a solution that achieved immediate results. By changing its position, the index mark was detected later in the index. A simple parameter change informed the PTS system of this mechanical change. As a result, the response time of the photocell was less critical as the index was not as fast as the point of detection. The performance benefit is shown in Figure 3c, with the timebase expanded to observe one index closely. The photocell was again adjusted to tweak the performance further.

Monitoring the performance of the machine showed that this adjustment of the photocell had achieved the required product tolerance (Figure 3d). The machine was working and installation was complete.

Quin Tel: 01734 771077