Principle of weighing sensors

2024-04-16

1. Deactivate the speed sensor
For a non speed regulating belt conveyor, the variation in belt speed is approximately ± (0.3-0.5)%, which depends on the frequency, voltage, and load rate of the power supply. Among them, the frequency of the power supply has the greatest impact. But for users powered by the large power grid, due to the relatively stable frequency of the power grid, the rate of change in belt speed is usually less than ± 0.2%. We measured the belt speed multiple times on site in the morning, middle, and evening under different load rates, and the changes were all less than ± 0.2%.
According to the recommendation of the American company Thermo Ramsay, when the belt speed changes by more than ± 0.2% or the weighing accuracy is required to be higher than ± 0.5%, a speed sensor should be used. On the other hand, when the speed change of the belt is less than ± 0.2% or the weighing accuracy requirement is less than ± 0.5%, it is possible to consider not using a speed sensor. So, what is the way to replace the signal of the speed sensor in the entire electronic Belt scale system?
In almost all electronic Belt scales, there is a function in the display instrument, which is to select internal and external pulses. When selecting an external pulse, the speed sensor installed on the belt conveyor provides a pulse signal to the display instrument; When selecting internal pulses, the pulse signal is provided by the signal pulse source inside the display instrument.
We saw during a visit to a beneficiation plant that a critical measuring belt scale had been discontinued. After inquiring about the cause, we found out that it was caused by a damaged speed sensor, which was temporarily abandoned due to difficulties in purchasing spare parts. We ask the user to power up the display instrument and call up the menu. When we switch to "Tacho. Active?", we see that we can choose "yes" or "no" (where "yes" indicates that the speed sensor installed on the belt conveyor provides a pulse signal to the display instrument, and "no" indicates that the pulse signal is provided by the signal pulse source inside the display instrument). After changing the selection from "yes" to "no", we set the actual speed value in the next step of the menu, "Normal Speed". The small light point on the upper left side of the belt scale display instrument starts rotating again, indicating that there is a speed signal. The belt scale has started working again.
One or two days later, the on-site maintenance personnel had new troubles. When the belt conveyor stopped running, the display instrument continued to slowly skip due to residual materials on the belt or uneven tare weight of the belt, causing the material to not be transported and the accumulator continued to accumulate. We have two more ideas: one is to appropriately increase the small signal cutoff value (when the instantaneous material flow rate is lower than this value, the accumulator will no longer accumulate), and the other is to connect the electrical operating contacts of the belt conveyor to the power supply circuit of the display instrument. When the belt conveyor stops running, the power supply of the display instrument will also be disconnected. Of course, this is just a temporary measure, and in the long run, more reliable methods should still be adopted.
2. Replacement of components for speed sensors
A certain factory originally used a speed sensor assembled by OMRON rotary encoder, with a resolution of 1024P/r (P-pulse). Due to damage to the rotary encoder, it needs to be replaced. To match the original speed measuring roller, it is hoped to use the original model of rotary encoder. However, the original model of rotary encoder has already been phased out. The resolution of the existing product with the same external dimensions is not 1024P/r, but 1000P/r, and users have never dared to buy it. After understanding the situation, we informed the user that a 1000P/r rotary encoder can be used, and asked the user to make appropriate adjustments during range calibration. Although the resolution of the rotary encoder has decreased by 2.4% (equivalent to a decrease of 2.4% in the measured value of the speed signal), it can be compensated for by adjusting the range coefficient (such as increasing the range coefficient by 2.4%).