What is weigh feeder and why is it used in PID loop

Explain the weigh feeder and what is the purpose of weigh feeder in PID loop

Weigh feeder is feeding equipment with the simultaneous weighing of that material. The product of speed and weight ie tph as the rate is compared with set reference in weigh feeder. The error is compensated with speed and to some extent weight variables. This way weigh feeder operates in a PID loop.

Integrating scale. Weight loss measurement as a function of engine speed and weight on the belt.