Below, a manufacturer of egg beaters explains how the measurement and control modes of an egg beater work.

During operation, the mixer’s beaters rotate at high speed, forcing the ingredients to come into full contact with one another and undergo vigorous friction. This achieves the functions of mixing, emulsification, aeration and the removal of some moisture. As the viscosity of the ingredients processed by an egg beater is lower than that of those processed by a dough mixer, the rotational speed of an egg beater is higher than that of a dough mixer, generally ranging from 70 to 270 r/min; it is therefore referred to as a high-speed mixer. Egg beaters are mostly manual, upright units, comprising a mixer, a container, a drive mechanism, a container lifting mechanism and a base. As for how they are measured and controlled—as you are no doubt aware, ‘measurement and control’ refers to measuring and regulating—what is measured is the concentration of the solid-liquid mixture within the container, i.e. its viscosity, whilst what is controlled is the rotational speed of the drive mechanism. The most common method for measuring viscosity is to use an accelerometer. I am not certain what type of sensor is used in this case, but if I were to design it, I would certainly use an accelerometer. Based on physics, we know Newton’s second law: F = ma. Furthermore, the motion of an object varies depending on the viscosity of the liquid it is moving through, and there is a relatively straightforward relationship between the two. The sensor on the rotating mechanism collects viscosity data and sends it to the controller. Through feedback regulation—typically in the form of a voltage signal—the control unit adjusts the rotational speed based on the voltage level. This utilises the concept of negative feedback from the principles of automatic control.
Put simply, when the egg beater detects a high concentration, it needs to accelerate; once it has been running for a while and the eggs are nearly beaten, the feedback signal indicates that the concentration has decreased, so the rotational speed is reduced.