Force Sensor
This is a sensor that provides an electrical signal which is proportional to the amount of force being applied to the specimen that is being tested. Typical force sensors are load cells and load washers.
Internally, a force sensor consists mainly of a high-strength metal column (or columns) with strain gages bonded to it. The gages are electrically connected to form a balanced Wheatstone bridge. An applied force changes the column's dimensions: this unbalances the bridge and produces an output voltage. By convention, the voltage is positive when the applied force is tensile and negative when the force is compressive.
Linear Variable Differential Transformers
These are commonly called LVDTs. They provide an output voltage that is proportional to the displacement of a movable core. An LVDT consists of a transformer with one primary and two secondary coils wound on a common cylindrical form. This assembly, along with a core, is usually mounted inside the hydraulic actuator so that the core moves with the piston rod. (There are also external versions.)