In the past, devices with controlled strain or strain rate (CR rheometers) were distinguished from rheometers with controlled stress (CS rheometers) depending on the measuring principle.
In a controlled strain (CR) rheometer, the sample is subjected to displacement or speed (strain or strain rate) using a DC motor, and the resulting torque (stress) is measured separately using an additional force-torque sensor (torque compensation transducer). The electric current used to generate the displacement or speed of the motor is not used as a measure of the torque acting in the sample. This mode of operation is also referred to as separate motor transducer mode (SMT).
* Deflection angle/strain and shear rate are set by the motor based on the position control of the  optical encoder in the lower part. 
* Sample reaction  (the stress acting within the sample) is measured by an additional force-torque transducer (torque re-balance transducer)
* The separation of drive and torque measurement has advantages in strain-controlled tests, since the motor's moment of inertia has no influence on the measured torque.
* Limitations of the SMT mode can be found in  stress-controlled measurements (e.g. creep tests)
In a controlled-stress (CS) rheometer, the torque acting in the sample is determined directly  from the electrical torque generated in the motor. With such a design, no separate torque sensor is required. Usually, this mode of operation is described as combined motor-transducer mode (CMT).
* The stress acting in the sample is determined directly from the torque generated in the motor, which is required  to deform the sample.
* Deflection angle/strain and shear rate are determined by the use of an optical encoder.
* Single-motor rheometers allow  characterization of samples in either strain/shear rate or shear stress-controlled tests
* Since only one actor (motor) is required, the single-motor rheometer can be easily combined with additional application-specific accessories that enable the study of material properties in a variety  of different applications.
* Limitations may occur from less precise data evaluation in the transient regime of start-up shear tests.
Nowadays, there are device concepts that allow both working modes, the combined motor transducer mode and the separate motor transducer mode, by using two motors in one device. The use of only one motor enables measurements to be made in the combined motor transducer mode. Using both motors allows working in the separate motor transducer mode, where one motor is used to deform the sample while the other motor is used to record the torque acting in the sample. Furthermore, this concept allows for additional modes of operation, such as counter-rotating mode, where both motors can rotate or oscillate in opposite directions. This mode of operation is used, for example, to increase the maximum achievable shear rate range or for advanced rheooptical characterization of samples.