ANALOG SENSORS AND TRANSDUCERS
Proper selection and integration of sensors and transducers are crucial in instrumenting a mechatronic system. Sensors may be used in a mechatronic system for a variety of purposes.
In particular, output signals are measured for feedback control; input signals are measured for feed f forward control; output signals are measured in system monitoring, tuning and supervisory control; and input-output signal pairs are measured for experimental modeling and evaluation of a plant.
Even though real sensors and transducers can behave quite differently in practice, when developing a mechatronic system we should use the ideal behavior as a reference for the design specifications.
Potentiometers, differential transformers, resolvers, synchros, strain gages, tachometers, piezoelectric devices, bellows, diaphragms, flow meters, thermocouples, thermistors, and resistance temperature detectors (RTDs) are examples of sensors used in mechatronic systems.
In a mechatronic control system, sensors are used to measure the system response, and it enables the controller to take corrective actions if the system does not operate properly. A mechatronic system may have unknown excitations and disturbances, which can make the associated tasks (performance monitoring, experimental modeling, control, etc.) particularly difficult.
Removing such excitations at the source level is desirable, through proper design or system isolation. But, in the context of control, if these disturbances can be measured, or if some information about them is available, then they can be compensated for within the controller itself.
This is in fact the approach of feed forward control. In summary, sensors may be used in a mechatronic system in several ways:
1. To measure the system outputs for feedback control.
2. To measure system inputs (desirable inputs, unknown inputs, and disturbances) for feed forward control.
3. To measure output signals for system monitoring, diagnosis, evaluation, parameter adjustment, and supervisory control.
4. To measure input and output signals for system testing and experimental modeling (i.e., for system
The variable that is being measured is termed the measurand. Examples are acceleration and velocity of a vehicle, torque into robotic joint, temperature and pressure of a process plant, and current through an electric circuit.
A measuring device passes through two stages while measuring a signal. First, the measurand is felt or sensed. Then, the measured signal is transduced (or converted) into the form of the device output. In fact the sensor, which “senses” the response automatically converts (i.e., transduce) this “measurement” into the sensor output—the response of the sensor element.
For example, a piezoelectric accelerometer senses acceleration and converts it into an electric charge; an electromagnetic tachometer senses velocity and converts it into a voltage; and a shaft encoder senses a rotation and converts it into a sequence of voltage pulses.
Hence, the terms sensor and transducer are used interchangeably to denote a sensor-transducer unit. Sensor and transducer stages are functional stages, and sometimes it is not easy or even feasible to separately identify physical elements associated with them. Furthermore, this separation is not very important in using existing devices. Proper separation of sensor and
Transducer stages (physically as well as functionally) can be crucial, however, when designing new measuring devices.
Typically, the measured signal is transduced (or converted) into a form that is particularly suitable for transmitting, recording, conditioning, processing, activating a controller, or driving an actuator. For this reason, output of a transducer is often an electrical signal.
The measurand is usually an analog signal, because it represents the output of a dynamic system. For example, the charge signal from a piezoelectric accelerometer has to be converted into a voltage signal of appropriate level using a charge amplifier.
For use in a digital controller it has to be digitized using an analog-to-digital converter (ADC). In digital transducers the transducer output is discrete. This facilitates the direct interface of a transducer with a digital processor.
A complex measuring device can have more than one sensing stage. Often, the measurand goes through several transducer stages before it is available for control and actuating purposes. Furthermore, filtering may be needed to remove measurement noise. Hence signal conditioning is usually needed between the sensor and the controller as well as the controller and the actuator.