Control of Mechatronic Systems. Patrick O. J. Kaltjob

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Figure 1.5 Generic control systems and instrumentation block diagram.

      Source: Based on Kaltjob P.

      1.3.1 The Data Processing and Computing Unit

      The data processing and computing unit is used: (i) to control and regulate machine operations; (ii) to monitor machines and processes operations; and/or (iii) to coordinate operations within the same process. Data processing and computing could be performed either:

Commonly encountered data processing and computing devices are: digital signal processing devices, programmable logic controllers, microcontrollers, field programmable gate arrays among others, and a distributed control system (DCS) (consisting of a historian server connected to a network of field controller devices). Those devices execute program routines for: (i) the acquisition of process variables; (ii) process condition monitoring and exception handling (i.e. executing process safety operations); (iii) the control of machine operations (e.g. activation/deactivation of motor, tracking of motor speed); and (iv) the archiving and sharing of process data with other control devices through the communication network.

      1.3.2 Data Acquisition and Transmission Units

      Data acquisition and transmission units are used: (i) to interface with various control devices (e.g. operator panel, detecting and measuring field devices); (ii) to transport process data between network nodes; (iii) to integrate process data from different sources on a single platform; and (iv) to integrate control functions (e.g. machine control and process control). These units operate through data transfer platforms and their data distribution service protocols. They can be designed based on the Open Systems Interconnection model, which is summarized as:

      1.3.3 Electrically-driven Actuating Units

      Electrically-driven actuating units convert voltage or current signals from the computing unit into appropriate input forms (mechanical, electrical, thermal, fluidic etc.) for the execution of machine's and process operations. Then those converted signals produce variations in the machine's physical variables (e.g. torque, heat, or flow), or amplify the energy level of the signal, causing changes in the process operation dynamics. Some examples of actuating elements are relays, magnets, and servo motors.

      1.3.4 Measuring and Detecting Units

      Measuring and detecting units consist of low-power devices, such as sensors and switch-based detectors interfacing with electrically-driven machines involved in process operations. As such, they convert related physical output signals from the actuating unit into voltage or binary signals ready to be used within the data processing and computing unit. Some key functions of these devices are: (i) data acquisition related to the change of machine variables; and (ii) conversion of the machine-gathered signal into electrical or optical signals. Depending on the nature of the process signal generated, a signal conditioner can be added.

      1.3.5 Signal Conditioning Units

Signal conditioning elements convert the nature of the signal generated by the sensing device into another suitable signal form (usually electrical). The signal conditioning units can also be embedded within the sensing devices. An example of such a unit is a Resistance Temperature Detector (RTD). Here, a change in the temperature of its environment is converted into a voltage signal reflecting its resistance change through a Wheatstone bridge and the bridge is a signal conditioning module.

1.4 Functions and Examples of Controlled Mechatronic Systems and Processes

      Mechatronic systems and processes have built-in intelligence through either their advanced information processing systems such as multifunctional control systems or intelligent electromechanical systems (including thermal, fluid, and mechanical processes) such as power-efficient multi-axis actuation with motion precision and detection features or miniaturized smart devices with embedded information processing capabilities. The resulting controlled mechatronic systems and processes aim to achieve various objectives: synchronize, control and sequence process operations, or detect and monitor process status.

Table 1.1 presents some typical process control objectives and their corresponding control functions along with some illustrative examples.

Table 1.1 Functions and implementation strategies for controlling mechatronic systems and processes.