Real-time systems. Sensors and Actuators 2 1

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1 Real-time systems Systems which monitor and control their environment. Use sensors and actuators - Sensors: Collect data from the system environment - Actuators: Change the system's environment Time is critical. Real-time systems must respond within a specified time. Sensors and Actuators 2 1

2 A real-time system model Sensors and Actuators 2 2

3 Sensor/Actuator Processes Sensors and Actuators 2 3

4 Sensor controller System elements Collects information from sensors. Can buffer information collected. Data processor Processes the collected information and computes the system response. Actuator controller Generates control signals for the actuators. Sensors and Actuators 2 4

5 Actuators Examples of Actuators Motor (impose a torque) Pumps (impose pressure or fluid velocity) Electro-hydraulic systems in aircraft Actuators may be powerful, large, and complicated Normally used to produce force or motion convert electrical signal to motion Sensors and Actuators 2 5

6 Types of Actuators Some of the most common actuators are: Electric motors: DC motors are the most common actuators in robots, used for location (by powering wheels or legs) and for manipulation (by actuating robot arms). Pneumatic actuators: Used in industry for manipulation by pumping compressed air through chambers Hydraulic actuators: Used in industry for manipulation by pumping pressurized oil Sensors and Actuators 2 6

7 Electric Motors Electric motors are the most common source of torque for mobility and for manipulation in robotics The physical principle of all electric motors is that when an electric current is passed through a conductor (usually a coil of wire) placed within a magnetic field, a force is exerted on the wire causing it to move Sensors and Actuators 2 7

8 Stepper Motors When incremental motion is required in a robot, it is possible to use stepper motors A stepper motor possesses the ability to move a specified distance in order to achieve a fixed and consistent movement Example in integrated circuit fabrication Sensors and Actuators 2 8

9 Stepper Motor Stepper motors advantages: Their control is directly compatible with digital technology They can be operated open loop by counting steps, with an accuracy of ±1 step. Sensors and Actuators 2 9

10 Actuator Summary Actuators provide force or motion and interact physically with the environment in which they operate The main issues with regard to actuators include: Required force/power/torque Speed of motion Sensors and Actuators 2 10

11 Actuator Control Control systems can be classified by control method into servo and non-servo systems Non-servo systems are open-loop systems where the actuator movements are limited to predetermined mechanical stops Servo systems use closed-loop computer control to determine the actuator movements Sensors and Actuators 2 11

12 Open-Loop Control An open-loop controller (non-feedback controller) is a type of controller which computes its input into a system using only the current state and its model of the system The system does not observe the output of the processes that it is controlling it is ignorant of its own output Sensors and Actuators 2 12

13 Open-Loop Control Input Controller Motor Output Sensors and Actuators 2 13

14 Open-Loop Control Open-loop control is useful for well-defined systems where the relationship between input and the resultant state can be modeled by a mathematical formula For example, determining the voltage to be fed to an electric motor that drives a constant load, in order to achieve a desired speed would be a good application of open-loop control Sensors and Actuators 2 14

15 Open-Loop Control An open-loop controller is often used in simple processes because of its simplicity and low-cost, especially in systems where feedback is not critical Generally, to obtain a more accurate or more adaptive control, it is necessary to feed the output of the system back to the inputs of the controller Sensors and Actuators 2 15

16 Closed-Loop Control A closed-loop controller uses feedback to control states or outputs of a dynamic system Process inputs have an effect on the process outputs, which is measured by sensors and processed by the controller. The result is used as input to the process, closing the loop Sensors and Actuators 2 16

17 Closed-Loop Control Input Output Measurement Controller Motor Output Feedback Sensors and Actuators 2 17

18 Closed-Loop Control Closed-loop systems have the following advantages over open-loop systems: Greater accuracy over a wider range of conditions Reduced sensitivity to parameter variations Can respond more quickly to changes at the system output Sensors and Actuators 2 18

19 Review Definition Examples Sensor Properties Sensor/Controller Interface Application Circuits Actuators Open-Loop and Closed-Loop Control Sensors and Actuators 2 19