MSD: Case Studies D R. T A R E K A. T U T U N J I P H I L A D E L P H I A U N I V E R S I T Y, J O R D A N 2 0 1 4
Outline Elements and design of mechatronic systems have been described in previous sections This section presents the design process of two mechatronic systems Liquid Level Control CNC Machine
Liquid Level Control
Liquid Level Control Liquid level control systems are commonly used in many process control applications to control, for example, the level of liquid in a tank. Liquid enters the tank using a pump, and after some processing within the tank the liquid leaves from the bottom of the tank. The requirement in this system is to control the rate of liquid delivered by the pump so that the level of liquid within the tank is at the desired point
Specifications Tank size is 1m x 2m x 1m Desired level between 0.8 to 1.2 meters Settling time < 20 min Rise time < 2 min Flow Rate Plant Height Level
Tank Level Control System Plant output Fluid Height Plant input Flow rate [Ref.] Dogan Ibrahim
System Model v1(t) is the input flow rate v2(t) is the output flow rate h is the height A is the surface area R is the outlet pipe resistance Note that the plant has a first order model
Open Loop Simulation Assume the following values Qin = 10 liter/min (1.66e-4 m 3 / sec) A = 2 m 2 TF = 15 / (30 s + 1) Desired height is 1 meter
Amplitude Open Loop >> num=15; den=[30 1]; >> sys=tf(num,den); >> step(sys) 15 Step Response System: sys Settling Time (sec): 117 Note again that there is no overshoot because the model is first order 10 5 0 0 20 40 60 80 100 120 140 160 180 Time (sec)
Amplitude Closed Loop >> syscl=feedback(sys,1); Step Response >> step(syscl) 1 0.9 0.8 System: syscl Settling Time (sec): 7.34 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 2 4 6 8 10 12 Time (sec)
Sensors Choice Controller Ultrasonic Potentiometer with float Electrodes Resistance probes Photo sensors Comment Needs added circuitry Works well Works Discrete applications Discrete applications Also, might not work with water
Assume 20 PSI conditions 12volts provides 2 GPM 12-Volt Water Pump
Power Op-Amp
Controller Choice Controller Microcontroller PLC DSP FPGA PC with DAQ Electronics Only Comment Works well Too expensive Not usually used for SISO Too complex Not enough inputs to justify No need for fast response to justify Too complex No need for fast response Too expensive No need to visual graphics Works well, but difficult to modify
Schematic Diagram [Ref.] Dogan Ibrahim
Components Water pump 12V water pump drawing about 3A when operating at the full-scale voltage. Level sensor. A rotary potentiometer type level sensor with a floating. The level of the floating arm, and hence the resistance, changes as the liquid level inside the tank is changed. The resistance changes from 430 to 40 Ohm
Components Microcontroller A PIC16F877 type microcontroller is used in this project as the digital controller. In general, any other type of microcontroller with a built-in A/D converter can be used. D/A converter An 8-bit AD7302 type D/A converter is used in this project. Power amplifier An LM675 type power (30W of power) amplifier is used to increase the power output of the D/A converter and drive the pump.
System Model
System Block Diagram [Ref.] Dogan Ibrahim
Hardware Set-up [Ref.] Dogan Ibrahim
Computer Numerical Control
CNC Machine Computer Numerical Control (CNC) system Specifications: Speed Accuracy Working material Power and Torque Work area size CNC machine size User Interface Cost
CNC Block Diagram [Ref.] Saluki Engineering Company
Motors for positioning Motor for drilling CNC Machine
CNC Machine: Positioning Actuator Options [Ref] Goodfry
Stepper Advantages over Servo Motor Lower cost All of the components associated with stepper systems (i.e., motors, drives, etc.) are less costly Very accurate and dependable under normal circumstances Intrinsic to stepper motors is their ability to achieve high positional accuracy. No tuning required Other than operating just under maximum capabilities of your drive/motor, no tuning is necessary. Less mechanical reduction needed These motors operate best at lower speeds, lower reduction ratios are required. Simpler system to understand Straightforward and easy to implement. [Ref] CNC Machining Handbbook by Overby
Motor Torque Calculations Determine the motion profile and calculate acceleration, deceleration and maximum velocity required to make the desired move. Select mechanical drive mechanism to be used and calculated inertia, friction and load torque. Determine required motor torque for the specific application. Select proper motor and driver based on their speedtorque characteristics [Ref.] www.t2cnc.hu
Motor Torque Calculations [Ref.] www.t2cnc.hu
Selecting Stepper Motor Calculate the total torque needed at the output shaft of the motor. Use the motor performance curve to select a motor with at least 50% more torque than that calculated at the required maximum Speed. Select a driver that is capable of supplying the needed current to the motor of choice and providing the resolution needed.
NEMA34 Stepper Motor
Stepper Motor Torque-Speed Curves
Stepper Motors
Speed Profile
Cytron Tech SD02B Stepper Motor Driver
CNC Machine Specs Design The controller coordinates all the system actions. Its output is connected to an interface card that sends signals to the stepper motors. The stepper motors carry the worktable on which the work-piece is supported. Load on steppers is 30 N Torque of 1.75 N.m (250 oz-in) Two stepper motors are required to provide movement, one in the x-direction and one in the y-direction. The stepper motors are synchronous 1.8 motors giving a half step angle of 0.9 per revolution.
Conclusion Mechatronics Design follows well-defined iterative steps that include synergistic design. It is composed of three stages: 1. Define the Objective and Specifications Includes customer needs and engineering specs 2. Analyze and Design Understand I/O to select appropriate sensors and actuators Choose controller algorithm and hardware Model and Simulate 3. Build and Test Build prototype and measure performance according to specs