The signed system will have a meaner of overturn protection as a form of motor protection and will also use the rotary method of motor speed sensing. Also motor speed displayed 7 segment displays. Within project there will be algorithms that are to be implemented and the interfacing circuitry that is to be used when interfacing with the microprocessor. Table of Contents Abstract I Introduction 6 Background theory 7 DC motor 7 Control schemes 7 Current sensing 9 Motor protection 10 Implementation of control algorithm Motor speed measurement 12 Design Brief 13 Production Design Specification 15
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Performance 16 Environment 16 11 Safety and standards 17 Testing 17 Conceptual Design 17 Control design 19 Microprocessor design 20 Controller design 21 Mathematically modeling the system using the step response 21 Determination of suitable performance specs 23 Determination of key circuit parameters 26 Finding the Armature Resistance 26 Finding the Armature Inductance 26 Finding the motor torque constant and the back NEFF constant 27 Finding the Time Constant for the system in the electrical domain Finding the Inertia 28 Justification of choice of the controller 29 Implementation of a Pl controller 29
Other Design Considerations 34 Implementation of Motor Speed input routines 34 Implementation of display output routines 35 Implementation of Speed output routines Improvement of resolution of speed sensors 37 Method for current sensing and motor protection 37 Justification of choice oscillator 38 Justification of component values chosen for driver circuit 38 Lab Protocol 39 Dress code 39 Conduct and behavior 39 Industrial standards 40 Safety standards 40 Component standards 41 Codes of Practice 42 Risk Assessment 44 Presentation of results 48 Key system variables and parameters 48
Response of the system to Pl control 49 Accuracy of speed measured 50 Test to Justify motor protection 50 Response to different inputs 50 Discussion 51 Interpretation of results 51 Justification of load and system response 51 Demonstration of the achievement 52 Shortcomings and errors 53 Conclusion 53 28 List of Tables Table 1 showing results of the lock rotor testes Table 2 showing the results form testing the motor 26 Table 3showing the risk definition 43 Table 4 showing Risk assessment matrix 44 Table 5 showing the findings of the risk assessment 46 Table 6 showing the results obtained 47
Page 2 Pi Control of a Dc Motor Essay
Table 7 showing the achieved parameters 48 Table 8 showing the specifications obtained 51 List of Figures Figure Showing a blockdiagram of the system 17 Figure 2 showing a block diagram of controls section 18 Figure 3 showing the block diagram of the micro-p system 20 Figure 4 the output response of the system 21 Figure 5 showing the flowchart of the Pl algorithm 29 Figure 6 showing the setup of the interrupter switch 34 Figure 7 showing the setup of the segments 35 Figure 8 showing the configuration of the driver circuit 36 Figure 9 showing the step response of the system 48
Figure 10 showing ramp input 49 Figure 11 showing the output of the system 50 List of Abbreviations PICK- Peripheral Interface Controller DC- Direct Current RPM- Revolutions per Minute PAID- Proportional Integral Derivative Pl- Proportional Integral Kip- Proportional Gain Ski – Integral Gain LED- Light Emitting Diode NEFF- Electromotive Force QUART- Universal Synchronous Asynchronous Receiver Transmitter Introduction This project is based on the design and implementation of a Pl control strategy on a DC motor. This DC motor will have a form of motor protection in the form of current limiting.
This current limit was chosen for an application of a cooling fan that will take into consideration the torque that will be required to spin the fan blade. This control scheme will be implemented on the FISTICUFF but the principles of the operation of the system can be transfer to any other microprocessor that has the display that comprises of four 7 segment displays that will display the speed at which the motor is rotating in terms of RPM. This speed is measured by meaner of a photojournalist optical interrupter switch (H2O AY) and an interrupter disc that has a resolution of eight holes.
Background theory DC motor A DC motor is made of two basic components that are called armature and the field coils. The armature is the moving part of the motor and is also known as the rotor. The field coils are the stationary component of the motor and are known as the stator. The stator will induce a magnetic north and south pole that will repel the north and south pole in the armature. As the armature moves the opposing pole will then begin to attract the armature pole. This attraction will continue until the armature reaches the locked position.
The ends of the armature are connected to a device known as a communicator. This is a metal ring that has two splits at either end. These are to switch the polarity of the armature when it reaches locked position. This is aided by carbon brushes that are used to connect the armature to its supply. When this polarity shift occurs the motor will continue to spin it that direction since the pole that was now a south will become a north and the north will become a south and cause the communicator to repel the poles that are at each end.
Control schemes There are mainly two control schemes that are normally implemented for DC motor which are open loop and closed loop. Open loop control is when signals are being sent to the driver circuitry without the system knowing if the operation is being performed as instructed. This meaner that the system will need to have some form of external controller which is normally human to help it obtain the correct results. The closed loop method of control is when the controller is fed information about the state of the system after it carries out the instruction.
This can either be the number of revolutions that the shaft makes in a given time frame or the angle displacement that the shaft made with a given input. This type of control is known as “feedback” since it utilizes information that is being sent back to the controller from the system output. There are many ways that this feedback can be used to correct the output of the controller so that the system will give the desired response. One of the most popular ways is the implementation the PAID theory.
This will use implement the use of three elements which are the P element which is proportional to the current error in the system, the I element which takes into consideration all of the past errors that have occurred in the system and the D element which takes into consideration a reduction of the future error that will occur in the system. These three coefficients are thus referred to as proportional, integral and derivative gains and need to be varied in order for the system to achieve optimum response. The PAID theory is based on the principles of disturbance rejection.
This is the process where the response of a given control system to disturbance that has occurred. The proportional gain (Kip) determines the speed at which the system will produce the output response to a given disturbance. As this value increases the rises time of the system will decrease UT this will begin to compromise the other parameters of the system like the to increase and as such oscillations will become larger which will decrease the stability of the system. The integral component gives a summation of the error over a given time period.
The integral response of the system will increase over time and in so doing will drive the steady state error to zero. The steady state error is the final difference between the system variable and the standpoint. When the integral action saturates the controller before the system has a chance to reduce the Steady state error to zero, it is called wind-up. The derivative component acts as a buffer to prevent the process variable from increasing too quickly. This meaner that the derivative response of the system will be proportional to the rate of change of the process flow.
This term will cause the system to react more strongly to changes in the error that is being produced. In practice, it is better to use a very small derivative component as it increases it; the sensitivity of the system to noise will increase and thus reduce the overall system response and may cause the system to become unstable. In this project, we choose to use closed loop method of control since it allowed for the controller to accurately controller the DC motor by the use of feedback. We shall only implement the use of two of the components in the PAID theory. These are the P and I elements.
This is because of the fact that even thou the D element is useful in some applications it can easily influenced by noise and the desired system response could be obtained by use of PI control strategy. Current sensing In the project description, one of the requirements is to limit the current that is being supplied to the DC motor. In order to accomplish this we must first identify a method o accurately measure the current that the motor is using at any given point in time. This can be done by measuring either measuring the voltage drop though a known value of resistance of by measuring the strength off known inductor.
If the use off resistor is to be implemented then there are two positions that current can be sensed. This either on the supply side of the driver which is known as high side current sensing or the ground side of the driver known as low side current sensing. The disadvantage to low side current sensing is thou it is simpler to be implemented he driver circuit will no longer be grounded so therefore will lead to safety issues. If high side current sensing is to be implemented a differential amplifier would have to be used. This is more complicated to set up that the normal amplifier that is required for low side current sensing.
Another method that can be done is the use off Hall Effect Sensor. This is a type of sensor that measures the magnetic field that is surrounding the wire that the motor current is flowing through. This sensor can be located either on the high or low side of the driver since its position does not matter. This is because it does not need any voltage drop to determine the current that is flowing through the wire at any given moment. For this project, low side current sensing was implemented due to the simplicity in nature and the fact that it requires the less complex circuitry than the high side current sensing.
Motor protection The DC motor like any electrical device must be protected from being damaged both electrically and mechanically. Some of the conditions that can result in motor windings being damaged are excessive moisture, mechanical damage, high electrical stress and high temperatures. In order to prevent the motor from being damaged we protected. One method of protection is the use of thermal overload relays of which there are two types. Inherent thermal overloads where the switch is built into the system and will trip at some given standpoint.
When the circuit trips it will either alarm the controller or De-energies the motor. The second type is type is external thermal that use of heaters. These will break the circuit by either melting away when the heat reaches a certain value like the solder pot heaters or can be like the inherent thermal overloads with a bi-metallic strip. Another method of protection is over current protection that is designed to protect the circuit from ground faults that are created by unintentional meaner such as dust, water and worn insulation.
To protect from this we can use fuses and circuit breakers that will cause the circuit to break under cases where the current value reaches to high. Thus arose the need to protect the motor from the effects high currents thereby the need for a method whereby the current that flows through the motor could have been monitored. Hence this was the reason that it was included in the project description. The implementation of the control algorithms can be done by many different meaner. These different types will allow for different levels of control and can have different cost to be implemented.
The first method is the use of analog components that will be able to perform the functions of the different types of control components that are needed. The main component that will be used in this type of system is an operational amplifier. Using this type of electronic device you are able to create integrators and differentiators. These can be used by the designer to implement the PAID algorithm. The system can also be done by using logic gates. This is done by using FAGAN technology where a digital circuit can be created in order to represent the system.
This digital system will then be responsible for the manipulation of the data in order to produce the desired effect. This system there might be a level of complexity to the manner in which the digital implementation must take place. This will make this method very difficult to implement. The final manner in which controls system can be implemented is via the use of a microelectronic. This option will provide the designer with more flexibility in the implementation process. Also with he increase in the clock speed of the modern microprocessors with make it easier and even cheaper to replace the older analog system.
Also because of the variety of ways in which the system may be implemented on the microprocessor this will make the implementation process even simpler. In this project, the microprocessor was chosen as the meaner for implementation of the control algorithm since it was simpler and easier to use that the other methods. Motor speed measurement In order to provide feedback to the controller the output of the motor must be measured. There are two things that can be measured in order to be fed back into he system.
These are the NEFF produce by the motor due to the motion of the shaft and the actual speed at which the shaft is rotating. For this application we chose to measure the speed at which the motor is moving. This can be done by various methods. The first method is by using the rotary encoder sensing. This method uses a uses a shaft encoder which in most applications are encoder disks. This is used along with sender and photo transistor as the receiver. The shaft encoder is connected to the output shaft of the motor at the back and then placed in between the sender and receiver of the photo-couple.
In this configuration, as the encoder disk rotates along with the shaft it will create a resulting sequence of light pulses that will be converted by the photo transistor into pulses of current flow. This series of pulses can then be fed to the controller that will be able to determine the speed of the motor. This is because the frequency the output pulses that are being produce is directly proportional to the speed at which the shaft is rotating that is the RPM of the motor. Also the number of the pulses will correspond to the angular displacement that the shaft is experiencing.
The resolution at which the encoder can detect the speed corresponds to the number of holes that are present on the decoder disk. As such the greater the number of holes the more precise the readings will be. The overall performance of the shaft encoder is very highly affected by the position at which the encoder is being placed. To ensure that the value of the speed is accurate the encoder must be connected to the back shaft of the motor. Another method of finding the speed of the motor is by using an analog tachometer. This is an electrical generator that has a linear output response to a pacific RPM range.
By simply knowing what are the output characteristics of the tachometer the RPM of the shaft can be found by measuring the voltage across the tachometer terminals. For this project, the speed of the DC motor that had to be controlled was measured using the rotary encoder sensing. This was found to be the more accurate method of the two methods of speed sensing because of the resolution that can be obtained when using the encoder disk. Design Brief In this project we were asked to design and implement a system that uses Pl control strategy to control the speed of a DC motor.
Some of these were specified in the design brief of the reject such as the type of control strategy that was to be implemented, but in order to set the other type of specifications of the project a suitable application must be determined. This application will demonstrate how this type of control scheme can be implemented in real life. The application that I chose was the design and implementation of an automated cooling system. This system will make use of a fan that would be providing cool air to the system.
After a change is made to the desired standpoint the cooling system will perform Pl control to ensure it reaches the correct speed. In this project we will consider a three speed fan. For this application we would now have to determine the specifications that would have to be implemented in this project. These different specifications of the system are categorized in the following subheadings and was chosen when a proper about of research was done into the application. Performance * The system must have a settling time of 0. As or less * The overshoot of the system must not be more than 10% * The operating speed of the motor should be operate at 1800, 4000 and 6000 RPM * The motor must be operate at a current that does not exceed 0. AAA Environment * The system must be able to function in a normal working environment with no special conditions needed. * System must operate in a temperature range of less than ICC to prevent the system from unnecessary damage. Product life cycle * The system must require only small amounts of maintenance like changing of brushes hence it will be a feasible system to be built. The system must therefore have a life expectancy of the main component which is the motor. This was approximated to be around 5-10 years given the brand and that the durability of the motor that is being used. Safety and standards * The system must be able to conform to the ENEMA MGM -Motors and Generators Revision 1 2010. This meant that the operation of the motor will be too particular standard. * When building the system it had to be able to comply with the OSHA Act. 2004. This is to improve the safety of the system.
Testing * The system must not only be designed but implemented and well tested so that it meets all the specifications of the project. * The appropriate test should be carried out to see if the system can be used from the application that was chosen above. Conceptual Design In order to meet all the specifications of the project that were identified in the section before, we would have to employ the use of suitable peripherals either internal or the external to the microprocessor. There is also the need for necessary calculations to be done in order for the system to perform as required.
This refers to data conversions and the implementation of the algorithm that is to be used as a meaner of control. Hence in the design the FISTICUFF will be used as the controller. This microprocessor was equipped with enough of the internal peripherals so that the implementation of this project was made simpler than having to use many external peripherals. Some of the peripherals that can be used are timers/counters of varying sizes (bit and 16 bit), a 10 bit multi-channel Analog to digital converter and the PAM modules.
The motor that was being used in the project is a DC V motor that has a load. It meant that a driver circuit would be needed to control the motor speed using the PAM signal from the Pl. This is supply the operating current and voltage for the motor. The following diagram shows a block diagram of the main system components. In the making of this diagram all of the system requirements were taken into account. Hence, in this diagram we can clearly see the areas that are implemented in the PICK and the peripheral devices that were used.
In this general plan, it can be seen that the PICK is the major component that is Just interacts with different peripherals which include motor, speed sensor and displays. Some types of these peripherals cannot interact directly with the PICK and thus transistors were used to bridge the gap created. These were not shown in the block diagram but be detailed further down in the report. PICK PICK Figure Showing a block diagram of the system Control design One of the most important aspects of this project was that it must employ the use of the Pl control strategy.
This is method involves the negative feedback of the output to produce the error that the system is producing. This error was found by using a fixed standpoint that needed to be set by the user and a sample of the current speed at which the motor is spinning. By simply subtracting the two values you will get the error speed of the motor. This error then passes through both Proportion Gain and the Integral Gain only since for a Pl strategy Derivative Gain is zero. The summation of these two new error values will produce the required output speed in order for the correction of the error that was being experienced.
The values of Ski and Kip had to be first found using the system model and then manual tuning was performed so the desired performance requirements that were required were met. The output speed had to therefore be converted to a PAM and sent the control signal to motor. Figure 2 showing a block diagram of controls section Microprocessor design The processor that is responsible for the data manipulation and control of the peripherals is housed within the FISTICUFF. The design of how the system will be used is shown below.
In this system, timers will be used as counters to keep track of the pulse count and also control the rate at which the display is updated. Also that is controlled by timers is the rate at which the samples are being taken and sent to be processed by the control algorithm. This therefore will therefore control how effective the control scheme is being implemented by the controller. The control therefore as a compensator to the motor and does the correction to the input value. The control mechanism will utilize PAM to vary the speed of the motor. The method chosen to input the set-point used the A/D converter within the PICK EFFIE.
This was done to limit the amount of external components that would be needed to implement the design. The disadvantage to this method though is that the processor would know be required to perform extra calculations that would need to be done if an extra components like the MAXIMA that would be needed for the serial communication. Controller design Mathematically modeling the system using the step response The first step in determining how the system is going to perform is to get a transfer function for the system. This transfer function can be modeled to represent the output response to a given input.
Hence if we accurately plot the output of the system we would be able to determine the key system parameters that can then be used to derive the transfer function. In order to implement this principle a section of the design project was built. We would have to get the output of the system which in this case is the motor hence we used to the interrupter switch and the encode disc to get the speed. This speed was then going to be sent to a computer via the QUART which is also known as a Serial communications interface. This was then received on the Hypercritical interface.
Using this method a step of Vass applied to the motor. This was done by stepping the power supply to the desired voltage and turning it off. Then the QUART was then configured to receive signals and the power supply turned on. At this time the output speed of the motor could have be measured and the recorded. It this experiment we were mainly considered with the settling time of the system so the values taken as the output was taken a few readings after steady state was reached. These values were then converted the radar/s and the graph of speed vs.. Mime was plotted in MUTUAL.
Since the sampling rate was every looms hence in order to convert the readings into radar/s we first multiplied by a factor often. Since this transfer function was found using the encoder disc in therefore became a section of the system and was already considered for in the transfer function. Figure 4 the output response of the system Now knowing that the transfer function of the motor is: The gain of the system, K is the ratio of the output speed to the input voltage. This gave us a gain of 80. 5. The time constant, is the time taken to reach 63% of the maximum output. This was form to be 0. Sass.
Using these parameters we are to determine the transfer function of the motor. This was found to be: G’s=Speedups(s)Voltage(s)=80. 50. Sass G’s=Speedups(s)Voltage(s)=429. As+5. 39 This gave an original transfer function that is seen below: Determination of suitable performance specs The general form of the controller in a PI strategy is: Ski+sips We will now derive the transfer function for the entire system which will include the both the planter (motor) and the controller. Has= Ski+sips x 429. As+5. 39 H(S) =429. Kiss+429. SKIP+5. ASS+429. Ski The maximum overshoot that was given in specifications was 5%.See More on Control theory