Real Time Stepper Motor Control Using ARM Microcontroller and MATLAB GUI For Satellite Ground Station Tracking

Transcription

1 Real Time Stepper Motor Control Using ARM Microcontroller and MATLAB GUI For Satellite Ground Station Tracking Arjun R 1, Harshith Patte 2, Harshith V 3, Karthik K R 4, Narayana T Deshpande 5, Dhruti Ranjan Gaan Student, 5 Assistant Professor, Department of ECE, BMSCE, Bangalore 6 Research Engineer, ISAC Abstract: The bird's-eye view that satellites have allows them to see large areas of Earth at one time. This ability means satellites can collect more data, more quickly, than instruments on the ground. Before satellites, TV signals didn't go very far. TV signals only travel in straight lines. With satellites, TV signals and phone calls are sent upward to a satellite. Then, almost instantly, the satellite can send them back down to different locations on Earth. Hence a reliable and fast communicating satellites are important in today s world. This is achieved in the project by using an accurate antenna arm. In this project, a stepper motor is implemented instead of BLDC motor which is currently used. This results in lesser beam width, hence faster data transfer rate and minimal data spill. This is an open loop system which makes it more dynamic and more responsive. In this project, the error will be reduced and the precision will be increased. The stepper motor can give a minimum error of ±0.1 degree can be achieved either by full stepping, half stepping or micro stepping of the stepper motor. A synchronous serial communication is established between TEENSY 3.1 and optical encoder to control and receive data from the optical encoder as commanded by the master, TEENSY 3.1. Since the board cannot provide the necessary voltage and current levels to drive the motor, IC L298 and IC L6506 together are used as a constant current driver for the motor and the commutation signals to the motor are generated by the Teensy board. MATLAB based GUI is used to control the motor. With each step of motor, optical encoder is made to read and send the angular position to the MATLAB to calculate the difference to find the error. The error calculated in saved in as Excel sheet and reproduced in graphs. Keywords: Low Earth Orbit (LEO), Advanced RISC Machine (ARM), Graphical User Interface (GUI), Synchronous Serial Interface (SSI). 93 I. INTRODUCTION A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. Communications satellites are used for television, telephone, radio, internet, and military applications. There are over 2,000 communications satellites in Earth s orbit, used by both private and government organizations. Wireless communication uses electromagnetic waves to carry signals. These waves require line-of- sight, and are thus obstructed by the curvature of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated points. Communications satellites use a wide range of radio and microwave frequencies. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" certain organizations are allowed to use. This allocation of bands minimizes the risk of signal interference. In satellite communication, signal transferring between the sender and receiver is done with the help of satellite. In this process, the signal which is basically a beam of modulated microwaves is sent towards the satellite. Then the satellite amplifies the signal and sent it back to the receiver s antenna present on the earth s surface. In satellite communication, signal transferring between the sender and receiver is done with the help of satellite. In this process, the signal which is basically a beam of modulated microwaves is sent towards the satellite. Then the satellite amplifies the signal and sent it back to the receiver s antenna present on the earth s surface. Satellite Communication utilisation has become wide spread and ubiquitous throughout the country for such diverse applications like Television, DTH Broadcasting, DSNG and VSAT to exploit the unique capabilities in terms of coverage and outreach. The technology has matured substantially over past three decades and is being used on commercial basis for a large number of applications. Most of us are touched by satellite communication in more ways than we realise [1]. Low Earth Orbit (LEO) satellites are used to collect less frequent but more detailed information. An orbit is defined as LEO when it is at any altitude between 100-1,240 miles (160km - 2,000km). Here are several types of low earth orbits but the most common for earth and atmospheric science is the polar orbit [2]. LEOs are mostly used for data communication such as , paging and videoconferencing. Because LEOs are not fixed in space in relation to the rotation of the earth and since they orbit at a lower altitude compared to other satellites, they move at very high speeds and therefore data being transmitted via LEOs must be handed off from one satellite to the next as the satellites move in and out of range of the earth-bound transmitting stations that are sending the signals into space. The antenna of the polar satellites (LEO) should track the ground station in real time with minimal error for successful and fast data transmission. For a polar satellite to load and dump data, it has only few minutes of span. For example, a Satellite with 2 Hour orbit period has 10 minutes to transfer the data collected to a ground station. The beam width has to be minimal in order to increase the data transfer speed, minimise data spill/data loss. Hence, we need an accurate antenna arm. The antenna drive with minimal error and BLDC

2 motors are used for the same. The rotation of antenna on a satellite is done by BLDC motor. In a BLDC motor, commutation is done by electronic means. In that case the instantaneous rotor position must be known in order to determine the phases to be energized. The angular rotor position can be known by: Using a position sensor (Hall sensor, optical encoder, resolver) Electronically analysing the back-emf of a nonenergised winding. This is called sensor less commutation. In general, BLDC motor have three phase windings. The easiest way is to power two of them at a time, using resolver to know the rotor position. Commutation may be done very simply or it may be more complex by modulating sinusoidal currents in the three phases. This is called vector control, and its advantage is to provide a torque ripple of theoretically zero, as well as a high resolution for precise positioning. The minimum error with which the antenna could track the ground station now is ±1º degree. In order to make it more accurate and precise, Stepper motors will be implemented instead of BLDC motors. This results in lesser beam width, hence faster data transfer rate and minimal data spill. The project work consists of 5 modules and they are Full step, Half step excitation of stepper motor. Real-Time Control Platform for MATLAB and Simulink Stepper Motor Control SSI interfacing of 16-bit optical encoder to teensy 3.1 USB-HID Real-Time Data Acquisition into MATLAB MATLAB GUI for real time plotting and data acquisition. II. THE LITERATURE SURVEY Stepper motors have been used to drive mechanical systems for many decades. Stepper motors require a current-limited multiphase driver, and techniques have evolved from simple resistive drivers to complex high efficiency, high-performance switching devices. This has caused a shift in driver technology from a quaddrive system to an H-bridge system, and a change in motor design from four windings to two [4]. The open loop control type eliminates the need for feedback sensors, and since the cost reduction is a continuous demand in industrial applications, the open loop stepper motors remain suitable in many practical application fields. The main disadvantage of the open loop control mode is the steps lost followed by the motor stall. The stall can be detected by estimating the motor speed in real time by measuring motor currents and voltages. The estimated speed is compared with the speed command, and stall error protection is signalized if the error exceeds a certain range [5]. Stepper motors are 94 used in applications like Bonding and Laser trimming, it is necessary to control the stepper motor from remote places. The angular position of the stepper motor can be controlled remotely using DTMF (Dual-Tone Multi- Frequency) technology. DTMF Technology provides acoustic communication for controlling the angular position of the stepper motor remotely anywhere in the world through mobile phone network [6]. In polar satellites the antenna should track the ground station with minimum error in real time for effective and fast data transfer. Currently BLDC motors are used to align the antenna to track the ground station in real time which have very low precision. Hence stepper motors can be used to do the same with high precision. Tracking Quality Measurements of Ground Station for Low Earth Orbit Satellite. The poor quality position tracking of the satellite Ground Station (GS) for the Low Earth Orbit (LEO) satellite is due to the low quality RF signal received [8]. Also a satellite tracking station (STS) must be able to track a satellite at any position in the sky above a few degrees elevation. The best quality of data reception is obtained when the telemetry signal is stronger, i.e., where the satellite is closest to the ground station. In addition, it is most important that the station can be capable of continuing a good tracking through and near the local zenith to assure quality reception of the telemetry data If the antenna is aligned with minimum error with in the available beam width the strength of the received RF signal can be increased by a considerable amount [3], [8]. III. EQUIPMENT USED AND BLOCK DIAGRAM A. An Optical Encoder: An encoder is an electromechanical device used to monitor the motion or position of an operating mechanism, and to translate that information into a useful output. The output can take the form of a simple system status indication, or it can provide feedback control information or in other ways interface with related devices. There are several position sensing techniques available to the system designer. The most widely used types of sensors fall into one of the following types: capacitive, magnetic, contact or optical. Figure 1 Components of Optical Encoder B. Stepper Motor: A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. The motors rotation has several direct relationships to these applied input

3 pulses. The sequence of the applied pulses is directly related to the direction of motor shafts rotation. The speed of the motor shafts rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied. C. Gear: Spur Gears: Spur gears or straight-cut gears are the simplest type of gear. They consist of a cylinder or disk with teeth projecting radially. Though the teeth are not straight-sided (but usually of special form to achieve a constant drive ratio, mainly involute but less commonly cycloidal), the edge of each tooth is straight and aligned parallel to the axis of rotation. These gears mesh together correctly only if fitted to parallel shafts. Backlash is defined as the amount by which a tooth space exceeds the thickness of an engaging tooth. Excessive backlash in gears is objectionable, particularly if the drive is frequently reversing. Harmonic Gears: A harmonic gear is a specialized gearing mechanism often used in industrial motion control, robotics and aerospace for its advantages over traditional gearing systems, including lack of backlash, compactness and high gear ratios. In our project, we have used gear assembly that comprises of Harmonic gear and Spur gear. Harmonic gear has a gear ratio of 100:1 and an efficiency of 95%. Spur gear has a gear ratio of 83:21(~ 4:1) and an efficiency of 75%. The overall efficiency of the gear assembly is 71.25%. D. Teensy 3.1: The Teensy is a complete USB-based microcontroller development system, in a very small footprint, capable of implementing many types of projects. All programming is done via the USB port [9]. The key features are: Compatible with Arduino Software & Libraries, USB can be any type of device, Single pushbutton programming, Easy to use Teensy Loader application, Works with Mac OS X, Linux & Windows, Tiny size, and perfect for many projects, Available with pins for solder less breadboard. be synchronized using the sync pin. In this mode of operation, the oscillator in the master chip sets the operating frequency in all chips. F. L298: The L298 is an integrated monolithic circuit in a 15lead multiwatt and PowerSO20 packages. It is a high voltage, high current dual full-bridge driver designed to accept standard TTL logic levels and drive inductive loads such as relays, solenoids, DC and stepping motors. Two enable inputs are provided to enable or disable the device independently of the input signals. The emitters of the lower transistors of each bridge are connected together and the corresponding external terminal can be used for the connection of an external sensing resistor. An additional supply input is provided so that the logic works at a lower voltage [13]. G. AM26C31: The AM26C31 is a differential line driver with complementary outputs, designed to meet the requirements of TIA/EIA-422-B and ITU (formerly CCITT). The 3-state outputs have high-current capability for driving balanced lines, such as twistedpair or parallel-wire transmission lines, and they provide the high-impedance state in the power-off condition. The enable functions are common to all four drivers and offer the choice of an active-high (G) or active-low (G) enable input. Bi CMOS circuitry reduces power consumption without sacrificing speed [11]. H. AM26C32: The AM26C32 device is a quadruple differential line receiver for balanced or unbalanced data transmission. The enable function is common to all four receivers and offers a choice of active-high or active-low input. The 3-state outputs permit connection directly to a bus organized system. Fail- safe design specifies that if the inputs are open, the outputs always are high. The AM26C32 devices are manufactured using a Bi CMOS process, which is of bipolar and CMOS transistors. This process provides the high voltage and current of bipolar with the low power of CMOS to reduce the consumption to about one-fifth that of the standard AM26LS32, while maintaining AC and DC performance [12]. I. Block Diagram: 95 Figure 2 Teensy 3.1 Pin Configuration E. L6506: The L6506/Dis a linear integrated circuit designed to sense and control the current in stepping motors and similar devices. When used in conjunction with the L293, L298, L7150, L6114/L6115, the chip set forms a constant current drive for an inductive load and performs all the interface function from the control logic thru the power stage. Two or more devices may Figure 3 Block Diagram

4 IV. METHODOLOGY AND IMPLEMENTATION The clock required for SSI is generated using manual toggling. The clock generated by the teensy board is in TTL logic (i.e. 5V for HIGH and 0V for LOW). But the optical encoder uses RS 422 standards (voltage range is between +6V to -6V). So a quadruple differential line driver and a quadruple line receiver is to be placed between the teensy board and optical encoder for the communication to happen. The line driver converts the clock signals from teensy board to clock signals that are understandable by optical encoder. The line receiver converts the data sent from the encoder to voltage levels understandable by the teensy board. Figure 4 SSI Hardware Interface The commutation signals to the motor are generated by the Teensy board. Since the board cannot provide the necessary voltage and current levels to drive the motor, IC L298 and IC L6506 together are used as a constant current driver for the motor. IC L298: The L298 accept standard TTL logic levels from L6506 and drive the stepper motor. Two enable inputs are provided to enable or disable the device independently of the input signals. The emitters of the lower transistors of each bridge are connected together and the corresponding external terminal is used for the connection of an external sensing resistor. An additional supply input of 18V is provided so that the logic works at a lower voltage. IC L6506: L6506 senses and controls the stepper motor. L6506 together with L298 forms a constant current drive for the motor and performs all the interface function from the control logic through the power stage. The current in the windings reaches either peak value or low value which is sensed by voltage across the sense resistor (Rsense) and either decreases or increases the current in the coil respectively, in order to maintain the normal functioning of the motor. Here we establish a serial communication between the MATLAB and the Arduino to send the command and data to the Arduino and also receive the output data sent from the encoder to plot it in MATLAB. The steps include setting the baud rate, selecting the com port and creating serial object with all these features. The buffer size of the MATLAB is increased as per our requirement as there will be lot of samples received from the teensy board. The MATLAB based motor control is achieved by a GUI that provides user a means to enter the inputs to control the motor. In the GUI, the COM port, Baud rate and other necessary information required to create a serial port are entered. Connect and disconnect option are provided to establish and terminate the connection. Specific numbers are assigned for each mode and on user s instruction these numbers are sent from MATLAB to Arduino serially through the port established. Based on the data received from the MATLAB at the Arduino, mode selected is found out. Once the selection is done, code takes the arguments as per the user input. Before selecting mode or transferring any other data used in controlling the motor, serial communication is to be established and same should be terminated, once used. After every step the motor takes, the encoder is read to get the position information which in turn is processed the get the angular position. This angular position is sent to the MATLAB for error calculation. The current angular position is compared with the reference position and the difference, which is the error in rotation is calculated and is saved in an excel sheet. The error is calculated the same way for every step taken. The calculated error is plotted on the GUI in appropriate way. V. RESULTS Figure 6 Error in Excel Sheet 96 Figure 5 Motor Interface Figure 7 Error Plot

5 VI. CONCLUSION The Codes which were written separately to SSI Interfacing and MOTOR interfacing was merged together into a single code and was tested with the device. MATLAB GUI has been developed with code to each block in it and is working fine. The problem which we faced during the midterm review of sending data from MATLAB to Arduino has been rectified now. Stepper motor with gear assembly having harmonic and spur gear have been used for performance demonstration. As spur gear is having inherent backlash, the expected error performance couldn t be shown. VI. FUTURE WORK Motor has to be tested with load (antennas assembly). Sinusoid profile with microstep (1/8, 1/16, 1/32) mode has to be carried out. Optical encoder could be used in closed loop mode for assessment of variable load or motor step losses for dynamic friction. For future work, motor assembly has to be backlash free. So antibacklash mechanism has to be added to gear assembly. VII. REFERENCES [1] Isro.gov.in. (2016). PSLV-ISRO. [online] Available at: [Accessed 4 Apr.2016]. [2] Anon, (2016). [online] Available at: [Accessed 5 Apr. 2016]. [3] A. Taheri, M. Hossein Fatehi, H. Bahrami and M. Shoorehdeli, "Implementation and Control of X-Y Pedestal Using Dual-Drive Technique and Feedback Error Learning for LEO Satellite Tracking", IEEE, vol. 22, no. 4, [4] N. Greenough and C. Kung, "A New High- Efficiency Stepper Motor Driver For Old Technology Stepper Motors", IEEE, [5] I. Stika, L. Kreindler and A. Sarca, "A Robust Method for Stepper Motor Stall Detection", IEEE, [6] S. Pal and N. Tripathy, "Remote Position Control System of Stepper Motor Using DTMF Technology", International Journal of Control and Automation, vol. 4, no. 2, [7] "Novel Position Detection Method for Permanent Magnet Stepper Motors Using Only Current Feedback", IEEE Transactions On Magnetics, vol. 47, no. 10, pp. 3590,3591,3592,3593, [8] M. Stolarski, "Tracking Quality Measurements of Ground Station for Low Earth Orbit Satellite", Space Research Centre, Polish Academy of Sciences, [9] Pjrc.com, "Teensy USB Development Board", [Online]. Available: [Accessed: Jan- 2016]. [10] Implementation Of SSI Master Interface Application Note, 3rd ed. Posital Fraba, [11] Quadruple Differential Line Driver, 6th ed. Texas Instruments, [12] AM26C32 Quadruple Differential Line Receiver, 8th ed. Texas Instruments, [13] Dual Full Bridge Driver, 1st ed. ST Microelectronics,