Conference Paper · October 013 doi: 10. 1109/cobep. 2013. 6785223 citations reads 2,638 authors
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DRIVES
A.
Digital Circuit The digital portion circuit is the core of the developed system and it is based on a microcontroller, produced by Microchip, namely dsPIC33FJ12GP201, which is a general purpose 16-Bits microcontroller, with several built-in peripherals, such as Timers, Analog Digital Converter (ADC), Universal Synchronous/Asynchronous Serial Receiver/Transmitter (USART) and others. The main functionalities executed by the microcontroller are: Selectable drive schemas, being available full step, half step and microstep 1/4 and 1/8; Current reference, controlled according to the active drive scheme; Digital interface for pulse, direction and enable signals; signal comparison; Digital control of power transistors. Figure 4 presents some variables and services that run within the microcontroller in order to implement such functionalities. There are four lookup tables, one for each available driving schema and only one can be active at a time. Current reference comes from the active lookup table in a given index, stored at the variable Npos. D ir e ç ã o P a s so Figure 4 – Block diagram of variables and services that run on dsPIC33FJ12GP201. An Acquisition service runs at 50 KHz. After acquisition cycle, outputs that control the power switches are turned on and off according to the comparison result of the current reference and the read current. A Pulse monitoring service runs whenever the state of the pulse digital signal chances from high to low - the falling edge - and increments or decrements the variable Npos, according to the state of the direction digital input. The variable Npos is bounded according to the size of the active table and wraps around whenever it reaches its upper or lower limits. The real change on the stepper motors' position will take place on the next acquisition cycle, where the switches are to be controlled according to the new index on the active lookup table. B. Power Circuits At the power stage, it was employed and monolithic IC, namely L298, which integrates two H-Bridges and its transistors drives circuits. The H-Bridge is based on Bipolar Junction Transistor (BJT). Its maximum voltage is 40 V and is capable of carrying 2 A by each bridge. External free- wheeling, fast recovery diodes were used, to allow current flow on the opposite direction of the BJT’s orientation. The switches are controlled by digital inputs, turning them on and off according to the drive’s logic. In series with the load, more exactly under the low voltage side transistor, a low resistance resistor was placed, generating a voltage signal proportional to the load current. Such voltage is used as input to the analog conditioning circuit. C. Analog Circuit There are a few operations needed to be carried out on the current-proportional voltage signal before it can be used as input to the ADC. These operations are: scaling and filtering. Signal shifting was not necessary on this case, since the signal of interest is positive only. Filtering is necessary to preserve the spectral content of the interest signal on the sampling process. Scaling is necessary to reduce the error introduced on the discretization process and to allow the signal to swing through the entire range of the ADC. The circuit on Figure 5 presents this functionality. ( ) * ( forms a first order, low pass filter, with a cutoff frequency given by: + ,-$!(( = 1 ( . * ( . 2. . (2) The op-amp /01 the resistive network 1 ) 2 forms a non-inverting amplifier, whose gain is given by: = 1 + 13 (3) Figure 5 – Schematics of the current conditioning circuit. The transfer function of the whole circuit is given by: (4) = 5!"#$ . ( + ) 6 5!"#$ + ( 7. * ( . 4 + 1 . 1 (4) Since the conditioned signal is sampled at 50 KHz, 15 Khz band limiting would work just fine to avoid aliasing. Since 15 KHz cutoff frequency is hard to achieve with regular discrete components, the cutoff frequency was approximated to 15.157 KHz. To yield such frequency, along with an overall gain of 0.5 V/A, the employed values were: ( = 7 9Ω = 40 9Ω * ( = 1,5 > = 10 9Ω !"#$ = 0.1 Ω IV. RESULTS The system was built in a printed circuit board and is part of a CNC milling/router system. The built system is illustrated by Figure 6. There were made two tests with the system: current regulation; and the overall current waveform for three of the driving schemas. All measurements were made with an oscilloscope, with the gain of 0.47 V/A. The current signal could not be directly measured, due to lack of correct instrumentation – a high performance Hall Effect current clamp –, so the current was measured through !"#$. itself. Due to this fact, all the negative current swing was reflected and observed as a positive swing, i.e. its absolute value was observed. Figure 6 – Stepper motor driving system developed. Figure 7 shows the current ripple for one of the phases with a set-point of 2A. The output voltage, given a 2 load current, is 0.94 @, which is close to the mean value shown in Figure 7. The value of 0.950 @ A5B was read, that yields a load current of 2.0212 , that in turns corresponds to an absolute error of C DEF = 0,0212 and a percentual error of C % = 1,1 %. The peak-to-peak load current ripple read was H# IJ = 0,097 and, percentually, H# IJ % = 4,888 %. Figure 7 – Ripple of load current. Next, the results achieved with two driving schemas are shown. Figures 8 and 9 presents the actual current waveform measured and the ideal current waveform, respectively, for a half-step driving schema. Figure 8 – Measured current waveform for half step driving schema. Figure 9 – Ideal current waveform for half step driving schema. Figures 10 and 11 present the actual current waveform measured and the ideal current waveform, respectively, for a micro step 1/4 driving schema. Figure 10 – Measured current waveform for micro step ¼ driving schema. Figure 11 – Ideal current waveform for micro step ¼ driving schema. V. CONCLUSION The stepper motor drive is base of the low cost positioning system. Currently, this topology is being largely employed on low cost CNC systems, whose applications are as varied as possible, being successfully applied to machining, pick- and-place, 3D plastic printing and many others. A stepper motor driving system was developed described in this paper. The designed system is based on low cost devices and performed as expected, with its performance parameters within the expected. The experimental results obtained from the developed system are presented. As indicated by the measured current waveform plots, the developed system could regulate the load current according to the selected driving schemas. VI. ACKNOWLEDGEMENT The authors would like to acknowledge: CNPq, for supporting this research; Oyamota do Brasil, for supporting the manufacturing of mechanical system; CEAMAZON, for supporting high-end research and continuously effort on producing human resources. VII. REFERENCES [1] N. Dahm, M. Huebner, and J. Becker, "Approach of an FPGA based adaptive stepper motor control system," Download 0.84 Mb. Do'stlaringiz bilan baham: |
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