Turn the potentiometer counter clockwise to reduce

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Unformatted text preview: JUMPER DIAGRAM Brushed DC Lead FIGURE 2-10: DS51557B-page 30 © 2005 Microchip Technology Inc. Example Projects FIGURE 2-11: PROJECT 5: SCHEMATIC VDD LCD1 PIC16F917 11/32 COMX Common pins SEGX VDD Segment pins VDD R10 POT1 10 kΩ 1 kΩ 2 AN0 C26 1000 pF VIM-332-DP VDD VDD VDD U11:B R11 20 kΩ R20 470 Ω Q6 RD7 R22 30 24 RC5 470 Ω 10 kΩ D12 Motor U10:A 12/31 VSS CCP2 21 10 k Ω Simplified circuit shows equivalent functionality Instructions Use POT1 to adjust the speed of the motor. Turn the potentiometer counter-clockwise to reduce speed; turn it clockwise to increase motor speed. Multiply the LCD display by 1000 to obtain speed in RPM. Discussion With the CCP module operating with 8 bits of resolution, the module can output a PWM signal at a frequency of 31.2 kHz. This is sufficiently high frequency that motor whine is outside of the audible frequency for humans. This is especially important in applications where the motor may turn at slow speeds. Using an optical sensor to gather feedback from a motor is an integral part of many mechatronic systems. Some systems require a constant speed over varying loads. An optical encoder is similar to the optical interrupter circuit used in this project. The difference is that an optical encoder uses more than one optical sensor (as many as four) to deduce speed and shaft position. Note: © 2005 Microchip Technology Inc. The optical interrupter circuit is susceptible to interference from outside infrared light sources (i.e., incandescent lights, sunlight, etc.). It may be necessary to cover the optical sensor to obtain an accurate reading. DS51557B-page 31 PICDEMTM Mechatronics Demo Board User’s Guide 2.3.6 Project 6: Brushed DC Speed Control with Back EMF Feedback Project 5 uses an optical encoder to provide motor speed feedback. In this project, another form of speed measurement will be explored, Back Electromotive Force (Back EMF). You may be aware that a brushed DC motor, when turned by hand, will produce voltage at its leads, becoming a generator. When a brushed DC motor is being driven, and then the drive voltage is removed for a brief amount of time, the voltage generated by the inertia of the motor will be proportional to its speed. This voltage is the Back EMF. Objectives 1. Effectively read Back EMF using the on-board Analog-to-Digital Converter. 2. Interpret the Back EMF into a speed for the motor. Applicable Technical Documents Low-Cost Bidirectional Brushed DC Motor Control Using the PIC16F684 Application Note, AN893 (DS00893) Jumper Configuration • • • • • • • DS51557B-page 32 RD7 (J10) to P1 (J1) CCP2 (J10) to N2 (J1) AN0 (J13) to POT1 (J4) AN1 (J13) to BACK EMF (J16) Attach the motor leads to DRIVE1 (P9) and DRIVE2 (P10). J2 and J3 should be unpopulated. Connect the right and center pins of JP8 using a shunt. © 2005 Microchip Technology Inc. Example Projects PROJECT 6: JUMPER DIAGRAM TM Brushed DC Lead FIGURE 2-12: © 2005 Microchip Technology Inc. DS51557B-page 33 PICDEMTM Mechatronics Demo Board User’s Guide FIGURE 2-13: PROJECT 6: SCHEMATIC VDD LCD1 PIC16F917 11/32 COMX Common pins SEGX VDD Segment pins VDD R10 POT1 10 kΩ 1 kΩ 2 AN0 C26 1000 pF VIM-332-DP VDD U11:B R32 Back EMF 1 kΩ 7 AN4 RD7 30 C36 1000 pF 10 kΩ Back EMF Motor U10:A 12/31 CCP2 21 VSS 10 kΩ Simplified circuit shows equivalent functionality Instructions POT1 adjusts the speed of the motor. Set POT1 at 100% to run the motor at full speed. The speed indicated on the LCD should be similar to the speed shown in Project 5. Next, run the motor at half speed by setting POT1 to 50% and do the comparison again. Note: Different motors have different speed characteristics. The motor used at the time this manual was written may be different from the motor shipped with your PICDEM Mechatronics Demo Board. Verify that the motor data sheet included on the CD-ROM matches the motor on the board. If they are different, go to Microchip’s web site for updates to this project. Discussion Back EMF is typically not as accurate as speed feedback from an optical encoder. However, in many applications the accuracy does not have to be very precise and measuring Back EMF is more cost effective than any other speed feedback mechanism. DS51557B-page 34 © 2005 Microchip Technology Inc. Example Projects 2.3.7 Project 7: Stepper Motor Control: Single Stepping, Half Stepping and Microstepping This project demonstrates the various ways to drive a bipolar stepper motor. There are several ways to step a stepper motor, the most basic of which is single stepping, or moving the motor in one-step increments. If a motor is specified as a 7.5 degrees-per-step motor, then single stepping the motor will result in moving the shaft of the motor 7.5 degrees per step. Half stepping the same motor would result in a 3.75 degrees step. Torque and current are linearly related for a stepper motor. Therefore, if two sinusoidal currents are applied to the windings, offset by a 90 degree phase shift relative to one another, then the stepper motor will have constant torque as it turns. This results in a very smooth rotation of the shaft. Applying current to the windings in this way is referred to as sine-cosine microstepping. Objectives 1. Single-step a stepping motor. 2. Half-step the motor. 3. Micro-step the motor. Applica...
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This note was uploaded on 01/29/2014 for the course AA AA taught by Professor Aa during the Winter '10 term at ENS Cachan.

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