Wireless Control of PMDC Motors, Through Variable Speed Drives

Wireless Control of PMDC Motors, Through Variable

Speed Drives

Dnyanesh P. Joshi, Rahul S. More, Dr. Neelima S. Iyer

Instrumentation & Communication Unit. CSIR-National Chemical Laboratory, Pune-411008

Abstract – The work focuses on development of Wireless Embedded System for remote monitoring and controlling of a PMDC motor, by controlling the driving voltage to the Variable Speed Drive. The system broadly comprises of two modules, one at user side and other at field (motor) side. The user side module is used to enter required rpm value as well as get the feedback from field side. With this arrangement, attempt has been made to develop a motor RPM control system which varies motor speed as per load requirement, resulting in energy and cost savings; perfectly suitable for industrial applications and installation in hazardous & congested environments.

Keywords – ARM7, Fixed Field Optical Sensor (Q12AB6FF50), IEEE 802.15.4 Standard, MCP4922 (12-Bit DAC), RPM Measurement Techniques, VSD.

I.

I

The recent advances in wireless technology, finds its applications in various fields and industries. Replacing the existing wired network with wireless network has many advantages including but not limited to flexibility, scalability& simplicity.

The incorporation of wireless network in the existing complex wired system is quite challenging and rewarding. The wireless technology could be useful in application ranging from monitoring and controlling of industrial processes and plants, to monitoring important parameters of geographically distributed system [6]. Recently, several wireless protocols such as ZigBee, Bluetooth, WirelessHART have been developed which uses IEEE 802.15.4 standard as a baseline and adds additional routing and networking functionality to it. Few of these are proprietary, but ZigBee is an open standard protocol having various advantages, include product interoperability, vendor independence and accessibility to broader markets.

The IEEE 802.15.4 standard was developed keeping in mind following important points namely lower data rate, simple connectivity and longer battery life. The standard specifies that communication can occur in the 868MHz, 915 MHz or 2.400-2.4835 GHz Industrial Scientific and Medical (ISM) bands. ZigBee uses the ISM band for wireless data transfer with maximum data rate of 250kbps [4][8].

We have developed a prototype module to control the PMDC motor RPM, by varying the voltage supplied to Variable Speed Drive (VSD). This controlling signal is passed to VSD using ZigBee wireless protocol, thereby reducing the wiring, providing flexibility especially when the motor installations are in cramped spaces. VSDs offer the greatest opportunity for energy savings when driving

loads because motor horsepower varies as the cube of speed and torque varies as square of speed for these loads. For example, if the motor speed is reduced 20%, motor horsepower is reduced by a cubic relationship (0.8 x 0.8 x 0.8), or 51%, thus providing greater energy savings. Following are the typical uses of variable speed drive. 1) Speed control, torque, position, acceleration or braking. 2) Developing closed loop system using feedback sensors. 3) Energy optimization using speed control etc.

4) To allow accurate and continuous process control over a wide range of speeds.

The paper is organized as follows:  Section II - SYSTEM DESCRIPTION  Section III - SPI INTERFACE For LPC2148  Section IV - RPMMEASUREMENT TECHNIQUES  Section V - SYSTEM TEST RESULTS &ANALYSIS  Section VI - CONCLUSION AND FUTURE SCOPE

II.

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D

Generally a process control system consists of two major components; viz. controlled variables and manipulated variables. The controlled variables are physical quantities to be measured like temperature, flow, humidity; while the manipulated variables comprise of the valve position, motor speed, state of heater etc. The system changes manipulated variables so that set value is maintained for control variable. The focal point of this paper is one of the vital elements of process control system i.e. motor speed control. The system is broadly divided into two parts; one part consists of Wireless Control Circuit (fig. 1) that encompasses a microcontroller, wireless ZigBee module and DC motor drive, the other part is a Wireless Embedded Module that provides user interface for motor control.

A.

System Flow:

User enters a desired value of RPM from remote location, which is transmitted to control circuit via wireless ZigBee devices [7]. The microcontroller then converts the RPM value into corresponding voltage value as per the motor characteristic equation. The computed value is then applied to the DAC, which gives out an analog voltage. The voltage is further amplified and applied to VSD. Then motor runs at a specific speed, which is measured by the optical sensor. Feedback from the sensor is used to calculate an error between desired and measured RPM value.

B.

ZigBee Module:

Copyright © 2013 IJECCE, All right reserved Fig.1.Wireless Control Circuit for PMDC Motor

C.

Microcontroller:

The LPC2148 is an ARM7 family microcontroller; low power consumption & high performance are the key features of it. The on chip UART and SPI block are used to interface ZigBee & MCP4922 respectively. The external interrupt pin of controller is used to interface an optical sensor [3]. An optical sensor has been implemented so as to produce a pulse per rotation, which acts as an interrupt for the controller. For every interrupt, controller reads timer register TC that gives a time period between two successive interrupts [5]. Controller then evaluates a frequency, in terms of RPM, from that time period as shown below.

RPM(PCLKTimePeriod) 60

PCLK is peripheral clock for controller that is divided by Time Period to get revolutions per second, which is multiplied by 60 to convert it into RPM.

D.

Digital-to-Analog Converter:

MCP4922 is 12 bit SPI compatible converter, used to produce analog control voltage for VSD, from entered RPM values. It utilizes a resistive string architecture which is fast & simple for 12 bits. Low DNL error and fast settling time are the key features of MCP4922. The single IC package houses two DACs, which can be configured individually. The SDI (MOSI), SCK & CS are part of serial interface, while LDAC is latch DAC synchronization input pin. The MISO pin of controller is unused for this interface. All write operations to MCP4922 are 16-bit words, out of which higher four are configuration bits followed by 12-bit data. Asserting chip select (CS) low initiates the write cycle, followed by 16-bit of data transfer on SDI. CS goes high at the end of the cycle, after which LDAC is asserted low. As soon as LDAC goes low the data gathered in the input buffers of MCP4922 is transferred to the output buffers.

E.

Voltage Amplifier:

It is a simple Op Amp based single stage voltage amplifier. The voltage amplifier with the gain of four has been implemented in order to overcome output voltage range and current sourcing capability of DAC.

F.

Variable Speed Drive:

It incorporates power components, which enables it to produce desired DC voltage and current. Control signal to VSD is low DC voltage 600–7000mV which linearly corresponds to 10-300 motor RPM.

G.

Optical Sensor:

Different sensor that can be used for this application, like Hall-effect sensor, optical sensor, proximity sensor, rotary encoders etc. In our application we have used Fixed-field photoelectric Sensor (Q12AB6FF50). It uses a bright, visible red (640 nm) light source for object detection that gives a reliable output. The Fixed-field type sensor compares the reflections of its emitted light beam from an object back to the sensor’s two differently aimed detectors, R1 and R2. If the near detector's light signal is stronger than the far detector's light signal, the sensor responds to the object. If the far detector's light signal is stronger than the near detector's light signal, the sensor ignores the object [2]. The cutoff distance for this sensor is fixed at 50mm. Objects lying beyond the cutoff distance are usually ignored, even if they are highly reflective.

Fig.2.Working Principle of Optical Sensor As can be seen from fig. 2, the object is detected only when it is in the detecting range of the sensor; fixed field type sensors are perfectly suitable in applications where there is a lot of background movement and constrained spaces, so that the object beyond the sensing range is completely neglected even if the object/background is reflective. Thus this sensor has a type of background suppression provision due to far-limit cutoff.

H.

Motor Assembly:

The actual motor assembly (fig. 3) has a bolt fixed onto the motor shaft & protruding outwards. The sensor beam is positioned such that when the bolt is aligned with the sensor beam, the sensor’s receiver detects its presence which repeats for every rotation. The shaft on the other hand goes undetected as it is beyond the sensing range of the photoelectric sensor. The sensor output is then given to the controller for rpm measurement.

III.

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F

LPC2148

The important configuration registers of SPI block are SPCCR, SPCR, SPDR and SPSR.

A.

SPCCR (Clock Counter Register):

The SPCCR is used to set data transfer rate (SPI Frequency). It is 8 bit register, and the value entered in this register is used to calculate frequency. The value entered must be even and greater than 8

SPIdataRatePCLKSPCCR

B.

Important bits of control register are BitEnable, CPHA, CPOL and MSTR. BitEnable bit is used to select the data length for SPI protocol. CPHA stands for Clock Phase Control, and plays an important role in deciding the relation between sampling of data and clock pulse. The important thing to be considered here is CPHA should be 0 when LPC2148 is used as a SPI Master. The SSEL signal is inactive during the data transfer when CPHA is 0, but when CPHA is 1 the SSEL signal becomes active and SPI block immediately transforms itself into slave. This results into a Mode Fault and data transfer terminates. In this case MODF bit of Status Register will set to 1. CPOL Stands for Clock Polarity Control. The bit decides the polarity of SPI clock.

Fig. 4: Flow chart for SPI data transfer

C.

SPDR (Data Register):

It is 16 bit register used in data transfer; the data length is selectable (8 bits to 16bits). The data length can be configured by using bit 2 (BitEnable) and bits 11:8 of control register. There is no buffer between the data register and the internal shift register. A write to the data register goes directly into the internal shift register.

Therefore, data should only be written to this register when a data transmit is not currently in progress. Otherwise a Collision Error may occur. On the other side read data is buffered and transferred to an internal data register, after the completion of data transfer.

D.

SPSR (Status Register):

This is 8 bit status register of SPI block. The last bit of this register is SPIF, which signifies completion of data transfer, while other four bits indicate an error (if any) occurred during that data transfer. In the normal operation these error bits should be 0. It is mandatory to read SPSR followed by SPDR in order to clear SPIF bit (fig. 4).

E.

SPI Master:

When the SPI block is configured as Master, both Read and Write operations are initiated by Master itself. The write operation begins when data is written into the data register and then SPI block generates clock along with the data bits. The read operation resembles with that of write; just in this operation dummy bits are written into the data register. In read operation data is transferred from Slave to Master, but as clock is generated by Master and it generates clock only when data is written into its data register. As the SPI is full duplex communication protocol, when one bit transfers from master to slave using MOSI line, the MISO is sampled to receive the bit transferred by slave. Thus both master and slave can transfer data at the same time, but during practical implementation of SPI, such duplex mode of communication can be utilized only if the data transferred by Mater & Slave are logically independent. In Master mode of LPC2148, SSEL cannot be used as a Chip Select line for slave, as it is an input pin and will be useful when controller is configured as a slave. In this case a GPIO port pin can be dedicated to act as CS.

IV.

RPM

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T

The output signal received from the photoelectric sensor is used by the controller to determine actual speed of the motor. The important RPM measurement techniques are-  Frequency Measurement Method

 Period Measurement Method

 Constant Elapsed Time Method (CET)

The selection of any of the above methods depends on the range of RPM and desired accuracy. The Frequency measurement method is better for higher RPM value found in motors and turbines that typically run at thousands of revolutions per minute. Period measurement method is well suited for low RPM values. The Constant Elapsed Time method gives more accurate output for both high & low RPM values. It is combination of both period & frequency measurement methods [1]. The method can be implemented by using on-chip timers of microcontroller.

V.

S

T

R

&

A

Copyright © 2013 IJECCE, All right reserved determine the system accuracy. A graphical representation

of the above readings is shown along with the approximate line equation to show the relation. The error and % error values are plotted for the specific instances.

A.

Mean Error:

The mean error for the above set of data is calculated by averaging the individual difference between measured RPM & tachometer RPM readings. Here the Mean Error is approximated to - 0.41

Fig.5. Graph showing Measured RPM against Tachometer RPM

Fig.6. Graph showing Absolute Error & % Error in RPM measurement

B.

Standard Deviation:

Standard deviation determines how accurately the system achieves desired RPM and its deviation from the standard RPM measurement tool i.e. tachometer reading. A smaller value of this parameter is desired for higher system accuracy. From the system test results and the mean error calculated, the standard deviation for the above set of error values was calculated to be 0.43, inferring that the error value’s are most likely to be between ± 0.43 of the central tendency.

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VI.

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The above work demonstrates a wireless control over PMDC motor, but the design can be imitated for any other applications. It can be inferred from the graphs in fig. 5 and fig. 6 that the RPM measured by embedded module is nearly equal to that shown by standard tachometer. The maximum value of absolute error is 1.5 which is within permissible limits for this application. The accuracy of RPM measurement can be improved further by implementing constant elapsed time method. In addition, the use of shaft encoders will provide higher resolution, as they produce (1 to 5000 PPR) clearly defined and symmetrical pulses.

The range of wireless control over DC motors is determined by the ZigBee which operates satisfactorily upto 65 meters of line of sight and 25 meters indoors (in presence of obstacles). However, this range over which the motors can be wirelessly controlled, can be further improved by using intermediate ZigBee nodes, incorporating higher dBi antenna (typically 5dBi). An extension to above application could be achieved by forming a ZigBee network to monitor and control multiple motors.

Fig.7. Snapshots of actual system

A

this application for Chemical Engineering process. To Dr. B.D. Kulkarni, Chairman, In house project committee, NCL, for granting the funds for this development at NCL.

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[1] D. Denić, G. Miljković, D. Živanović; “Microcomputer Based

Wide Range Digital Tachometer”.

[2] Q12AB6FF50 photoelectric sensor product manual by banner engineering (www.bannerengineering.com).

[3] LPC2141/42/44/46/48 – User Manual, Philips.

[4] Wang Yuan , Chen Keshan , Xue Chao, Li Hongjian, “Design and Implementation for ZigBee Long-distance Wireless Data Transmission System”, The Tenth International Conference on Electronic Measurement & Instruments - ICEMI’2011. [5] K.Sreelekha, B.Nagaraju & K.Raghavendra rao “Design and

development of a microcontroller based system for measurment of RPM”, Electronics Laboratory, Department of Physics, Sri Krishnadevaraya University.

[6] Bill Conley,“Solving Industrial Monitoring Challenges through Wireless I/O”, B&B Electronics

[7] XBee®/XBee-PRO® OEM RF Modules - Product Manual by Digi International.

[8] Muthu Ramya.C, Shanmugaraj.M, Prabakaran.R, “Study on ZigBee Technology”, Electronics Computer Technology (ICECT), 2011 3rd International Conference on (Volume:6 ) ISSN: 978-1-4244-8679-3.

[9] MCP4921/22, 12-bit DAC with SPI interface, Microchip Technology Inc.

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Dnyanesh P. Joshi

was born in Pune, India on 24th _{May 1990. He}

received his B.E. (Electronics) from University of Pune in 2011. He’s currently working as Research Intern at National Chemical Laboratory, Pune. His work focuses on design & development of embedded control systems for chemical process plants.

Email: [email protected]

Rahul S. More

was born in Pune, India on 5th _{August 1989. He}

received his B.E. (E&TC) from University of Pune in 2011. He’s currently working as Research Intern at National Chemical Laboratory, Pune. His work focuses on implementing wireless solutions for current systems, sensor interfacing and development of supporting hardware for the application. Email: [email protected]

Dr. (Mrs.) Neelima S. Iyer

was born in Pune, India on 8th _{October 1961. She}

received her M.Sc. (Electronics) & Ph.D. (Instrumentation) from University of Pune and MS (Software Systems) from BITS, Pilani. At present, she’s Senior Principal Scientist & Chair. Instrumentation & Communication Unit, National Chemical Laboratory, Pune. Her research area includes Embedded System development, LabVIEW & Wireless Instrumentation. Email: [email protected]