Six Step Trapezoidal Control of a BLDC Motor Using Back-EMF
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1. a period of time when both pulses are inactive to allow for switching time in the transistors See the R8C 25 hardware manual for more detail on TimerRD used in Complementary PWM mode As mentioned above we need to apply voltages which are centered about the middle of the bus in order to allow zero crossings to be detected at the half bus reference We can do this type of voltage driving to the motor phases using voltages symmetric about the half bus level We use complementary PWM and a reference level of 50 PWM duty for both the higher and lower side voltages on two of the phases We set up the PWM output near a value of 50 for both driven phases but increase one phase duty by some amount and lower the other phase the same amount For example if we drive phase U to 60 duty then we can drive phase V at 40 duty the difference is 20 as will be seen by the motor phases and the middle of the two will still be 50 where we are detecting the zero crossing on phase W A look at the voltage present at phase W will still have the PWM components of the other two phases but once filtered the PWM components of phases U and V will be removed leaving the correct representation of the phase W voltage When the phase W signal crosses the middle of the bus it is also the same angle where phase U and V are equal and opposite and also balanced around the middle of the bus so our assumption of symmetry allows us to detect that phase W has crossed zero w2. 5 of 10 R8C 25 Group L LEN ESAS Six Step Control of BLDC Motor Using Back EMF Since the BEMF detection is done by observing the phase voltage which is a function of speed and of the driving voltage of our PWM it is necessary to get the motor running at some speed before the BEMF voltage is large enough to be detected So for starting the motor from full stop where no BEMF voltage exists we have to force the motor to spin first before we can use BEMF detection to continue the commutation For that we use open loop alignment and start up techniques 4 Open Loop Alignment and Rotation Starting The method of commutating by detecting BEMF voltages requires that the motor is already spinning and generating a reasonable amplitude BEMF signal before commutation can be done To make that happen we first force the rotor into a known position in an alignment process This can be done by setting the voltage on Phase U higher than V and W That is done by setting PWM counts for U higher and the counts in V and W smaller Though alignment can be done with only two phases by applying the same voltage to two of the phases produces a natural damping effect to help the motor settle more quickly into the alignment position l Figure 7 Concept of alignment current We slowly increase this voltage that is the PWM duty to not impart an impulse to the motor which may cause it to oscillate We do this process for up to several seconds to allow
3. EMF Signals for COMMUtALION ccccccccseeseeeeeeeeeeeeeeeeseeeeeeeeesuaeeeeeeeeeeessaaaeeseeeess 3 3 Driving BLDC Motor for Back EMF Commutation cccccsseeseeeeeeeeeeeeeeeeeeeeeeeeaeaesceeeeeeeeseaaseeeeeeess 5 4 Open Loop Alignment and Rotation Starting cccccccccccsssssecceeeeeesseeseeeeeeeeeesseeeeeeeeeeeseseeaseeeeeeseeaas 6 5 How to deal with Inductive Effects cccccccccssssssseeceeeeeeeeeeseeeeeeeesseeeeceeeeeeesseeasseeeeeesseeesseeeeeeesnnaas 7 UU ce PEE AN T EE E E A AAEE ENT 8 le POOIER 9 REU05B0073 0101 Rev 1 01 February 2009 Page 1 of 10 R8C 25 Group E LEN ESAS Six Step Control of BLDC Motor Using Back EMF 1 Motor Back EMF Signal Characteristics A 3 phase Permanent Magnet Brushless DC PM BLDC motor generates 3 phase AC voltages shown in Figure 1 called a Back Electro Motive Force also Back EMF or BEMP as it rotates The amplitude and frequency of the generated voltage are proportional to the rotational speed If the motor has a single pole pair on the rotor one North and one South pole then the Frequency of the BEMF voltage will be the same as the rotational speed of the rotor If however it has multiple magnetic poles as motors often do then the frequency will be will be higher in proportion to the number of pole pairs on the rotor The voltage generated will be determined by the number of turns in the windings and the strength of the rotor magnet There will be a relati
4. MF signals during hall commutation if available or empirically by selecting arbitrary speeds and checking results Once the motor has accelerated in the open loop to our switch point it is just a matter of changing over to the BEMF modulation method where the comparator output state determines the motor phase drive Once BEMF commutation kicks in the motor current can be reduced since the voltage fed will now align with the motor BEMF signal increasing the torque It is now possible to increase the ramp of applied voltage and accelerate the motor at a higher rate to the speed desired Figure 8 shows a waveform of this as the motor begins with alignment current accelerates with open loop current then when switched to BEMF commutation the current required goes down significantly Figure 8 Speed and current waveforms 5 How to deal with Inductive Effects Commutating a BLDC motor where the windings always have some inductance means that there will be inductive effects from changes in current on the voltage waveforms The nature of inductance is to resist changes in current and larger changes in current require large voltage to be applied Likewise when current is forced to change during the switching action of the transistors large voltages can occur spontaneously in the windings often called inductive kick When a phase is switched to float the remaining current will decay during which time the inductive kick will force the voltage
5. Motor Using Back EMF Motor BEMF Figure 2 Rotating vector representation of motor phases 2 Detecting Back EMF Signals for Commutation The voltage present on the motor phases is a combination of those generated internally and of those being driven externally by our transistors with PWM voltages So detecting BEMF on an undriven motor is easy but not so when we are also trying to force current into the windings to generate torque and cause it to spin The internal voltages are due to the BEMF and to affects from the resistance and inductance of the motor which are functions of the current flowing in the windings The PWM voltages that we use to drive the motor windings externally will likely be much higher than the BEMF voltages we are trying to detect Despite this we need some way of seeing the BEMF voltage near the time when the phase crosses zero so we can decide when to commutate In order to get a clean representation of the BEMF voltage it is necessary to filter the high frequency PWM signals from the motor phase voltage leaving only the lower frequency BEMF signal After that we can also deal with the voltages due to the resistive and inductive effects For example if we use a common PWM carrier frequency of 12 kHz the BEMF frequency will likely be much lower around a few hundred Hz The exact value of the BEMF frequency can be found as BEMF_Hz RPM x 1 min 60 sec x Number_of_Pole_Pairs So for a motor with 5 pole pairs runnin
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9. ed we begin looking at the comparator output for a change which would indicate a valid change in step Once that is detected we begin the timer again to mask the next voltage pulse for the same period of time Figure 9 Filtered Back EMF and comparator output Figure 9 shows a graph showing the comparator output including the voltage pulse to be masked The blue trace bottom is the filtered BEMF and the red trace top is the comparator output with extra edges The first edge is the valid one the next two or three quickly following are the invalid edges caused by the inductive pulse If we imagine the same comparator signals but without the extra edges caused by the inductance then what we find is that the three comparator signals are nearly identical to what we would expect from Hall device signals So we can use the comparator signals in much the same way we would with Hall signals and use their state to determine the commutation state This is exactly what we do in the sample motor code supplied with the YMCRPR8C25 kit The BEME code uses the three comparator edges filters out the unwanted edges and uses their state in a lookup table to decide which transistors are switched to high low or float 6 Summary In this application note we described a method of commutating a 3 Phase Brushless DC BLDC motor using Back EMF BEMP signals The Renesas YMCRPR8C25 Motor Control kit is an excellent vehicle for evaluating BEMF control us
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11. g at 3000 RPM the BEMEF frequency is 250 Hz The difference between 12 kHz and 250 Hz allows us some room to create a low pass filter that will pass the BEMF component while attenuating the PWM component ilps Fpi AE Figure 3 Raw and filtered motor voltage Assuming the bus voltage is higher than the logic level voltages then the motor voltage should be also divided down to scale it to those that can be observed by the comparator logic The circuits used to scale down and filter the motor phases voltages are shown below in Figure 4 The U V and W are the motor lines with PWM and BEMF components REU05B0073 0101 Rev 1 01 February 2009 Page 3 of 10 R8C 25 Group L LEN ESAS Six Step Control of BLDC Motor Using Back EMF up to SOV divided by a factor of 10 and the V_U V_V and V_W lines are the scaled and filtered BEMF signals which are fed into comparators 10k 0 015uF 0 015uF 0 015 uF T V_U T Figure 4 Filtering and Scaling of phase voltages to match the logic levels Once we have the BEMF voltages filtered and scaled we compare it with a reference point derived from the bus voltage We do this because the motor phases are modulated from the bus and if the bus voltage varies by any amount the reference and phase voltage will remain proportional One difference from the motor filter and divider is that the bus reference is scaled at half the value In this way the bus reference will always be at a poin
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13. ing the R8C Tiny Microcontroller devices Offering a rich set of Motor Control peripherals the R8C25 series is specifically suited for 3 phase motor control including the BEMF method described here as well as Hall based commutation and AC Induction motor control REU05B0073 0101 Rev 1 01 February 2009 Page 8 of 10 R8C 25 Group L LEN ESAS Six Step Control of BLDC Motor Using Back EMF 7 Reference YMCRPR8C25 Kit User Manual Application Notes REU05B0074 0100 Rev 1 00 Six Step Trapezoidal Control of a BLDC Motor Using Hall Sensors REJ05B0845 0100 Rev 1 00 Timer RD in Complementary PWM Mode REJ05B0486 0100Z Rev 1 00 Solutions for Three Phase Motor Control Programmin Hardware Manual R8C 24 Group R8C 25 Group Hardware Manual Rev 1 0 YMCRPR8C25 Motor Control Demo Kit Use the latest version on the home page http www renesas com Website and Support Renesas Technology Website http www renesas com Technical Contact Details Global csc renesas com Inquiries http www renesas com inquiry Revision Record Description Rev Date Page Summary 1 00 Feb 05 08 First edition issued 1 01 Feb 02 09 10 Disclaimer changed REU05B0073 0101 Rev 1 01 February 2009 Page 9 of 10 R8C 25 Group E gE NESAS Six Step Control of BLDC Motor Using Back EMF Notes regarding these materials This document is provided for reference purposes only so that Renesas customers may select the appropriate Renesas products for their u
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15. ithout measuring the neutral point directly Voltage drops caused by the resistance in the other phases are cancelled since the current through the other phases must also be equal In the software this method of symmetrical modulation is easy to implement For any given duty desired we normalize it to the range of 50 duty then add or subtract that value from a set value of 50 for two of the phases So if we have 1024 counts of PWM total meaning that a value of 1024 will give us 100 duty PWM on the upper channel and 0 on the lower then 50 for both upper and lower in complement will be a timer compare value 512 For a motor needing apparent 50 duty we actually add 25 to 50 for one phase and subtract 25 from 50 for the other creating a total of 50 but each only 25 offset from the 50 reference We can now guarantee that the motor neutral will be very near the center of the bus when the undriven phase crosses zero on its BEMEF output Of course only two of the phases are being driven at one time the floating phase is the one being observed for BEMF crossing Once a decision is made to commutate and switch the transistors the floating phase should be the one next approaching the zero crossing We use the data from the present commutation state or Step along with the zero crossing detection to continue moving the motor to the next Step and so on Figure 6 Simulated PWM signals REU05B0073 0101 Rev 1 01 February 2009 Page
16. of a floating phase to discharge to one of the power rails through diodes parallel with the transistors The length of time that this current discharge happens is proportional to the amount of current to be dissipated and inversely to the difference in voltage between the phase BEMEF and the power rail Under low current conditions the voltage pulse generated by the inductive kick is short in duration Under larger current as when the motor load is larger the time for the voltage to decay is longer It is important to note that this time must not be so long as to prevent the detection of the normal zero crossing of the undriven phase or BEMF commutation is not possible Assuming the current discharge time is short enough then a masking interval can be used to ignore the pulse at the output of the comparator Selecting the right masking time is critical since if masking time is REU05B0073 0101 Rev 1 01 February 2009 Page 7 of 10 R8C 25 Group L LEN ESAS Six Step Control of BLDC Motor Using Back EMF too short invalid commutation signals will cause currents to be switched to the wrong phase and decrease motor efficiency If the masking time is too long the commutation rate is limited therefore the motor speed will be limited We use a timer in the software to mask the voltage pulses This count is defined in software as a count on the PWM timer and can be changed for different motors and load conditions Once the masking time interval has pass
17. onship then that can be established with the motor where it will generate a particular AC voltage when it is spinning at a particular speed This value can be expressed as a constant Ke it equals the volts generated at some RPM usually 1000 RPM Motor manufacturers usually provide the value of this constant Phase to phase Voltage BEMF Figure 1 Voltage and frequency measured externally at rotating BLDC motor In most BLDC motors we normally only have access to the three windings not to the center tap neutral so the voltages have to be seen between two phases where it is the difference of the voltages between those windings If one voltage is near maximum and the other is near minimum then the voltage seen between them will be larger than if the two phases were both positive or were both negative relative to the neutral For the purpose of commutation with the BEMF signals we want to know when a phase voltage crosses zero with respect to the neutral As it happens when one phase is crossing neutral the other two phases are at equal and opposite voltages so we can detect a zero crossing of one phase when its measured voltage is exactly between the other two For purposes of this document we will assume that BLDC motors are always constructed symmetrically with similar BEMF resistance and inductance in all three phases REU05B0073 0101 Rev 1 01 February 2009 Page 2 of 10 R8C 25 Group L LEN ESAS Six Step Control of BLDC
18. s products or if you have any other inquiries Note 1 Renesas Electronics as used in this document means Renesas Electronics Corporation and also includes its majority owned subsidiaries Note 2 Renesas Electronics product s means any product developed or manufactured by or for Renesas Electronics tENESAS APPLICATION NOTE R8C 25 Group Six Step Trapezoidal Control of a BLDC Motor Using Back EMF Introduction This document describes a method of commutating a 3 Phase Brushless DC BLDC motor using Back EMF BEMF signals This document is intended to supplement the understanding of 6 step BLDC motor control using BEMF detection as one of many possible methods of commutation The BEMF detection method as well as the Hall device method are implemented on the Renesas YMCRPR8C25 Motor Control Evaluation kit which provides an excellent platform for developing the algorithm for individual applications The reader should already have some understanding of six step commutation methods such as Hall commutation to provide background to the techniques described here The reader is encouraged to first read the 6 Step Motor Drive with HALL sensor Application Note referred to in the Reference section at the end of this document Target Devices R8C 24 and R8C 25 Contents 1 Motor Back EMF Signal Characteristics cccccccccssseecceeceeeesceeeeeeeeceeeeseeeeceeeesaeeseeeessaaeeeessseseeees 2 2 Detecting Back
19. se Renesas neither makes warranties or representations with respect to the accuracy or completeness of the information contained in this document nor grants any license to any intellectual property rights or any other rights of Renesas or any third party with respect to the information in this document Renesas shall have no liability for damages or infringement of any intellectual property or other rights arising out of the use of any information in this document including but not limited to product data diagrams charts programs algorithms and application circuit examples You should not use the products or the technology described in this document for the purpose of military applications such as the development of weapons of mass destruction or for the purpose of any other military use When exporting the products or technology described herein you should follow the applicable export control laws and regulations and procedures required by such laws and regulations All information included in this document such as product data diagrams charts programs algorithms and application circuit examples is current as of the date this document is issued Such information however is subject to change without any prior notice Before purchasing or using any Renesas products listed in this document please confirm the latest product information with a Renesas sales office Also please pay regular and careful attention to additional and different informa
20. t where we can detect when the phase voltage is above or below the 50 value which we will see is a convenient way for detecting the zero crossing in the BEMF voltages MOTOR PHASE VOLTAGE BUS VOLTAGE Figure 5 Simplified BEMF comparator signals The next step is to modulate the motor in such a way as to keep the motor neutral very close to half the bus voltage so that when a phase does a zero crossing it can be detected by the comparator at the half bus reference point REU05B0073 0101 Rev 1 01 February 2009 Page 4 of 10 R8C 25 Group L LEN ESAS Six Step Control of BLDC Motor Using Back EMF 3 Driving BLDC Motor for Back EMF Commutation For complementary PWM each comparator register in the PWM timer produces two PWM outputs not just one The two complementary signals are used in one phase of the three phase PWM so there are a total of three comparator registers and six PWM outputs or three pairs One of each pair is designated as the upper transistor driver and the other is the lower We use the convention that as the PWM compare value increases the upper transistor duty for that phase becomes larger and the lower duty becomes smaller The two signals are complementary in that they are the logical inversion or complement of the other so that the two signals are not active at the same time In reality the TimerRD also allows deadtime settings which introduces
21. the rotor to align with the currents and to allow any spontaneous rotational oscillation to dampen out Small motors with low inertia may align in less than a second while large inertia loads require more time Once we are sure the motor is in a known alignment state we begin sending a slowly accelerating stepped current into the motor phases to get the motor to spin in the desired direction We commutate as in Figure 6 above increasing and cycling through the Step count as we go The initial duty cycle is selected to overcome the frictional forces and inertia of the motor We increase the voltage sent to the phases as well as increase the step rate to force the motor to turn and accelerate overcoming the motor friction inertia and the increasing back EMF Because there is no commutation feedback during this stage to keep the currents aligned with the rotor the open loop currents must be larger than normal to maintain the torque It is important therefore to set the alignment voltage and time as well as the ramp rate and voltage to match the motor and load to ensure a reliable start REU05B0073 0101 Rev 1 01 February 2009 Page 6 of 10 CCECECENICO Aq R8C 25 Group EAT NS Six Step Control of BLDC Motor Using Back EMF The software should increase the open loop commutation switch rate until such time that the BEMF is large enough to generate reliable signals into the comparators That rotational speed can be found by examining the BE
22. tion to be disclosed by Renesas such as that disclosed through our website http www renesas com Renesas has used reasonable care in compiling the information included in this document but Renesas assumes no liability whatsoever for any damages incurred as a result of errors or omissions in the information included in this document When using or otherwise relying on the information in this document you should evaluate the information in light of the total system before deciding about the applicability of such information to the intended application Renesas makes no representations warranties or guaranties regarding the suitability of its products for any particular application and specifically disclaims any liability arising out of the application and use of the information in this document or Renesas products With the exception of products specified by Renesas as suitable for automobile applications Renesas products are not designed manufactured or tested for applications or otherwise in systems the failure or malfunction of which may cause a direct threat to human life or create a risk of human injury or which require especially high quality and reliability such as safety systems or equipment or systems for transportation and traffic healthcare combustion control aerospace and aeronautics nuclear power or undersea communication transmission If you are considering the use of our products for such purposes please contact a Renesas sales offi
23. uard against the possibility of physical injury and injury or damage caused by fire in the event of the failure of a Renesas product such as safety design for hardware and software including but not limited to redundancy fire control and malfunction prevention appropriate treatment for aging degradation or any other applicable measures Among others since the evaluation of microcomputer software alone is very difficult please evaluate the safety of the final products or system manufactured by you In case Renesas products listed in this document are detached from the products to which the Renesas products are attached or affixed the risk of accident such as swallowing by infants and small children is very high You should implement safety measures so that Renesas products may not be easily detached from your products Renesas shall have no liability for damages arising out of such detachment This document may not be reproduced or duplicated in any form in whole or in part without prior written approval from Renesas Please contact a Renesas sales office if you have any questions regarding the information contained in this document Renesas semiconductor products or if you have any other inquiries 2008 Renesas Technology Corp All rights reserved REU05B0073 0101 Rev 1 01 February 2009 Page 10 of 10
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