BLDC MOTOR BASED SYSTEM FOR DATA ACQUISITION AND CONTROL IN ORTHOTICS

Transcription

1 BLDC MOTOR BASED SYSTEM FOR DATA ACQUISITION AND CONTROL IN ORTHOTICS Stefan Karastanev Institute of Mechanics, Bulgarian Academy of Sciences Sofia, Bulgaria Radosveta Antonova Institute of Mechanics, Bulgarian Academy of Sciences Sofia, Bulgaria Mihaela Kalaidjieva Institute of Mechanics, Bulgarian Academy of Sciences Sofia, Bulgaria ABSTRACT. This paper presents application of BLDC motor based system for data acquisition and control in orthotics. Brushless Direct Current (BLDC) motors are one of the motor types rapidly gaining popularity. BLDC motors are used in industries such as appliances, automotive, aerospace, medical, industrial automation equipment and instrumentation. As the name implies, BLDC motors do not use brushes for commutation; instead, they are electronically commutated. BLDC motors have many advantages over brushed DC motors and induction motors. A few of these are: better speed versus torque characteristics, high dynamic response, high efficiency, long operating life, noiseless operation, higher speed ranges. In addition, the ratio of torque delivered to the size of the motor is higher, making it useful in applications where space and weight are critical factors. This method use high precision quadrant encoder in addition to Hall sensors built in the BLDC motor to increase precision of measurement of angles and creates corrective moments if necessary. In same time the system allows data acquisition of parameters of human gait and transfers collected data by wireless connectivity to the analyzing software. Proposed network architecture allows connecting of multiple controllers in a system which provides capability for analyze and control of complex movements of more than one joint. In this study covers two aspects of the system: software and hardware realizations. The proposed method estimation can improve measurement accuracy and commutation smoothness. KEY WORDS: Human gait, orthotics, BLDC motor, data acquisition 1. Introduction In the cases of neurological disorders (cerebral palsy, insult, spinal cord injury), muscularskeletal disorders (traumas, age changes) and pathologies of the ankle-foot complex can result in abnormal gaits and commonly treated with orthoses to partially compensate functional loss. For increasing stability and mobility of the ankle different type of orthoses are used. One of ankle-foot orthosis (AFO) function is to guide ankle in plantar and dorsiflexion. In some cases, such as cerebral palsy or spinal cord injuries (SCI), the AFO is used to avoid excessive plantar flexion which is one of the causes of drop foot gait [1]. The orthoses can be passive mechanisms that support weakened or paralyzed body segments, or active devices that assist joint motion using an external power source. Blaya and Herr [2] developed active AFO to assist drop foot based on linear serial elastic actuator to assist ankle motion and it uses plantar sensors and potentiometers to detect gait phases. To control the orthosis, it is necessary to identify the gait cycle and its phases. A gait cycle is divided into two phases: a stance and a swing. A stance phase starts with a heel contact and continues with subphases: loading response, mid stance, terminal stance and preswing. The separation of toes from the ground marks the beginning of swing phase which involves the following subphases: initial swing, midswing and terminal swing [3,4] (Fig. 1.). Recently Brushless Direct Current (BLDC) motors have gained tremendous popularity [5]. BLDC motors are used in industries such as appliances, automotive, aerospace, consumer, medical, industrial automation equipment and instrumentation. Quintero et al. [6] presented a powered lower limb orthosis that controls hip and knee motion with brushless DC motors. BLDC motors have advantages over brushed DC motors and induction motors. They have better speed versus torque characteristics, high dynamic response, high efficiency, long operating life, noiseless operation, higher speed ranges, rugged construction and so on. Also, torque

2 delivered to the motor size is higher, making it useful in applications where space and weight are critical factors. Fig.1. Gait phases in a normal gait cycle Similar to brushed motors, brushless DC motor works on the same principle of magnetic field switching. However brushless DC motors are electronically commutated rather than using brushes. In the servo motor version, this is accomplished using Hall Effect sensors [7] or encoder feedback. They are mainly designed for high performance applications where high speed and continuous torque are essential requirements. Also brushless DC motor allows for better heat dissipation because the windings are located on the stator. As opposite of brushed DC motor, the commutation of a BLDC motor is controlled electronically. To rotate the BLDC motor, the stator windings should be energized in a sequence. It is important to know the rotor position in order to understand which winding will be energized following the energizing sequence. Based on the above discussion, brushless DC motor was considered the best option for data acquisition system and control in orthotics. 2. Methods There is a need for system that can provide a wireless, real time and reliable result to measure and control in ankle foot complex movements. In this article we introduce system based on BLDC motor attached to ankle foot orthosis (AFO) and give capability for acquisition, analysis and control in complex movements of more than one joint (Fig.2). A method use high precision quadrant encoder in addition to Hall sensors built in the BLDC motor to increase precision of measurement of angles and creates corrective moments if necessary. Encoders are sensors that generate digital signals in response to movement. Optical encoders are devices that convert a mechanical position into a representative electrical signal by means of a patterned disk or scale, a light source and photo-sensitive element. Brushless DC motors require precise absolute position feedback in order to be properly electronically commutated. In order to minimize torque ripple introduced by the method of block commutation, sinusoidal commutation is used to smoothly commutate the motors. Torque ripple is highly undesirable in a system to be used for manipulation and locomotion. Fig.2. AFO and BLDC motor system with wireless communication XBee The block diagram on Fig.3 depicts how the BLDC motor controller is driven and how optical incremental encoder [8] read using a dspic30f2010. The actual speed value is determined via this encoder which ensures high precision of speed measurement especially in low speed ranges.

3 Fig.3. Block Diagram BLDC motor controller The six MC PWM outputs (PWM1L to PWM3H) are connected to three MOSFET driver pairs (IR2101S), which in turn are connected to six MOSFETs. These MOSFETs are connected in a threephase bridge format to the three BLDC motor windings. In the current implementation, the maximum MOSFET voltage is 70 Volts, and the maximum MOSFET current is 6 Amps. The motor is a 24V BLDC motor so the DC+ to DC- bus voltage is 24V. A regulated 5V is provided to drive the dspic33fjxxmcxxx. The three Hall effect sensor inputs are connected to input pins that have Change Notification circuits associated with them (CN). These inputs are enabled along with their interrupt. If a change occurs on any of these three pins, an interrupt is generated. To provide some current feedback to the motor, a low value resistor (50 milliohms) is connected between the DC- bus voltage and ground or Vss. The voltage generated by this resistor is amplified by an external op amp (MCP6002) and fed to an ADC input. Optical encoder is connected to dedicated inputs of QUADRATURE ENCODER INTERFACE (QEI) MODULE. The QEI module provides the interface to incremental encoders for obtaining mechanical position data. The operational features of the QEI include: Three input channels for two phase signals and index pulse 16-bit up/down position counter Count direction status Position Measurement (x2 and x4) mode Programmable digital noise filters on inputs Alternate 16-bit Timer/Counter mode Quadrature Encoder Interface interrupts Integrated USART module provides necessary capabilities to build communication interface for motor control, parametrization and data acquisition. Wireless communication is realized by Xbee 24 module which is engineered to meet IEEE standards and support the unique needs of lowcost, low-power wireless sensor networks. The modules require minimal power and provide reliable delivery of data between devices. In the same time the system allows data acquisition of parameters of human gait and transfers collected data by wireless connectivity to the analyzing software. In the Table 1 is given some of the parameters of the BLDC motor. Model 42BLDC 85A-240 Rated power Rated voltage Table 1. Specification of Brushless DC Motor Rated N of Rated Torque Peak Current poles speed rated torque speed Moment constant Lenght Weight W V DC A rpm N.m N.m N.m/A mm kg

4 2.1. Sensor position A position sensor is required to determine the angle of the ankle joint. Since the device is actuated along a single axis in the sagittal plane, a single position sensor can be placed on the mechanical system such that it directly measures the ankle joint angle flexion and extension. Other locations of the position sensors are possible, but this will increase the complexity of the controller as well as add possible error scenarios. Position sensor information can be mathematically manipulated to obtain velocity Sensor requirements Sensors are required to provide kinetic and kinematic data information that can be relayed for measurement purposes in order to be able use it in the controller algorithm. The number and the types of sensors depend on the data that will be required for the control implementation. In general, position feedback is required for a position controller and force feedback for force/torque controller. The following data/information is critical: Instantaneous position and velocity of the ankle joint angle during any given stage of the gait cycle. Instantaneous Torque/force at the ankle joint during any given stage of the gait cycle. Stage of the gait cycle. Parameters for sensor selection should include: Weight of the sensor: Since the sensor system is attached to the patient s lower extremity, any additional weight should have a minimum effect on their rehabilitation capabilities. Accuracy: The sensor has to provide accurate measurements. This is most critical in determining the ankle joint angle during hyperextension. Repeatability: The sensor has to provide consistent measurement for the data acquired every time in order to control algorithm to exhibit robustness. Range: The position sensor has to have range of motion of the ankle joint. The force/torque sensor should be capable of measuring the maximum torque at the ankle joint. Signal Processing: Most sensor or transducers will require signal amplification and signal condition (noise filtering). A/D conversion: Almost all analog sensors will require an A/D conversion depending upon the type of microcontroller being used. 3. Experimental results In this study covers two aspects of the system: software and hardware realizations presented on Fig.4, 5 and 6. Fig.4. Motor control window and position curve editor

5 Fig.5. Integrated development environment microchip Fig.6. Integrated development environment QT On the Fig7 is given the real designed controller board. The controller meets the block diagram requirement with possibility to connect wireless module directly via built in RS-485 interface. Fig.7 Brushless controller with RS-485 interface

6 4. Discussion It is proposed a BLDC motor based system for data acquisition, analysis and control in complex movements in ankle joint. A BLDC microcontroller is mounted to AFO for measuring of position and data acquisition for human gait parameters and then transfer collected data by wireless connectivity to the analyzing software. Brushless motor application is useful in highly compact spaces with small motors. A method use high precision quadrant encoder in addition to Hall sensors built in the BLDC motor to increase precision of measurement of angles and creates corrective moments if necessary. In Table 2 are discussed the advantages and disadvantages of brushless DC motor and brushed DC motor. Based on above discussions, brushless DC motor is considered the best option for data acquisition system and control in orthotics. Table 2. Comparing a BLDC motor to a brushed DC motor Feature BLDC Motor Brushed DC motor Commutation Electronic commutation based on Brushed commutation Hall position sensors Maintenance Less required due to absence of Periodic maintenance is required brushes Life Longer Shorter Speed/Torque Characteristics Flat Enables operation at all speeds with rated load. Moderately flat At higher speeds, brush friction increases, thus reducing useful torque. Efficiency High No voltage drop across Moderate Output Power/Frame Size Rotor Inertia brushes High Reduced size due to superior thermal characteristics. Because BLDC has the windings on the stator, which is connected to the case, the heat dissipation is better Low, because it has permanent magnets on the rotor. This improves the dynamic response. Moderate/Low The heat produced by the armature is dissipated in the air gap, thus increasing the temperature in the air gap and limiting specs on the output power/frame size Higher rotor inertia which limits the dynamic characteristics Speed Range Higher No mechanical limitation imposed by brushes/commutator. Lower Mechanical limitations by the brushes. Electric Noise Generation Low Arcs in the brushes will generate noise causing EMI in the equipment nearby. Cost of Building Higher Since it has permanent Low magnets, building costs are higher Control Complex and expensive Simple and inexpensive Control Requirements A controller is always required to keep the motor running. The same controller can be used for variable speed control No controller is required for fixed speed; a controller is required only if variable speed is desired R E F E R E N C E S [1] ROMKES J, BRUNNER R. Comparison of a dynamic and hinge ankle-foot orthosis by gait analysis in patients with hemiplegic cerebral palsy. Gait & Posture, Vol.15 (2002) [2] BLAYA J, HERR H. Adaptive control of a variable-impedance ankle-foot orthosis to assist drop-foot gait. IEEE Int. Neural Systems and Rehab. Engineering, Vol.12 (2004) [3] PARRY J. Gait Analysis Normal and Pathological Function, Slack Incorporated: Thorofare, NJ, (1992) USA. [4] ZIJSLTRA W., AMINIAN K. Mobility assessment in older people: New possibilities and challenges, Eur. J. Aging 4 (2007), [5] YEDAMALE P. Brushless dc (bldc) motor fundamentals. An885:Application note, Microchip Technology inc., 2003.

7 [6] QUINTERO HA, FARRIS RJ, GOLDFARB, M. Control and Implementation of a Powered Lower Limb Orthosis to Aid Walking in Paraplegic Individuals. Proc. IEEE Int. Conf. on Rehab. Robotics (ICORR 11), Zurich, (2011), [7] Simpkins A., Todorov E. Position estimation and control of compact bldc motors based on analog linear hall effect sensors. American Control Conference Marriott Waterfront, Baltimore, MD, USA (2010), [8] Optical Encoder Applications. Computer Optical Products, Inc.