In most industries, electric systems have been taking over the major role of a function in a whole system from conventional mechanical or hydraulic system. In this technical trend, the electric motor is by far the most important actuator, so it is used in vast range of industries.
Especially, in...
In most industries, electric systems have been taking over the major role of a function in a whole system from conventional mechanical or hydraulic system. In this technical trend, the electric motor is by far the most important actuator, so it is used in vast range of industries.
Especially, in automotive industry, the usage of electric motor is rapidly increasing to fulfill driver’s needs for more efficient and more comfortable feature. There are various types of electric motors in the market, but due to the great advantage of high efficiency and power density, the Permanent Magnet Synchronous Motor (PMSM) and its driving system are the most widely adopted in automotive industries.
With this PMSM advantage, nowadays, the Electric Power Steering (EPS) system is very popular in the market. EPS system reduces the driver’s steering effort by assisting it with the electric motor torque. When a driver turns steering wheel, torque sensor notices the driver’s steering intention and generates electric command signals with respect to the torsional angle between input and output shaft. The command signals are transmitted to the current controller, and the controller drives the electric motor to generate steering assist torque.
Despite all its merits, in an EPS system, sudden failures originated from the electric motor drive system can result in safety critical issues, such as unintended steering and sudden Loss Of Assist (LOA). In conventional safety management protocol, when the EPS system has critical failures, the electric drive system is to be shut down and system turns to the manual steering mode in which on motor torque is assisting, it is mainly to protect the system and drivers from any unintended incident. However, when a sudden LOA occurs in the EPS system through abrupt shut-down, the driver is to have unwanted disturbances, such as heavy steering torque and it can create a serious accident. To prevent this unwanted incident and enhance driver’s safety in the EPS system, these days, it is highly demanded that the sudden LOA should be precisely and effectively mitigated and managed.
To conform to this technical demand and trend, this paper suggests effective mitigation method of system failure in an EPS system with PMSM drive system. Electric motor drive failures can be categorized by failure entities as following three major area : 1) by failures by various sensors such as current sensor or position sensor, 2) by inverter component and interface such as open and short circuit and 3) by motor itself.
At first, the various sensor failures can be addressed by using sensor-less drive. For instance, in case that a rotor position sensor failure occurs, there are a couple of ways to estimate rotor position and speed information. One is to use phase currents to calculate the rotor position, and the other is to use back EMF which is generated while rotor rotates. As an another example, current sensor failure also can be assumed. When a current sensor is failed, there can be three different ways to obtain phase current information of motor. Phase current can be estimated either by sensor-less control method where current can be calculated using phase voltage, rotor speed and motor parameter such as stator resistance, stator inductance and flux linkage, or by using remaining current sensor which does not show any failure, or DC link current can be used to calculated 3 phase current of motor. However, these methods need high performance processor such as Digital Signal Processor (DSP) and much more effort to address electric noise along with higher system cost. Secondly, failures by inverter component and interfaces can be addressed by several studies which suggested modification of standard 3 phase inverter topology, so that it can obtain some tolerance to failure in its legs. The third failure mode by motor itself can be mitigated by using special motors with multiple phase winding or dual star winding. These mitigation methods also need hardware modifications and higher cost.
Among these three categories, this paper deals with mitigation methods for following two failures through ‘Control’ : 1) current sensor failure and 2) one phase open failure of motor and shows these control methods effectively reduce the driver steering effort without any hardware modification in the EPS system.
At first, for current sensor failure, the mitigation strategy is achieved by using current calculating method with the proposed simple current calculator and dead-time compensator. the d-q axes voltage references are needed to calculate phase current. However, because of the influence of voltage drop due to dead-time effect of each phases, the voltage references are not equal to the actual phase voltage values supplied to the motor. Apparently, this voltage error can be reduced by sensing phase voltage directly, however, it requires additional hardware modification and cost increase as a downside. In this paper, the calculated current is achieved from electrical modeling of PMSM using dead-time compensation method which reduces calculated current error by dead-time effect.
Secondly, to mitigate one phase open failure through control method, a new torque control method for PMSM is proposed, which can control the motor continuously under one phase open failure condition. So far, once this one phase open failure occurs, the system is supposed to be shut down and turned into manual steering mode. In fact, in an EPS system point of view, the motor output torque can be generated by the remaining two phases like in single phase synchronous motor. However, it generates zero torque regions and large output torque fluctuations. This will make problems like sudden change of steering torque and eventually results in unstable driver feeling. The key point of the proposed torque control concept is to allocate target reference current, using phase shifting under open failure for each phase.
These two mitigation control methods are implemented for an EPS system with the rated 5.01 Nm PMSM torque. To verify the effectiveness and performance of the proposed method, the MATLAB simulations, experimental tests with real PMSM and EPS system are carried out. The load conditions can be achieved by measuring Rack and Pinion gear system with a spring forced load which corresponds to equivalent vehicle conditions. In terms of safety view points, there are two simulation and test conditions representing real-life. One is the city drive mode and the other is the lane change mode on the highway. The experimental results show that the proposed failure mitigation methods demonstrate good performance result with an EPS system.
In most industries, electric systems have been taking over the major role of a function in a whole system from conventional mechanical or hydraulic system. In this technical trend, the electric motor is by far the most important actuator, so it is used in vast range of industries.
Especially, in automotive industry, the usage of electric motor is rapidly increasing to fulfill driver’s needs for more efficient and more comfortable feature. There are various types of electric motors in the market, but due to the great advantage of high efficiency and power density, the Permanent Magnet Synchronous Motor (PMSM) and its driving system are the most widely adopted in automotive industries.
With this PMSM advantage, nowadays, the Electric Power Steering (EPS) system is very popular in the market. EPS system reduces the driver’s steering effort by assisting it with the electric motor torque. When a driver turns steering wheel, torque sensor notices the driver’s steering intention and generates electric command signals with respect to the torsional angle between input and output shaft. The command signals are transmitted to the current controller, and the controller drives the electric motor to generate steering assist torque.
Despite all its merits, in an EPS system, sudden failures originated from the electric motor drive system can result in safety critical issues, such as unintended steering and sudden Loss Of Assist (LOA). In conventional safety management protocol, when the EPS system has critical failures, the electric drive system is to be shut down and system turns to the manual steering mode in which on motor torque is assisting, it is mainly to protect the system and drivers from any unintended incident. However, when a sudden LOA occurs in the EPS system through abrupt shut-down, the driver is to have unwanted disturbances, such as heavy steering torque and it can create a serious accident. To prevent this unwanted incident and enhance driver’s safety in the EPS system, these days, it is highly demanded that the sudden LOA should be precisely and effectively mitigated and managed.
To conform to this technical demand and trend, this paper suggests effective mitigation method of system failure in an EPS system with PMSM drive system. Electric motor drive failures can be categorized by failure entities as following three major area : 1) by failures by various sensors such as current sensor or position sensor, 2) by inverter component and interface such as open and short circuit and 3) by motor itself.
At first, the various sensor failures can be addressed by using sensor-less drive. For instance, in case that a rotor position sensor failure occurs, there are a couple of ways to estimate rotor position and speed information. One is to use phase currents to calculate the rotor position, and the other is to use back EMF which is generated while rotor rotates. As an another example, current sensor failure also can be assumed. When a current sensor is failed, there can be three different ways to obtain phase current information of motor. Phase current can be estimated either by sensor-less control method where current can be calculated using phase voltage, rotor speed and motor parameter such as stator resistance, stator inductance and flux linkage, or by using remaining current sensor which does not show any failure, or DC link current can be used to calculated 3 phase current of motor. However, these methods need high performance processor such as Digital Signal Processor (DSP) and much more effort to address electric noise along with higher system cost. Secondly, failures by inverter component and interfaces can be addressed by several studies which suggested modification of standard 3 phase inverter topology, so that it can obtain some tolerance to failure in its legs. The third failure mode by motor itself can be mitigated by using special motors with multiple phase winding or dual star winding. These mitigation methods also need hardware modifications and higher cost.
Among these three categories, this paper deals with mitigation methods for following two failures through ‘Control’ : 1) current sensor failure and 2) one phase open failure of motor and shows these control methods effectively reduce the driver steering effort without any hardware modification in the EPS system.
At first, for current sensor failure, the mitigation strategy is achieved by using current calculating method with the proposed simple current calculator and dead-time compensator. the d-q axes voltage references are needed to calculate phase current. However, because of the influence of voltage drop due to dead-time effect of each phases, the voltage references are not equal to the actual phase voltage values supplied to the motor. Apparently, this voltage error can be reduced by sensing phase voltage directly, however, it requires additional hardware modification and cost increase as a downside. In this paper, the calculated current is achieved from electrical modeling of PMSM using dead-time compensation method which reduces calculated current error by dead-time effect.
Secondly, to mitigate one phase open failure through control method, a new torque control method for PMSM is proposed, which can control the motor continuously under one phase open failure condition. So far, once this one phase open failure occurs, the system is supposed to be shut down and turned into manual steering mode. In fact, in an EPS system point of view, the motor output torque can be generated by the remaining two phases like in single phase synchronous motor. However, it generates zero torque regions and large output torque fluctuations. This will make problems like sudden change of steering torque and eventually results in unstable driver feeling. The key point of the proposed torque control concept is to allocate target reference current, using phase shifting under open failure for each phase.
These two mitigation control methods are implemented for an EPS system with the rated 5.01 Nm PMSM torque. To verify the effectiveness and performance of the proposed method, the MATLAB simulations, experimental tests with real PMSM and EPS system are carried out. The load conditions can be achieved by measuring Rack and Pinion gear system with a spring forced load which corresponds to equivalent vehicle conditions. In terms of safety view points, there are two simulation and test conditions representing real-life. One is the city drive mode and the other is the lane change mode on the highway. The experimental results show that the proposed failure mitigation methods demonstrate good performance result with an EPS system.
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#PMSM EPS Mitigation
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