Chapter 4 Design of the Optimal Switching Mechanism
4.3 Switching Strategy for Both the MTPA and the HE Operation
4.3.2 Experimental verification of the designed strategy
To verify that the designed switching strategy is effective, we have performed various experiments with practical driving conditions. The first one was carried out with step 2.576 pu of is*
, and operation with MTPA only, HE only, and the designed switching strategy are compared in Fig. 4-14 and Table 4-4.
As seen from Fig. 4-14 and Table 4-4, operation with the switching strategy possesses both charateristics of MTPA (γ = 1.625) and HE (γ = 0.625) as: At start-up of the vehicle, the system is operated at the MTPA so that the maximum acceleration can be obtained. Then, the system is switched to the HE operation for higher efficiency and final speed as the speed increases until it exceeds 10 km/hr.
Fig. 4-13 Block diagram of overall control program implemented on the DSP controller
The second experiment for the verification was launched such that the driver can step on the pedal as one wants to control the vehicle. Results of control schemes with and without the switching strategy are shown in Fig. 4-16 and 4-15, respectively.
When the vehicle is driven without the switching strategy, the maximum acceleration can be obtained since γ is set as 1.625 to provide the MTPA. However, Fig. 4-15 (c), (d), (i), and (j) exhibit that the vehicle speed is limited to approximately 14.7 km/hr no matter how large the command is*
is fed. The reason is that strong rotor flux linkage is excited when the system is operated at the MTPA and it causes large back-emf. From the results shown in Fig. 4-3 we also know that the efficiency at such an operation is not desirable.
(a) Speed response (b) γ
Fig. 4-14 Experimental results of operation with the MTPA, the HE, and the switching strategy
0 2 4 6 8 10 12 14
0 5 10 15 20 25
Time (sec)
Speed (km/hr)
MTPA HE
Switching strategy
0 2 4 6 8 10 12 14
0.5 1 1.5 2
Time (sec)
Slip factor
Switching strategy MTPA HE
Table 4-4 Comparison of with the MTPA, the HE, and the switching strategy
Operation Operating Condition Acceleration (rad/s2)
Efficiency coefficient (rad/s.A)
Final speed (km/hr)
MTPA only γ = 1.625 39.27 38.11 14.7
HE only γ = 0.625 18.36 49.88 21.6
Switching strategy γ is automatically adjusted 39.27 49.88 21.6
Stator current magnitude command (pu)
0 2 4 6 8 10 12 14 16 18
Stator current magnitude command (pu)
0 2 4 6 8 10 12 14 16 18
Fig. 4-15 Experimental results of control schemes without the switching strategy
Stator current magnitude command (pu)
0 2 4 6 8 10 12 14 16 18
Stator current magnitude command (pu)
0 2 4 6 8 10 12 14 16 18
Stator current magnitude command (pu)
0 2 4 6 8 10 12 14 16 18
Stator current magnitude command (pu)
0 2 4 6 8 10 12 14 16 18
Fig. 4-16 Experimental results of control schemes with the switching strategy
Stator current magnitude command (pu)
0 2 4 6 8 10 12 14 16 18
Stator current magnitude command (pu)
0 2 4 6 8 10 12 14 16 18
After the switching strategy is added to the control scheme, γ can be automatically adjusted as shown in Fig.4-16(e), (f), (k), and (l) according to the driving conditions such as the vehicle speed and the stator current magnitude command controlled by the accelerator in order to obtain both the MTPA and the HE operations. Also, the vehicle speed is no longer restricted to 14.7 km/hr. The highest vehicle speed that can be obtained with the switching strategy is 21.6 km/hr as shown in Table 4-4.
In Chaper 3, we have already verified that the approach of equal d-q current command is not suitable for the real vehicle application, since the desirable MTPA operation is lost due to severe flux saturation. When the electric vehicle is driven along the 75-m straight path fed by the maximum stator current 2.576 pu, we notice that the operation with the switching strategy is the best among three operations as shown in Fig. 4-17. It can also be recognized from Table 4-5 that vehicle performance is signicantly improved by the desgined switching strategy.
Fig. 4-17 Speed responses obtained with different control strategies
0 2 4 6 8 10 12 14
0 2 4 6 8 10 12 14 16 18 20 22
Time (sec)
Speed (km/hr)
(1) Operation of the switching strategy (2) Operation of gamma-adjustment MTPA (3) Operation of equal d-q current command
(1)
(2)
(3)
To prove that the MTPA obtained by the proposed γ-adjustment and the switching strategy is better than the operation of the equal d-q current command, we also have performed an experiment on an inclined road locating at the campus for testing the capability of uphill climbing, where the system was fed by the maximum stator current as shown in Fig. 4-18.
It is obvious that the operation of the equal d-q current command cannot produce adequate torque to overcome the rolling resistance so that the vehicle gets stuck on the way and fails to climb the inclined road as seen from the experimental results shown in Fig. 4-19. On the other hand, the operation of the switching strategy does provide sufficiently large torque for uphill climbing, and the vehicle can steadily climb the road with around 10 km/hr. Note that γ is not switched to 0.625 since the condition for the switching where the speed is larger than 10 km/hr is not satisfied.
Table 4-5 Summary of performance obtained by different approaches
Operation Acceleration (rad/s2)
Efficiency coefficient (rad/s.A)
Final speed (km/hr)
Finish time of 75-m moving (s)
Equal d-q current command 23.51 13.35 10.2 25
γ-adjustment MTPA 39.27 38.11 14.7 18
Switching strategy 39.27 49.88 21.6 15
Improvement 67% 273.6% 111.8% 40%
(a) in front of the vehicle (b) behind the vehicle Fig. 4-18 View of the inclined road for testing uphill climbing ability
4.4 Summary
In this chapter, the development of a switching control strategy designed for obtaining both the maxmium torque and the highest efficiency operation of the electric vehicle is presented. Some important results are reviewed as follows.
(1) The γ-adjustment control approach that is proposed for a smooth driving characteritic possesses the following two merits:
The maximum torque per amperage (MTPA) and the highest efficiency (HE) operation can still be obtained by properly setting γ even though ids*
is set as 0.62 pu which is not at both optimal operating points as 0.82 pu for the MTPA and 0.42 pu for the HE:
It provides smoother operating performance than the ids*
-adjustment when the switching machanism is applied so that it is more suitable to traction control of the electric vehicle
Operation Slip factor setting
MTPA 1.625
HE 0.625
Fig. 4-19 Experimental results of the uphill climbing test
0 2 4 6 8 10 12
0 2 4 6 8 10 12 14 16
Time (sec)
Speed (km/hr)
(1) Operation of the switching strategy (2) Operation of equal d-q current command
(1): Successfully finish
(2): Fail to climb
(2) Switching strategy which can be automatically adjusted according to the driving conditions is properly designed with significant improvement on both developed torque and operating efficiency as follows:
When the vehicle is driven along the 75-m straight path fed by the maximum stator current, all the acceleration, efficiency coefficient, and final speed can respectively be improved by 67%, 273.6%, and 111.8% if the switching strategy is applied compared with the operation of equal d-q current command. Finish time of 75-m moving can also be shortened from 25 seconds to 15 seconds.
When the vehicle is driven along the inclined road fed by the maximum stator current, it can successfully climb the road with 10 km/hr if the switching strategy is applied while the operation of equal d-q current command is not able to provide sufficient torque for uphill climbing.
Chapter 5
Conclusion
In this research work, the high performance servo traction control of the 0.75-kW IM has been successfully implemented on the TMS320F28335 DSP micro-controller, and it is further improved by the designed automatic switching control strategy to obtain both the maxmium torque and the highest efficiency operation to provide satisfactory performance of the electric vehicle.
Accomplishments and contributions of this Thesis are summarized as follows.
1. High performance servo control of the IM is realized for the traction application (1) The rotor time constant of the IM is precisely identified
An acceleration-based identification process which can be directly performed on the vehicle with reasonable accuracy is developed to obtain the rotor time constant. The identified result is 0.08s, which is close to its nominal value 0.074s.
(2) The maximum torque per amperage (MTPA) operation is obtained by the rated flux excitation
The conventional approach of equal d-q current command is verified unreliable for the vehicle application in this study, since the desirable MTPA operation is lost owing to severe flux saturation of the IM. By the standard experimental process, it can be proved that MTPA operation can be obtained when the rotor flux linkage is exited at the rated value. With such arrangement, all the acceleration, efficiency coefficient, and the final vehicle speed can be improved by 65.5%, 173.2%, and 44.1%, respectively, when the maximum stator current is applied.
(3) The highest efficiency (HE) operation is obtained by setting proper flux current
In spite of the maximum torque produced with the MTPA operation, its operating efficiency and vehicle speed are quite limited. Therefore, the HE operation is found by the standard experimental process for the improvement of the efficiency and the vehicle speed obtained by the MTPA
operation. The former one is improved by 32.7% while the latter one is increased by 45.6%, but the price is the degraded acceleration performance obtained.
2. A switching control strategy which can be automatically adjusted based on the driving conditions is designed to obtain both the maximum acceleration and the highest operating efficiency of the electric vehicle.
A novel control method called the γ-adjustment control is developed for a smooth driving characteritic. Its merits are described as follows.
The MTPA and HE operation can still be obtained by properly setting γ even though ids*
is originally not set at both optimal operating points as 0.82 pu for the MTPA and 0.42 pu for the HE.
It provides smoother operating performance when the switching machanism is applied, and thus it becomes more reliable in traction control of the electric vehicle. Then, a switching strategy with which the MTPA operation is applied at a low speed for the maximum acceleration while the HE operation is applied at a high speed for improved operating efficiency and vehicle speed is designed based on the γ-adjustment.
Two experimetal testings have been applied to verify validity of the present control strategy to indicate the following improvements.
The first experiment is performed for testing vehicle performance with the maximum supplied stator current along a 75-m straight path. When the switching strategy is applied, all the acceleration, efficiency coefficient, and final speed can respectively be improved by 67%, 273.6%, and 111.8%, compared with the operation obtained by the equal d-q current command.
The time of 75-m driving can also be shortened from 25 seconds to 15 seconds.
The second experiment is launched for testing vehicle performance with the maximum supplied stator current along an inclined road. Results show that it can steadily climb the road with 10 km/hr if the switching strategy is applied while the operation of equal d-q current command is not able to provide sufficient torque for uphill climbing.
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