Abstract
The hydraulic drive unit (HDU) applied in a hydraulic drive legged robot joints adopts an outer loop impedance control method based on the hydraulic control's inner loop during motion so that the entire system obtains certain compliance characteristics. There are two types of inner loops, namely, the position closed loop control and the force closed loop control; this paper studies a system in which the inner loop is a force closed loop control system and improves its accuracy in order to provide references for the outer loop impedance control. Therefore, designing a compensation method that improves the inner loop force control's accuracy has important research significance. In view of the above research significance, this paper first deduces a mathematical model for the force closed loop control and simplifies the sixth-order mathematical model to find the transfer function of each part. Second, combined with influence factors, such as pressure-flow nonlinearity, friction nonlinearity, and complex and variable loads on the system, the feedforward compensation controller of the force control input is derived. Considering the practical application in engineering, the controller order is reduced, and a partial compensation is achieved. Finally, on the HDU performance test platform, the force control performance is quantitatively analyzed by inputting typical signals and random signals. The experimental results show that the feedforward compensation controller can greatly improve the system force control performance with different input signals. The above research results can be combined with a corresponding disturbance rejection strategy for the force control system to provide an important reference and experimental basis for the hydraulic inner loop control strategy of force-based impedance.
Highlights
Due to the rapid development of the robotic industry in recent years, there are many kinds of robots available to provide services for human society
In the author’s previous research, the force control performance and parameters and many aspects of the sensitivity characteristics of hydraulic drive unit (HDU) with PID controllers were studied [5], [22], but only the traditional PID controller is used to control the HDU force control system; its performance can not meet the requirements of the high accuracy of control required of the impedance control inner loop on robots’ legs [23]
After adopting the feedforward controller Gff (s) (1) By observing Fig. 6-Fig. 11, in combination with the data in Table 3, it can be seen that the tracing error of the HDU force control system is greatly improved when the input signal is sinusoidal, and the elimination rate does not obviously decrease under the different working conditions, which indicates that the feedforward compensation controller has certain robustness
Summary
1) An experimental test using the sinusoidal force signal the HDU force control system performs sinusoidal motion with amplitudes of 1000 N and 1500 N as shown in Table 2 without any position disturbance; the frequencies are 0.5 Hz, 1 Hz and 2 Hz, respectively. 2. After adopting the feedforward controller Gff (s) (1) By observing Fig. 6-Fig. 11, in combination with the data, it can be seen that the tracing error of the HDU force control system is greatly improved when the input signal is sinusoidal, and the elimination rate does not obviously decrease under the different working conditions, which indicates that the feedforward compensation controller has certain robustness. (2) Observing Fig. 12-Fig. 14, combined with Table 3, it can be seen that the HDU force tracing error is greatly reduced with the random input signal. It can be seen that the influence on the control effect of the system is significant, which verifies the effectiveness of the controller
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