# Design and Development of an Embedded Controller for a Hydraulic Walking Robot WLBOT
###### Abstract
MBBOT owns a PC104 main controller and four leg servo controllers with a 1 kHz joint control frequency [27]. [PERSON] et al. developed a low-cost FPGA-based decentralized control system for a MEMS-based distributed air-flow micromanipulator [28]. Dynamic motions of the legged robots have been studied for years, and many control methods have been proposed. HyQ uses dynamic torque control to be actively compliant, with an inner PID torque loop and an outer PID position loop [29]. It also proposes a position control combined with an admittance block to handle the environmental interaction efficiently while the robot is not-walking [30]. MIT Cheetah I is controlled by the proprioceptive impedance controller to achieve a stable gallop at 3.2 m/s which can allow reflex responses to external forces according to the predefined compliance [31]. ANYmal develops a cascaded joint torque, position, and impedance control to realize the joint tracking very accurately [32]. JINPOONG uses a PD plus feed-forward controller to walk stably on uneven terrain [33]. [PERSON] et al. utilized a virtual model control (VMC) in SCalf-II to reduce the contact force by 35.7% [15]. [PERSON] et al. designed a position/force control law on the base of the operation space dynamic equation and the maximum sinusoid tracking error was less than 8 mm [34]. [PERSON] et al. acquired a foot contact estimation method in the impedance control and proposed a compensation scheme using neural network in the real quadruped robot system [35]. [PERSON] et al. presented a VMC for quadrupedal dynamic locomotion, which consisted of a stance phase virtual model control for full control of the robot body and a swing phase virtual model control for control of swing legs [36]. [PERSON] et al. proposed an impedance controller to solve the ball-on-plate (BoP) problem for six-parallel-legged robots walking on irregular terrains based on seven six-dimensional(6D) force/torque(F/T) sensors [37]. [PERSON] et al. implemented a real-time position and force feedback control for the artificial muscle with microfluidic sensors [38]. [PERSON] et al. presented a PDMS micro-optofluidic chip [39] and identified a class of models representing the two-phase microfluidic flow in different experimental conditions [40]. This paper gives an introduction of the quadruped hydraulic wheeled-legged robot WLBOT, including its structure, hydraulic system, and control system. It concentrates on the embedded controller, including the hardware, software design, and control strategy. Both the joint position control and joint force control are introduced respectively. In joint position control, a PID controller with velocity feedforward compensation (VFC) is proposed and the Chebyshev filtering algorithm is utilized to reduce the impact of joint jitter. What's more, an impedance controller is designed to realize better locomotion behavior in joint force control. Finally, the test platform is constructed, where the feasibility of the design and the validity of the control algorithm is verified. This paper makes the following contributions:  
* The relationship between ADC-DMA interrupt cycle, data volume, and the sampling rate is researched and a 2 kHz high-speed control loop for the embedded controller is proposed to realize better control performance;
* A proportional-integral-derivative (PID) velocity feedforward compensation (VFC) controller is designed to realize the joint position control. Compared with the general PID, VFC can reduce the maximum joint tracking error by 39.13% and 71.31%;
* The Chebyshev filtering algorithm is utilized to reduce the vibration noise of joint signals;
* An impedance controller in the joint space is proposed to get the better locomotion behavior and compliant effect in the walking experiments.