# Design and Development of an Embedded Controller for a Hydraulic Walking Robot WLBOT
###### Abstract  
In order to meet the requirement for the real-time of the hydraulic walking robot (WLBOT) and the stability of its movement, an embedded controller is proposed, which takes charge of multi-sensor information processing and signal output of the servo valve. The controller is capable of receiving control command and sending processed information while communicating with an embedded single board computer PCM-3365 via Control Area Network (CAN) bus at a 200 Hz frequency. In this paper, an appropriate interrupt cycle is selected and a 2 kHz high-speed control loop is run after we research the relationship between analog-to-digital converter direct memory access (ADC-DMA) interrupt cycle, data volume, and sampling rate. Significantly, the control strategy of WLBOT joint is introduced and a proportional-intergral-derivative (PID) compound controller with velocity feedforward compensation (VFC) is realized. Meanwhile, the Chebyshev filtering algorithm is utilized to attenuate the vibration noise of joint signals. What's more, an impedance controller is designed to gain better locomotion behavior and compliance in joint force control. Finally, the joint angle tracking and robot walking experiments are implemented, where the feasibility of the design and the validity of the control algorithm is verified. The results show that the PID velocity feedforward compensation controller can reduce the maximum tracking error by 39.13% and 71.31% in the knee and hip joint and the impedance control can reduce the standard deviation (SD) of the foot force by 36.06% and 72.79%. hydraulic actuation; legged robot; embedded controller; control frequency; sampling +
Footnote †: journal: Applied Sciences  
MBBOT [19], National University of Defense Technology's (NUDT) quadruped robot [20] and Beijing Institute of Technology's (BIT) bionic quadruped robot [21] also continued to obtain advances in the fields of dynamic locomotion and control in recent years. Since legged robots usually need to implement multiple tasks at the same time, such as data collection, posture calculation, gait planning, actuation control, they often adopt a hierarchical control system, where the embedded controller often plays an important role. BigDog is controlled by a layered onboard computer that contains a PC104 Pentium processor (Intel Corporation, Santa Clara, California, U.S.) running a multi-rate real-time operating system under QNX as well as I/O cards and I/O interfaces. The high-level control system coordinates the behavior of the legs to regulate the velocity, attitude, and altitude of the body during locomotion in 200 Hz while the lower-level control system servos positions and forces at the joints in 1 kHz [22]. HyQ's control system architecture consists of a CPU board and five multifunction I/O boards. The CPU board is a Kontron MOPSPM104 board (Kontron S&T AG, Augsburg, Germany) running a real-time patched version of Linux (Linux kernel patched with real-time Xenomai), while five multifunction I/O boards are responsible for four legs and other sensors in a 1 kHz control frequency [23]. qRT-2 has a control architecture composed of the task level controller, the function level controller, the local controller, and the power controller [11]. The MIT Cheetah, whose control system is comprised of four layers: the motor drivers, the parallel UART emulators, the real-time multicore controller, and the monitoring PC, uses an i7 dual-core 1.33 GHz CPU as the real-time controller running two 4 kHz high priority loops [24]. ANYmal has three Intel NUC PCs (Intel Corporation, Santa Clara, California, U.S.) running the robot operating system (ROS) with a 400 Hz actuator control frequency, including inspection PC, navigation PC, and locomotion PC [25]. SCalf has a control system comprised of an industrial personal computer and a TMS320f28335 digital signal processor (DSP) (Texas Instruments Incorporated, Dallas, Texas, U.S.) with a 5 kHz sampling frequency and 1 kHz servo rate [26].