Background

Imagine: robotic partners assist everyone with their work, help travelers carry luggage at airports, and even provide comprehensive daily care for the elderly. This is the future life that PAL Robotics SL, based in Barcelona, Spain, aims to create. This innovative company is located in the heart of Barcelona's old town, just a few steps from the world-famous La Rambla pedestrian street.

PAL Robotics is a leader in the robotics industry; within their bustling office space, the design, programming, and assembly of robots proceed in an orderly manner, with engineers gathering in small groups to discuss how to continuously improve robot functionality.

Challenge  

PAL Robotics Chief Technology Officer Luca Marchionni points out that one of the major challenges is maintaining balance while the robot walks, something that seems taken for granted in humans.

Robot walking involves contact between the bipedal feet and the surrounding environment, as well as concurrent coordinated movements of various body parts across multiple degrees of freedom. The control system for a bipedal robot must handle transitions between two phases: double support (both feet on the ground) and single support (one foot on the ground).

Due to the nonlinear characteristics of robot motion kinematics, it is difficult to achieve walking control through simple program rules. Moreover, control via real-time state analysis methods is often not feasible because the trial-and-error process is too complex. Instead, the industry commonly employs a numerical method called "trajectory optimization," which predefines an "ideal walking path" for the robot and then uses this numerical method to calculate the best approximation closest to this path. The criterion for "best" is determined by a specific cost function that accounts for both the robot's ideal walking path and its physical constraints.

The joint design of humanoid robots has strict space and weight limitations, aiming to minimize the robot's volume and inertia. Most robots manufactured by PAL Robotics are similar in size to humans and possess up to 40 degrees of freedom.

PAL Robotics with RLS encoders

PAL Robotics with RLS encoders

Solution

The REEM-C and other humanoid robots manufactured by PAL Robotics feature fully articulated joints capable of performing a series of complex movements according to task requirements. High-quality encoder feedback data is required for servo control of torque, speed, and position at each joint. After gaining a deep understanding of PAL Robotics's various business needs and products, Renishaw provided encoder selection recommendations. PAL Robotics ultimately selected non-contact magnetic encoders produced by Renishaw's associate company RLS, including AksIM™ and Orbis™ rotary encoders integrated into the knees, wrists, and elbows, as well as component-level incremental RoLin™ encoders.

To achieve balance control, each foot of the robot is equipped with a force feedback measurement system that calculates the Zero Moment Point (ZMP)—a metric used to assess the stability of the REEM-C humanoid robot. The measured ZMP is then transmitted to a "fuzzy logic" PD controller to track the desired ZMP, thereby achieving motion balance and preventing tipping. The goal of the PD controller is to adjust the position of the robot's Center of Mass (CoM) to keep the ZMP within the support area (i.e., the contact area between the soles of the feet and the ground). Successfully achieving dynamic bipedal walking requires precise control of the angles of the robot's leg joints (including motion parameters such as position, velocity, and acceleration), all of which rely on precise feedback from rotary encoders.

Results

Balance control is crucial for ensuring stable movement of bipedal robots. Using data output from the encoders, the robot's "brain" calculates the current body posture and continuously issues new commands (including position, velocity, and acceleration) to each joint to maintain motion balance. RLS magnetic encoders provide PAL Robotics with flexible position feedback solutions that meet stringent requirements for space and performance. The carefully selected RLS encoders are powerful, allowing designers to fully unleash their creativity and talent. Balance control and stable walking are achieved by adjusting the instantaneous torque applied to each joint to ensure the robot's limbs remain in the correct positions. High-precision encoders ensure minimal control signal errors, enabling the controller to rapidly adjust the robot's body position, thus keeping the ZMP within the robot's foot support area.