Modeling Actuators of SARCOS robot

I think a good model is important, though very fundamental. As known to all, hydraulics is typically nonlinear. My goal for this part is to find which factors are important or not for our case and get an accurate but compact model for easy control use. Now I study the LHFE joint actuator to get some features that can be generalized to the whole robot.
There are nonlinearity from two parts, mechanics and hydraulics. I have got a good understanding of the mechanics especially the changing moment arm. You can see here. For hydraulics, I have a preliminary master by simulation and get some conclusions. However, these conclusions still need more data to verify by experiments of real robot. You can see here.

Changing Moment Arm Problem

The figure above is from CAD drawing of LHFE(Left Hip Flex/Extension Joint). It's a system of five bars. What we are interested in are two thing: 1. relation between actuator length(x) and leg angle(th); 2. relations between dx/dt and dth/dt or dx/dth, which is actually the same value as moment arm. I get the solution of 1 by solving kinematic equations and of 2 by analysing force and moment equalibrium(or directy using dx/dt from 1). The following are the simulation results from Matlab, previous source code is here, my version is here.
We can see that moment arm is close to a constant of 0.375 rad. within the range of 0 and 1 rad.

Modeling the Actuaor Dynamics and Feature Analysis

Physical-based Model,

Some formula used in simulation,

Some params used in simulation,

Simulation results,
around different angles,

no gravity and piston bolume V changes with angles,

with gravity and piston bolume V = Constant,

moment arm changes,

different length between CoG and hip pivot,

different bulk modulus,

Conclusion:
1. Moment arm and gravity effects are most important,
2. Piston and ring volume have less effects for our case,
3. Bulk modulus has great effects,
4. Mass distribution affects dynamics intensively,
5. The dynamics of the system is position-dependent and varies obviously when angle limit is approached.

Time

Subject

1) 2010-10-1

Design of a PID Controller to Hydraulic-Actuator

2) 2010-10-8

Design of LQR Controller to Hydraulic-Actuator

3) 2010-10-15

Controllers to Hydraulic-Actuator

4) 2010-10-22

Actuator Controller and Velocity Estimator

5) 2010-10-29

Actuator Controller and Velocity Estimator--Ⅱ

6) 2010-11-19

Single Joint Controller and Velocity Estimator--Ⅲ

7) 2010-12-10

Improved Dynamic Model and Velocity Filter

8) 2011-1-21

Model Hydraulic System

9) 2011-1-28

Model Hydraulic System

10) 2011-2-11

Modelling Hydraulic System

11) 2011-2-18

Modelling Hydraulic System

12) 2011-3-4

Modelling Hydraulic System

13) 2011-4-2

Modelling Hydraulic System

14) 2011-4-8

Modelling Hydraulic System

15) 2011-4-15

Modelling Hydraulic System

16) 2011-4-29

Hydraulic System Simulation and Study

17) 2011-5-6

Hydraulic System Modeling and Analysis

18) 2011-5-13

Hydraulic System Modeling and Analysis