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IVR: Control Theory

OVERVIEW

- Given desired behaviour, determine control signals
- Inverse models:
- Inverting the forward model for simple linear dynamic system
- Problems for more complex systems
- Open loop control: advantages and disadvantages
- Feed forward control to deal with disturbances

Control 2

Keypoints:

- Given desired behaviour, determine control signals
- Inverse models:
- Inverting the forward model for simple linear dynamic system
- Problems for more complex systems
- Open loop control: advantages and disadvantages
- Feed forward control to deal with disturbances

The control problem

- For given motor commands, what is the outcome? →Forward model
- For a desired outcome, what are the motor commands? →Inverse model
- From observing the outcome, how should we adjust the motor commands to achieve a goal? →Feedback control

Action

Outcome

Goal

Motor command

Robot in environment

Differential Equations

Using known relations between quantities and their rate of change in order to predict the behaviour of these quantities

x(t) is an unknown

function

f is a given analytical

expression

Equation:

Initial

condition:

and an initial condition (see last lect.)

Examples:

Pendulum equation simplified pendulum equation

Matrix form of the

simplified equation

Each of these: two initial conditions, e.g. x(0)=x0 and v(0)=v0

Differential equation of a two-link planar arm

Equations are classical physics.

Here copied from the paper:

approximate

new value

x(t)

old value

tangent to x(t)

Δt

t

t

t+Δt

Solving differential equations numerically**This is the most simple way of integrating a differential equation. Even for small step sizes considerable errors may accumulate.

- Given and
- Choose a small step size Δt
- Start at t = 0 andx(0) = x0
- Iterate x(t+Δt) = x(t) + Δt dx/dt until t = T

VB

Vehicle speed

v

?

IR

VB

e

Example from last lecture: Electric MotorSimple dynamic example –

We have a process model:

Solve to get forward model s(0)=s0:

- Derivation of this andmore general cases using e.g. Laplace transformation

Solving differential equations numerically

Example from last lecture:

- Given andRewrite:
- Choose a small step size Δt
- Start at t = 0 ands(0)= s0
- Iterate s(t+Δt) = s(t) + Δt ds/dt until t = T

?

The control problem- Forward: Given the control signals, can we predict the motion of the robot?

Forward model

Predicted output

Actual output

Motor command

Robot in environment

- Inverse: Given the desired motion of the robot Can we determine the right control signals?
- Control theory aims at unique or easy exact solutions
- Difficult in the presence of non-linearities or noise

Open loop control

Inverse model

Motor command

Robot in environment

Examples:

- To execute a memorised trajectory, produce appropriate sequence of motor torques
- To obtain a goal, make a plan and execute it
- means-ends reasoning could be seen as inverting a forward model of cause-effect
- ‘Ballistic’ movements such as saccades

Open loop control

- Potentially cheap and simple to implement e.g. if solution is already known.
- Fast, e.g. useful if feedback would come too late.
- Benefits from calibration e.g. tune parameters of approximate model.
- If model unknown, may be able to use statistical learning methods to find a good fit e.g. neural network.

Open loop control

- Neglects possibility of disturbances, which might affect the outcome.
- For example:
- Change in temperature may change the friction in all the robot joints.
- Or unexpected obstacle may interrupt trajectory

Inverse model

Motor command

Robot in environment

Disturbances

Feed-forward control

Inverse model

Motor command

Robot in environment

Disturbances

Measurement

- One solution is to measure the (potential) disturbance and use this to adjust the control signals.
- For example
- Thermometer signal alters friction parameter.
- Obstacle detection produces alternative trajectory.

Feed-forward control

- Can sometimes be effective and efficient.
- Requires anticipation, not just of the robot process characteristics, but of possible changes in the world.
- Does not provide or use knowledge of actual output (for this need to use feedback control)

Open loop control:

Inverse model

Motor command

Robot in environment

Disturbances ?

Feed-forward control:

Inverse model

Motor command

Robot in environment

Disturbances

Measurement

Feed-back control:

Control Law

Motor command

Robot in environment

Disturbances

Measurement

Combining control methods

- In practice most robot systems require a combination of control methods
- Robot arm using servos on each joint to obtain angles required by geometric inverse model
- Forward model predicts feedback, so can use in fast control loop to avoid problems of delay
- Feed-forward measurements of disturbances can be used to adjust feedback control parameters
- Training of an open-loop system is essentially a feedback process

Feedback control

- Example: Greek water clock
- Requires constant flow to signal time
- Obtain by controlling water height
- Float controls inlet valve

Compare: room heating

- Open loop: for desired temperature, switch heater on, and after pre-set time, switch off.

- Feed forward: use thermometer outside room to compensate timer for external temperature.

- Feedback: use thermometer inside room to switch off when desired temperature reached.

NO PREDICTION REQUIRED!

Control laws in feed-back control

Desired

output

Control Law

Motor command

Robot in environment

- On-off: switch system if desired = actual
- Servo: control signal proportional to difference between desired and actual, e.g. spring:

Disturbances

Actual

output

Measurement

VB

Vehicle speed

s

?

Servo controlSimple dynamic example:

Control law:

So now have new process:

gain=K

sgoal

s

With steady state:

And half-life:

Say we have sgoal =10. What does K need to be for s∞ = 10?

- If K=5, and s∞ = 8, and the system takes 14 seconds to reach s=6 starting from s=0, what is the process equation?

?

?

Desired

output

Control Law

Motor command

Robot in environment

- May involve multiple inputs and outputs
- E.g. ‘homeostatic’ control of human body temperature
- Multiple temperature sensors linked to hypothalamus in brain
- Depending on difference from desired temperature, regulates sweating, vasodilation, piloerection, shivering, metabolic rate, behaviour…
- Result is reliable core temperature control

Disturbances

Actual

output

Measurement

Negative vs. Positive Feedback

- Examples so far have been negative feedback: control law involves subtracting the measured output, acting to decrease difference.
- Positive feedback results in amplification:
- e.g. microphone picking up its own output.
- e.g. ‘runaway’ selection in evolution.
- Generally taken to be undesirable in robot control, but sometimes can be effective
- e.g. in exploratory control, self-excitation
- model of stick insect walking (Cruse et al 1995)
- (requires negative feedback from the environment)

6-legged robot has 18 degrees of mobility.

- Difficult to derive co-ordinated control laws

- But have linked system: movement of one joint causes appropriate change to all other joints.
- Can use signal in a feed-forward loop to control 12 joints (remaining 6 use feedback to resist gravity).

Advantages

- Major advantage is that (at least in theory) feedback control does not require a model of the process.
- E.g. thermostats work without any knowledge of the dynamics of room heating (except for time scales and gains)
- Thus controller can be very simple and direct
- E.g. hardware governor does not even need to do explicit measurement, representation and comparison
- Can (potentially) produce robust output in the face of unknown and unpredictable disturbances
- E.g. homeostatic body temperature control keeps working in unnatural environments

Issues

- Requires sensors capable of measuring output
- Not required by open-loop or feed-forward
- Low gain is slow, high gain is unstable
- Delays in feedback loop will produce oscillations (or worse)
- In practice, need to understand process to obtain good control:
- E.g. James Clerk Maxwell’s analysis of dynamics of the Watt governor
- …more next lecture

Cybernetics

- Feedback control advanced in post WW2 years (particularly by Wiener) as general explanatory tool for all ‘systems’: mechanical, biological, social…
- E.g. William Powers –

“Behaviour: the control of perception”

- Jay Wright Forrester –

“World dynamics” – modelling the global economic-ecological network

Most standard robotics textbooks (e.g. McKerrow) discuss forward and inverse models in great detail.

For the research on saccades see:

Harris, C.M. & Wolpert, D.M. (1998) Signal dependent noise determines motor planning. Nature, 394, pp. 780-184.

For an interesting discussion of forward and inverse models in relation to motor control in humans, see:

Wolpert, D.M. & Ghahramani, Z. (2000) “Computational principles of movement neuroscience” Nature Neuroscience 3, pp. 1212-1217.

WolpertD.M., Ghahramani Z, & Flanagan J. R. (2001) Perspectives and problems in motor learning. Trends in Cognitive Sciences5:11, pp 487-494.

Further reading on feedback control:

Most standard robotics textbooks (e.g. McKerrow) discuss feedback control in great depth.

For the research on stick insect walking see:

Cruse, H., Bartling, Ch., Kindermann, T. (1995) High-pass filtered positive feedback for decentralized control of cooperation. In: F. Moran, A. Moreno, J. J. Merelo, P. Chacon (eds. ), Advances in Artificial Life, pp. 668-678. Springer 1995

Classic works on cybernetics:

Wiener, Norbert: (1948) Cybernetics. or Control and Communication in the Animal and Machine, MIT Press, Cambridge

Powers, WT: (1973) Behavior, the Control of Perception, Aldine, Chicago

Forrester, JW (1971) World Dynamics, Wright and Allen, Cambridge

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