### Feedback control

```Introductory Control
Theory
CS 659
Kris Hauser
Control Theory
• The use of feedback to regulate a signal
Desired
signal xd
Controller
Control input u
Signal x
Plant
Error e = x-xd
(By convention, xd = 0)
x’ = f(x,u)
What might we be interested in?
• Controls engineering
• Produce a policy u(x,t), given a description of the
plant, that achieves good performance
• Verifying theoretical properties
• Convergence, stability, optimality of a given policy
u(x,t)
Agenda
• PID control
• LTI multivariate systems & LQR control
• Nonlinear control & Lyapunov funcitons
• Control is a huge topic, and we won’t dive into
much detail
Model-free vs model-based
• Two general philosophies:
• Model-free: do not require a dynamics model to be provided
• Model-based: do use a dynamics model during computation
• Model-free methods:
• Simpler
• Tend to require much more manual tuning to perform well
• Model-based methods:
•
•
•
•
•
Can achieve good performance (optimal w.r.t. some cost function)
Are more complicated to implement
Require reasonably good models (system-specific knowledge)
Calibration: build a model using measurements before behaving
Adaptive control: “learn” parameters of the model online from
sensors
PID control
• Proportional-Integral-Derivative controller
• A workhorse of 1D control systems
• Model-free
Proportional term
Gain
• u(t) = -Kp x(t)
• Negative sign assumes control acts in the same
direction as x
x
t
Integral term
Integral gain
• u(t) = -Kp x(t) - Ki I(t)
• I(t) =

0
(accumulation of errors)
x
t
asymptotically to 0
Instability
• For a 2nd order system (momentum), P control
Divergence
x
t
Derivative term
Derivative gain
• u(t) = -Kp x(t) – Kd x’(t)
x
Putting it all together
• u(t) = -Kp x(t) - Ki I(t) - Kd x’(t)
• I(t) =

0

Parameter tuning
Example: Damped Harmonic
Oscillator
• Second order time invariant linear system, PID controller
• x’’(t) = A x(t) + B x’(t) + C + D u(x,x’,t)
• For what starting conditions, gains is this stable and
convergent?
Stability and Convergence
• System is stable if errors stay bounded
• System is convergent if errors -> 0
Example: Damped Harmonic
Oscillator
• x’’ = A x + B x’ + C + D u(x,x’)
• PID controller u = -Kp x –Kd x’ – Ki I
• x’’ = (A-DKp) x + (B-DKd) x’ + C - D Ki I
Homogenous solution
•
•
•
•
•
Instable if A-DKp > 0
Natural frequency w0 = sqrt(DKp-A)
Damping ratio z=(DKd-B)/2w0
If z > 1, overdamped
If z < 1, underdamped (oscillates)
Example: Trajectory following
• Say a trajectory xdes(t) has been designed
• E.g., a rocket’s ascent, a steering path for a car, a plane’s landing
• Apply PID control
• u(t) = Kp (xdes(t)- x(t)) - Ki I(t) + Kd (x’des(t)-x’(t))
• I(t) =

0
−
• The designer of xdes needs to be knowledgeable about the
controller’s behavior!
x(t)
xdes(t)
x(t)
Controller Tuning Workflow
• Hypothesize a control policy
• Analysis:
• Assume a model
• Assume disturbances to be handled
• Test performance either through mathematical analysis, or
through simulation
• Go back and redesign control policy
• Mathematical techniques give you more insight to improve
redesign, but require more work
Multivariate Systems
•
•
•
•
x’ = f(x,u)
x X  Rn
u U  Rm
Because m  n, and variables are coupled, this is not as easy as
setting n PID controllers
Linear Time-Invariant Systems
• Linear: x’ = f(x,u,t) = A(t)x + B(t)u
• LTI: x’ = f(x,u) = Ax + Bu
• Nonlinear systems can sometimes be approximated by
linearization
Convergence of LTI systems
• x’ = A x + B u
• Let u = - K x
• Then x’ = (A-BK) x
• The eigenvalues li of (A-BK) determine convergence
• Each li may be complex
• Must have real component between (-∞,0]
• x’ = Ax + Bu
 xTQ x + uTR u dt
Error term
“Effort” penalization
Over an infinite horizon
Closed form LQR solution
• Closed form solution
u = -K x, with K = R-1BP
• Where P is a symmetric matrix that solves the Riccati equation
• ATP + PA – PBR-1BTP + Q = 0
• Derivation: calculus of variations
• Packages available for finding solution
Nonlinear Control
• General case: x’ = f(x,u)
• Two questions:
• Analysis: How to prove convergence and stability for a
given u(x)?
• Synthesis: How to find u(t) to optimize some cost
function?
Toy Nonlinear Systems
Cart-pole
Mountain car
Acrobot
Proving convergence & stability
with Lyapunov functions
• Let u = u(x)
• Then x’ = f(x,u) = g(x)
• Conjecture a Lyapunov function V(x)
• V(x) = 0 at origin x=0
• V(x) > 0 for all x in a neighborhood of origin
V(x)
Proving stability with Lyapunov
functions
• Idea: prove that d/dt V(x)  0 under the dynamics x’ = g(x)
around origin
V(x)
t g(x)
t d/dt V(x)
Proving convergence with Lyapunov
functions
• Idea: prove that d/dt V(x) < 0 under the dynamics x’ = g(x)
around origin
V(x)
t g(x)
t d/dt V(x)
Proving convergence with Lyapunov
functions
• d/dt V(x) = dV/dx(x) dx/dt(x)
= V(x)T g(x) < 0
V(x)
t g(x)
t d/dt V(x)
How does one construct a suitable
Lyapunov function?
• Typically some form of energy (e.g., KE + PE)
• Some art involved
Direct policy synthesis:
Optimal control
• Input: cost function J(x), estimated dynamics f(x,u), finite
state/control spaces X, U
• Two basic classes:
• Trajectory optimization: Hypothesize control sequence u(t),
simulate to get x(t), perform optimization to improve u(t), repeat.
• Output: optimal trajectory u(t) (in practice, only a locally optimal
solution is found)
• Dynamic programming: Discretize state and control spaces, form
a discrete search problem, and solve it.
• Output: Optimal policy u(x) across all of X
Discrete Search example
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•
•
•
Split X, U into cells x1,…,xn, u1,…,um
Build transition function xj = f(xi,uk)dt for all i,k
State machine with costs dt J(xi) for staying in state I
Find u(xi) that minimizes sum
Value function for 1-joint acrobot
of total costs.
• Value iteration: repeated
dynamic programming over
V(xi) = sum of total future
costs
Receding Horizon Control (aka
model predictive control)
...
horizon 1
horizon h
Controller Hooks in RobotSim
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•
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•
sim = Simulator(world)
c = sim.getController(0)
By default, a trajectory queue, PID controller
• c.setMilestone(qdes) – moves smoothly to qdes
milestones and smoothly interpolates between them.
• Can override behavior to get a manual control loop. At every time
step, do:
• Read q,dq with c.getSensedConfig(), c.getSensedVelocity()
• For torque commands:
• Compute u(q,dq,t)
• Send torque command via c.setTorque(u)
• OR for PID commands:
• Compute qdes(q,dq,t), dqdes(q,dq,t)
• Send PID command via c.setPIDCommand(qdes,dqdes)
Next class
• Motion planning
• Principles Ch 2, 5.1, 6.1
```