Longitudinal Speed Control

Comprehend

What are we trying to control?

Develop a basic understanding of the QCar 2 longitudinal speed-control problem, including the desired speed, measured speed, feedback and error, the basic closed-loop arrangement, the vehicle speed response and the requirements used to describe satisfactory speed-control performance.

Learn, then apply

Complete each ACE-CORE topic, then apply the idea directly to the Quanser QCar 2 longitudinal speed-control problem.

Learn C01

What is a Control System Open ACE-CORE →

Consider the QCar 2 longitudinal speed-control problem. The aim is for the vehicle's actual speed to follow a desired speed.

Part A

Identify the speed-control quantities

What physical quantity is being controlled on the QCar 2?
What quantity represents how fast we want the QCar 2 to travel?
What quantity tells us how fast the QCar 2 is actually travelling?

Controlled quantity: longitudinal speed
Desired quantity: desired longitudinal speed
Measured quantity: actual longitudinal speed

Part B

Describe the speed-control objective

In one sentence, describe what the QCar 2 longitudinal speed-control system should achieve.
Why might applying the same drive command not always result in exactly the same vehicle speed?

The longitudinal speed-control system should make the actual QCar 2 speed follow the desired QCar 2 speed.

A drive command alone does not guarantee a particular speed because the physical vehicle can respond differently under different operating conditions.

Learn C02

The Role of Feedback in Error Generation Open ACE-CORE →

Assume the desired QCar 2 longitudinal speed is 0.80 m/s. The measured speed is fed back and compared with this desired value.

Part A

Calculate the longitudinal speed error

Desired QCar 2 speed (m/s)	Measured QCar 2 speed (m/s)	Speed error (m/s)
0.80	0.30	?
0.80	0.70	?
0.80	0.80	?
0.80	0.90	?

Using speed error = desired speed − measured speed :

0.80 − 0.30 = +0.50 m/s
0.80 − 0.70 = +0.10 m/s
0.80 − 0.80 = 0.00 m/s
0.80 − 0.90 = −0.10 m/s

Part B

Interpret the speed error

What does a positive QCar 2 speed error mean?
What does zero speed error mean?
What does a negative QCar 2 speed error mean?

Positive speed error means the QCar 2 is travelling slower than the desired speed.
Zero speed error means the measured speed is equal to the desired speed.
Negative speed error means the QCar 2 is travelling faster than the desired speed.

Part C

Why is speed feedback useful?

Why is feeding the measured QCar 2 longitudinal speed back into the control system useful?

Speed feedback allows the actual QCar 2 speed to be compared with the desired QCar 2 speed. This comparison produces the speed error and shows whether the vehicle is travelling too slowly, at the desired speed, or too quickly.

Learn C03

Configuration of a Feedback Control System Open ACE-CORE →

Now map the basic feedback-control structure onto the QCar 2 longitudinal speed-control problem.

Part A

Identify the QCar 2 speed-control elements

For the QCar 2 longitudinal speed-control loop, identify:

The desired input.
The measured output.
The error signal.
The generic controller block.
The physical system being controlled.
The feedback signal.

Desired input: desired QCar 2 longitudinal speed
Measured output: actual QCar 2 longitudinal speed
Error signal: desired speed minus measured speed
Controller: a generic speed-control block
Physical system: the QCar 2 drive system and longitudinal motion
Feedback signal: measured longitudinal speed

Part B

Connect the QCar 2 speed-control loop

Sketch the basic closed-loop arrangement for QCar 2 longitudinal speed control using the elements identified in Part A.

At this stage, do not choose a specific controller type.

The basic speed-control structure should follow:

Desired QCar 2 speed → speed comparison → speed error → controller → QCar 2 drive system → measured QCar 2 speed

The measured longitudinal speed is then fed back to the speed comparison point.

Learn C04

System Response and Performance Measures Open ACE-CORE →

Imagine that the desired QCar 2 speed changes from 0 m/s to 0.80 m/s. The measured longitudinal speed rises towards the target, briefly exceeds it and then settles close to 0.80 m/s.

Part A

Describe the QCar 2 speed response

What does rise time describe for the QCar 2 speed response?
What does overshoot describe?
What does settling time describe?
What does steady-state error describe?

Rise time: how quickly the QCar 2 speed rises towards the desired speed.
Overshoot: how far the QCar 2 speed exceeds the desired speed.
Settling time: how long the QCar 2 speed takes to remain sufficiently close to the desired speed.
Steady-state error: the remaining difference between desired and measured QCar 2 speed after the response has settled.

Part B

Recognise desirable speed-control behaviour

For the QCar 2 longitudinal speed response, would you generally prefer the following to be larger or smaller?

Rise time
Overshoot
Settling time
Steady-state speed error

In general, a well-behaved QCar 2 speed response would aim for:

shorter rise time
less speed overshoot
shorter settling time
smaller steady-state speed error

Learn C05

Requirements Capture Open ACE-CORE →

Consider the desired behaviour for the QCar 2: the vehicle should reach the requested longitudinal speed quickly, avoid excessive speed overshoot and then maintain the requested speed accurately.

Part A

Match QCar 2 speed behaviour to a measure

Which measure describes how quickly the QCar 2 reaches the requested speed?
Which measure describes how far the QCar 2 speed exceeds the requested speed?
Which measure describes the remaining difference between the requested and measured speed once the response has settled?

Time taken to reach the requested speed: rise time
Speed exceeding the requested value: overshoot
Remaining speed difference after settling: steady-state error

Part B

Make the QCar 2 speed requirements measurable

Rewrite each statement so that the QCar 2 longitudinal speed response could later be measured and tested.

"The QCar 2 should reach the requested speed quickly."
"The QCar 2 should not exceed the requested speed by too much."
"The QCar 2 should maintain the requested speed accurately."

Example measurable forms are:

QCar 2 speed rise time should be less than a specified time.
QCar 2 speed overshoot should be below a specified percentage.
QCar 2 steady-state speed error should remain within a specified speed tolerance.

The important idea at this stage is to turn the desired speed behaviour into something that can later be measured and tested.

✓

Comprehend done. Enough thinking for now. Time to make something happen.

On to Operate →

Operate

What practical understanding do we need?

Use ACE-CORE together with ACE-Lab to develop the practical understanding needed for longitudinal speed control, exploring how sensing, actuation and embedded control are implemented.

Refine

How can control performance be improved?

Continue using ACE-CORE and ACE-Lab to explore how an operating control system can be assessed, adjusted and retested to improve its performance.

Engineer

How can model-based design improve performance and reduce development time?

Apply a model-based design approach to develop the QCar 2 longitudinal speed-control system, using modelling, simulation and controller design to improve speed-control performance, reduce physical testing and accelerate implementation.