Category: Safety

System Safety with Ultrasonic Sensors

Safety is the highest priority at the Autonomous Valet Parking project. As we seek to demonstrate that Parkopedia’s maps are suitable for localisation and navigation within covered car parks, our safety case must ensure the safety of the people, vehicles and infrastructure inside our test vehicle and without. There have been some highly publicised incidents involving other organisations’ autonomous vehicles over the past year or two, so what is the AVP project doing around safety?

Our Safety Case involves a combination of System Safety and Operational Safety, to achieve the required assurance levels around our activities. System Safety covers any and all aspects of the system (hardware, software or both) that contribute to the safety case and is the focus of this post. Operational Safety is all the operational decisions taken to ensure that we are conducting the tests safely and you can read more about that here.

Our test vehicle, a StreetDrone.ONE is a converted Twizy, and comes with ultrasound range sensors. There are eight Neobotix ultrasound sensors in total, three to the front, three to the rear, and one in each door looking sideways. The front sensors are slightly fanned out, and in the rear the outer two are corner mounted. The signals from each sensor are gathered together by a Neobotix board and this publishes the range data as an automotive industry canbus signal. These can be monitored and action taken when a significant measurement is made.

The sensors create a “virtual safety cage”. This virtual safety cage can be imagined as an invisible cuboid around the StreetDrone.ONE, slightly wider and longer than the car itself. If anything, be it a car, pedestrian or wall, intrudes inside this cage, the car should stop immediately, thus acting as a belt and braces to the perception and navigation parts of the AI driving the car.

The video below shows a demonstration of the drive-by-wire system of the StreetDrone at 14 mph. The brake is applied by the actuators at the very beginning of the 3 metres wide white strip. Based on a 14mph start speed, we calculated the braking distance to be 4.5 metres. This is obviously an approximation, and the actual braking distance and time depends on many factors including brake disk wear and tear.

Applying 100% brake at 14mph

Remembering our high-school equations of motion:

Where “v” is the initial velocity, “u” the final velocity, “a” is the acceleration and “s” the distance.

Now we can rearrange this to obtain the acceleration, remembering in our case the final velocity is zero:

These numbers match well with the similar experiments carried out by StreetDrone using IMU data, which have shown peak deceleration of 0.67g and an average deceleration of 0.46g.

The maximum range of the Neobotix ultrasound sensor is 1.5 metres, so we could do the above calculation in reverse and calculate the maximum safe speed. Allowing for a safety buffer of 0.5m:

The distance “s” is 1.0 metres, the acceleration “a” is 4.3m/s^2, and again the final velocity “u” is 0 m/s.

The present AVP plan calls for a maximum speed within car parks of 5 mph which is well within the safety margin calculated above. The next step now is to process the data we’ve captured from the ultrasonic sensors and to develop the software that will automatically apply the maximum brake whenever the virtual safety cage is breached.

We are ready to test!

On the track at Turweston Aerodrome

Exciting times: this week marks the start of testing for our StreetDrone autonomous vehicle, as we build towards an Autonomous Valet Parking demonstrator.

This first testing phase will be in a controlled environment to minimise risk. For that reason, we’ve chosen Turweston Flight Centre, which has previously been used by our friends at StreetDrone who have done some of their testing there.

In accordance with our project Safety Plan and the Safety Case for this phase of the project, we’ve also been busy collecting safety evidence prior to starting the live tests. These documents will be made publicly available as part of our goal to be as transparent as possible, and for those who wish to use them as a starting point for their own safety case.

For this phase, our safety evidence documents are:

  • Safety Plan
  • Safety Case Summary
  • Risk Assessment and Method Statement
  • Review of the Requirements
  • StreetDrone User Manual
  • Incident Reporting Spreadsheet

Stay tuned for our next update!

Risk Assessment and Method Statements (RAMS)

One of the key objectives of this Autonomous Valet Parking project is to demonstrate our autonomous vehicle parking itself in a covered car park. The Transport Systems Catapult is responsible for the safety work package which ensures that all activities undertaken during the project are done in a systematic and safe manner. One of the important deliverables to ensure safety is the Risk Assessment and Method Statement (RAMS).

The RAMS document generally includes:

1.       An overview of the project and key objectives to provide the reader with a background of the project

2.       The activity being assessed, including:

  • Roles and responsibilities
  • Limits of the operation and trial details (route planned, scenarios, vehicle specifications, time of day, limits, weather, specifications)
  • Legal considerations such as vehicle insurance and laws
  • Emergency procedures (eg. vehicle breaking down, network error, sensor malfunction, accident)
  • Training achieved (eg. driver training on the StreetDrone.ONE vehicle, taking over manually)

3.       A risk assessment listing hazards, consequences, mitigation methods and detailing who might be harmed. Following the ISO 26262 standard, a hazard analysis and risk assessment is required in order to determine the criticality of a system.

The risk analysis is focused on:

  • Risks related to the ongoing operation of the vehicle
  • Risks related to the operation of external factors that affect current operation
  • Risks arising from the new equipment that may affect the safety of the vehicle or other
RISK MATRIX (To generate the risk level)
ACTION LEVEL (To identify what action needs to be taken)

The method statement part describes in a logical sequence how a task will be carried out in a safe manner. It includes all the risks identified and the measures needed to control those risks.

The purpose of the method statement is to ensure that:

  • The trial is carried in a structured, controlled and safe manner
  • The hazards and associated risks are understood and mitigated

While the ultimate goal of the project is to demonstrate Autonomous Valet Parking, we will build up to this demonstration through smaller manageable steps and a separate RAMS will be produced at each stage:

1.       Capturing data in car parks

2.       Testing in a controlled environment

3.       Testing in a covered car park

4.       Demonstration