Why Use FMCW radar?
What shape and size is the protection zone?
How is the protection zone created?
What is the maximum size of the protection zone?
What is the speed and direction sensing?
What is the speed threshold?
How is the radar information used?
What if the radar fails?
How easy is it to install?
Why Use FMCW radar? Back to questions
Numerous studies have shown that for the quarrying and mining environment, radar has the ability to see through all the dirt and dust and rain that defeat other techniques. FMCW (Frequency Modulated Continuous Wave) radar is a particular radar measuring technique ideally suited to this application.
Off highway vehicles work in close proximity to people and other vehicles. The hazard detection sensor must separately monitor multiple objects of very different sizes and identify the object which presents the immediate hazard. If that object is a man, his signal must be separated from that of a much larger object in the background, even when they are approaching at the same speed. FMCW radar can do this.
Reverse protection systems which are only active when reverse gear is engaged, will not warn the driver of something or someone behind his machine until he engages reverse. In the busy world of off highway machines the driver needs to know what is happening behind him all the time, whether he is about to reverse over someone or about to turn into the path of a small truck about to overtake.
But if you make the system active all the time you must avoid alarming all the time. You must separately identify stationary objects, objects approaching and a person standing immediately behind the machine. FMCW radar can do this.
What shape and size is the protection zone? Back to questions
The shape and size of the protection zone are both programmable. Some typical shapes are shown below.
How is the protection zone created? Back to questions
All objects within the beam produce radar reflections which are separated according to distance into range gates. A threshold table is set up so that the radar will respond according to the size of reflection in any range gate and the threshold set in the table. For short range you ignore the outer range gates.
The radar has two overlapping beams, both have a vertical spread of +/- 10o but one has a horizontal spread of +/- 50o while the other is much narrower at +/- 25o. Thus by setting up the threshold table to use different beams at different distances the shape can be tailored to suite the application.
The parameters which define the shape and size of the zone are held in a file. Each radar holds four files, each with a different set of parameters. The radar will use which ever set is selected by the cab unit. New files can be down loaded from a PC via the cab unit or existing files can be modified.
What is the maximum size of the protection zone? Back to questions
The protection zone can be built up out to the tenth range gate in each beam. The standard range gate spacing is 750 mm and each range gate is approximately 1500 mm deep, giving a maximum range of about 8 metres. However the range gate spacing can be increased and for large dump trucks it can be set to 1500 mm, giving a maximum range of about 17 metres.
What is the speed and direction sensing? Back to questions
When an object is moving with respect to the radar, this movement creates a Doppler effect on the return signal. FMCW radar uses frequency modulation to measure the distance but it is also possible to extract the Doppler information from the signal and measure both the speed and direction of movement. This is then used to determine whether objects within the protection zone are moving towards the machine or away from it, so that objects moving away do not trigger a response. In certain circumstances the speed of movement may be used to trigger an earlier response to objects closing more quickly.
The advantage of these measurements is that the radar response can be better related to real hazard conditions. Experience shows that safety is enhanced if the driver is aware of movement behind the machine before he engages reverse gear and if he is aware of other vehicles approaching his rear when driving forward.
By sensing the direction of movement the system can eliminate unwanted responses without limiting the response to only when reverse is engaged. It also removes a possible source of system failure caused by failure of the switch detecting reverse gear.
How is the radar information used? Back to questions
The radar divides the protection zone into three sections, inner, middle and outer. It compares the signal in each of these sections with the thresholds set for the two beams and calculates the speed and direction of movement compared with the threshold set for that section. It then sends this information to the cab unit via a serial link and receives from the cab unit, instructions as to which set of threshold tables to use. It analyses the radar reflections and repeats this calculation every 50 milliseconds. The cab unit uses this information either to issue a series of warnings as an object gets closer or to switch on the CCTV or as an input to an automatic braking system or for a combination of these, depending on the system installed. The same radar can be used with any of the systems by downloading the appropriate file from a PC.
What if the radar fails? Back to questions
The radar is a tried and tested design but no system can be guaranteed never to fail so we build in as many fail safe checks as possible.
If the radar is working correctly there will always be some reflected signal to analyse, even if it is only the reflection from the inside of the radar window. So if there is no signal to analyse the radar sends a fault message to the cab unit.
The radar sends a message to the cab unit every 50 milliseconds, then sends the same signal inverted so the cab unit can check it has received it correctly. So if any wire breaks, or the radar fails, the cab unit knows within 100 milliseconds.
What action the cab unit takes will depend upon the system. Careful consideration has to be given to the safest response to a failure, especially automatic braking systems. With each system the driver is warned and the machine set to the safest fall back position.
How easy is it to install? Back to questions
The automatic braking systems need to be tailored to suit the host machine and installation is a specialist field. The other systems are designed for easy installation. No gear input is required for the detection system and it will run on any voltage between 10 and 30 volts. The radar is connected to the cab unit with a standard quick release connector and a four core cable.
Speed Thresholds Back to questions
Speed thresholds define the minimum closing speed for the radar to respond. If the speed threshold is set to zero, any movement of a few millimetres in either direction will be detected. For a man within the protection zone, the movement of his breathing will be sufficient to trigger a response. However, any object, totally static with respect to the radar, will be ignored.