In recent years, there has been a lot of research on Radar-based Advanced Driver Assistance Systems (ADAS) as companies go head to head in the autonomous driving market. These systems use short range 24GHz and long range 77GHz radar in conjunction with other sensing or vision technology to characterize the driving environment and ensure safe autonomous driving. This technology is also finding its way to other markets where outdoor sensing is a requirement.
This article will go over
- The history of radar
- Radar basics
- Radar structure and block diagram
The history of radar would have to start with James Clerk Maxwell who produced the unified model of electromagnetism. Heinrich Hertz set out to prove this treatise. Along the way, he discovered radio waves in 1888. Then in 1904, German engineer Christian Hulsmeyer developed the world’s first radar system to serve as an early warning system for shipping. During WWII, radar technology was further developed by countries such as Japan, USA, Russia, and Great Britain to serve as early warning systems for planes. Today, radar systems are a critical part of ADAS.
- Radar stands for Radio Detection and Ranging.
- What does a radar actually do?
A Radar system transmits waves, and detects the echo signal which is bounced off a target. By using the properties of the transmitted wave and echo signal, the system calculates the range, velocity and the angle of the target.
There are various object-sensing technologies on the market: radar, infrared, ultrasonic and laser. Radar stands out as the go-to technology when the designer wants a robust detection system in an outdoor environment. Radar can see through snow, rain, mist, dust and darkness.
There are limitations to radar technology:
- Radar does not have detail resolution. For example, an image from an optical camera provides information to the user where a radar system cannot.
- Radar cannot provide information about color or heat
- Radar cannot “penetrate” through thick objects. In other words, it cannot detect targets behind buildings or walls.
Radar Structure and Block Diagram
There are two types of radar:
- Bistatic Radar
- Monostatic Radar
The bistatic radar consists of a transmitting section and receiving section. The transmitting section sends out the signal from the Tx antenna and circuit, while the receiving section receives the echo signal through a separate Rx antenna and circuit.
The monostatic radar has only one antenna for both Tx and Rx. Hence the need for a duplexer to connect the Tx circuit or Rx circuit to the only antenna in the system. Additionally, the transmitting power is several orders of magnitude higher than the receiving power. Thus a duplexer is required so that circuits remain independent.
To have some fundamental understanding of how radar works, let’s go over the range equation.
Let’s begin with a radar detecting a target object at range R. For an isotropic source, the power density is given by
The power density radiated by a given antenna with gain Gt
Let’s assume the target object area is ό, then the power density at target is given by
Thus the power density of the reflected signal is
The antenna has an effective area of Ae, thus the received power is
The maximum range obtained from the system happens when power is at the minimum detectable signal
Rearranging the formula,
Let’s review what we have learnt thus far:
The range of radar can be increased when we
increase the Tx power, Pt
increase the gain of the antenna, Gt
increase the target radar cross section, σ
increase the effective area of the antenna, AE
decrease the minimum detectable power, Smin
In a nutshell, when an engineer is designing a radar system, he has to consider factors such as the target object – is he trying to detect a car which has larger cross-section than a human? The antenna design will determine the gain and area.
The Doppler radar is used to measure the velocity of the target.
The transmitter section transmits a continuous sine wave of fo. From Doppler’s Effect, the frequency echo signal returning to Radar will increase if the Target is approaching the Radar or decrease if the Target is receding.
The receiving section will extract the frequency drift, fd.
The velocity of the target can be found using,
Obviously, if the Doppler Radar is capable of measuring the velocity of a moving object, it can be used to detect the presence of a moving object in a motion sensing application.
Frequency Modulation Continuous Wave
The Doppler Radar can neither measure Range nor detect a stationary object, as the frequency of the echo signal would be unchanged. There are different approaches to this problem, but the FMCW approach has established itself as the predominant method to obtain a distance measurement.
Let’s consider a static case
The FMCW radar sends a transmits a signal of increasing frequency as shown below. The Echo signal will return after time, τ.
The difference of frequency between the transmit and echo is fB. Geometrically, the following equation can be constructed from the above diagram.
Solving for τ
Range is given by speed of light time taken to travel
Plugging τ into the range equation,
By combining the Doppler Effect, the range of the moving Target can also be calculated.
Radar Detection vs PIR Motion Sensing
While the case for radar in an outdoor environment has been fully made, what about the case for radar in an indoor environment where neither rain nor fog is an issue?
Here are two use cases:
1. Aesthetics and Range/Distance
The most prevalent indoor sensor technology for presence detection has been the Passive Infrared (PIR) sensor. It is a motion detector, as it detects the change in infrared energy coming from the target. A PIR motion sensor consists of three parts:
- The infrared sensor, which is typically in a metal casing
- The lens, which focuses the light on the PIR sensor so that we can cover a larger area, as well as to protect the sensor from impact. It can also act as a filter to capture certain wavelengths
- The circuit, which can adjust sensitivity
The PIR motion sensor cannot be hidden, while a radar-based system can be encased in the furniture or light fixture as shown below. It is aesthetically pleasing, as it is fully hidden from the consumer.
2. Range and Distance
PIR motion sensors typically go up to 25’, while radar systems can go over 50’. The extra distance covered gives the designer more freedom to place the detection system. It is also possible to get the distance to a stationary target object using radar. A PIR motion sensor requires the object to be moving while a FMCW radar can detect a stationary object.
Radar in Tank Level Measurement
Clearly radar is advantageous when outdoor detection is mission critical. Another application where radar is proving its worth is level measurement. There are several ways to measure levels in a tank.
Here are some of the level measurement systems:
- Ultrasonic transmitter
- Differential pressure
- Capacitance transmitter
- Float transmitter
- Displacer Transmitter
The main challenges of these methods include
- The specific gravity has to assumed
- The gauge is in contact with the liquid
- Fumes and dust could affect readings
Measuring the level in a tank using radar is superior because it is measuring the level using the reflection of the wave – thus it neither comes in contact with the liquid nor is it affected by fumes or dust. In other words, accurate readings can be obtained in almost any environment inside the tank.
Radar Solution from Manufacturers
Now that you have a basic understanding of radar lingo and technology, let’s look at solutions from our manufacturers. The block diagram is a radar system – consisting of two blocks:
- 24Ghz transceiver chip – BGT24MTR11
- Microcontroller – XMC4200
Taking a look the receiver circuit, the Rx antenna receives fo + fd
*Source: Infineon data sheet
A LNA provides low noise figure and a RC polyphase filter (PPF) is used for LO quadrature phase generation of the homodyne quadrature downconversion mixer. Output power sensors as well as a temperature sensor are implemented for monitoring purposes.*
*Source: Infineon data sheet
The MCU modulates the VCO, and carries out the FFT and DSP.
The Sense2Go is a bistatic radar with 1 receiver and 1 transmitter with an integrated patch antenna.
*Source: Infineon data sheet
Radar technology has come a long way since the days of German engineer Christian Hulsmeyer. Radar stations were once deployed to detect planes and ships. Radar solutions are now being used in autonomous driving. The cost and size of radar technology is coming down, and this will allow new applications to be developed in the market where reliable outdoor detection is mission critical.