Laser Rangefinder Reference Design · 2020-07-08 · Laser Rangefinder Reference Design WAS-18A1EN...

Laser Rangefinder Reference Design

WAS-18A1EN V1.00 1 / 14 July 1, 2020


D/N: WAS-18A1EN

Introduction

There are many techniques for laser rangefinding. The Holtek laser rangefinder adopts a phase

method, which is one of the Time-of-Flight (TOF) technologies. The advantage of the phase method

is that it provides higher accuracy, which can reach up to the millimeter level. However, the phase

method requires more complex calculations, for which asynchronous sampling synchronization,

differential frequency phase measurement, Fourier transformation, distance synthesis and other

algorithms are required. Holtek has integrated these complex operating principles mentioned above

into this solution, providing users with a reference to enable product development cycles to be

effectively reduced.

Figure 1

Applications

Indoor distance measurements


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Solution Features

1. Laser ranging function: use a laser to quickly and accurately measure the distance to an object.

Laser measurements provides higher accuracy than ultrasonic measurement methods and is less

susceptible to external interference. However, laser rangefinders require a more complex and

quick calculation capability, therefore for this reason the 32-bit Arm® Cortex®-M0+ based MCU

HT32F52352 is used.

2. The phase method (TOF method): measures the phase delay generated by a laser round-trip to

calculate the actual object distance, here the theoretical measurement accuracy can reach up to

the millimeter level.

Algorithm Principle Description

Algorithm Process

Laser distance calculations can be divided into three parts. These are laser echo signal acquisition,

echo signal phase calculation and distance synthesis calculation. Attention should be paid to the

sampling time point of the laser echo signals on the internal and external optical path. A fast Fourier

transform (FFT) algorithm is then applied to the captured echo signals to obtain the echo signal phase,

after that, several groups of different measurement rulers are used to calculate the actual length.

Start

Laser echo signal acquisition(Four frequencies, internal & external optical path, 8 groups of data in total)

Echo signal phase ranging calculation(Four groups of direct rulers, six groups of indirect rulers)

Distance synthesis calculation

END

Figure 2


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Differential Frequency Phase Measurement

This algorithm uses the differential frequency phase measurement principle. A clock chip is used to

generate a high frequency main oscillation signal and local oscillation signal. The main oscillation

frequency is used to modulate the laser while the local oscillation signal is overlaid on the APD

backward voltage. The main oscillation signal and local oscillation signal are mixed by a frequency

mixer circuit to produce a invariant phase low frequency signal. This process provides a reduced

circuit complexity as well as making it easier for MCU echo signal acquisition.

Modulation Optical Frequency First Group Second Group Third Group Fourth Group Main Oscillation Frequency (MHz) 152 153 160 200 Local Oscillation Frequency (MHz) 151.99 152.99 159.99 199.99 Differential Frequency 10kHz

Table 1

It can be seen from the above table that the echo signal frequency passed through the frequency

mixer circuit is 10kHz, therefore the MCU can use the ADC to capture the echo signals.

Laser Echo Signal Acquisition

The laser rangefinder echo signal frequency is a 10kHz sinusoidal signal. By using the HT32F52352

internal ADC together with the PDMA function, the echo signals can be captured quickly and

accurately.

Figure 3

This algorithm uses four optical modulation signals with different frequencies to implement

distance measurement. The echo signal of each ruler corresponds to two signals on an internal

optical path and external optical path. By calculating the phase difference between the signals on

the external optical path and the internal optical path, the distance value measured by this

measurement ruler can be obtained.


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Figure 4

To implement the distance measurement of a ruler, it is necessary to capture the phase difference

between the signals on the internal and external optical paths with high accuracy. In this algorithm,

the ADC function is used together with the PDMA function to accurately control the time and

interval of the ADC capture operation. An FFT operation is then applied to the captured data to

calculate the phase value of each echo signal.

Phase Ranging Calculation Figure 5 shows the phase method laser ranging principle diagram. It can be seen that the distance

is given by D = 12

× c × t, (c: speed of light, t: time difference), assuming that the modulation

frequency is f, then the modulation signal wavelength λ = C𝑓𝑓 , the phase delay generated in a laser

round-trip over the distance to be measured D is φ, then the corresponding time difference will be

t = T × φ / 2π (T: modulation light period). As a result, the distance to be measured, D, can be

calculated by the following equation:

D =12

× 𝑐𝑐 × 𝑡𝑡 =12 ×

c𝑓𝑓 ×

φ2𝜋𝜋 =

λ2 �𝑁𝑁 +

Δφ2𝜋𝜋� = 𝐿𝐿𝐿𝐿(𝑁𝑁 + Δ𝑁𝑁)

(Ruler length 𝐿𝐿𝐿𝐿 =λ2

,𝑁𝑁: 2𝜋𝜋 integer multiple,Δ𝑁𝑁: 2𝜋𝜋 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝑓𝑓𝑟𝑟𝑟𝑟𝑐𝑐𝑡𝑡𝑟𝑟𝑓𝑓𝑟𝑟𝑟𝑟𝑓𝑓 𝑝𝑝𝑟𝑟𝑟𝑟𝑡𝑡)

Distance D Δϕ Wavelength λ

Laser Emitter

Laser Receiver

ϕ Figure 5


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Direct and Indirect Measurement Rulers

In this solution, the minimum measurement ruler frequency used by the rangefinder is 152MHz.

According to the measurement ruler length formula mentioned above, the corresponding maximum

direct measurement ruler length is 0.987m, which is not able to support long distance measurements.

For this reason, an indirect measurement ruler is utilised to implement long distance measurements.

The difference of any two direct ruler frequencies can be used as a new indirect ruler frequency, the

length of which is determined by the indirect ruler frequency. The phase difference is calculated by

the difference between the phase difference of the two original direct rulers.

The corresponding direct and indirect measurement rulers for this solution is shown in Table 2. For

example, an indirect ruler signal of 40MHz can be obtained from two ruler frequencies of 200MHz

and 160MHz, and the indirect ruler length is 3.75m; an indirect ruler signal of 1MHz can be

obtained from two ruler frequency signals of 152MHz and 153MHz, and the indirect ruler length

is 150m.

Measurement ruler length (𝐿𝐿) =Speed of light (𝑐𝑐)2 × frequency (𝑓𝑓)

Direct Ruler

Modulation Frequency 200MHz 160MHz 153MHz 152MHz

Ruler Length 0.75m 0.9375m 0.9805m 0.987m

Indirect Ruler

Modulation Frequency 48MHz 47MHz 40MHz 8MHz 7MHz 1MHz

Ruler Length 3.125m 3.1915m 3.75m 18.75m 21.42m 150m

Table 2

Distance Synthesis Calculation

This algorithm uses a total of 10 groups of measurement rulers, the length of which are 150m,

21.42m, 18.75m, 3.75m, 3.19m, 3.12m, 0.98m, 0.98m, 0.93m, 0.75m respectively. According to

the above formula, it can be seen that specific rulers can only reach the distance whose

corresponding phase difference does not fulfill 2π, i.e. the maximum measuring range of each ruler

will not exceed its ruler length.

In order to ensure ranging accuracy, a small measurement ruler is needed. However, in order to

ensure the measurement distance, a longer measurement ruler is needed. Therefore, it is necessary

to use the multiple measurement ruler connection algorithm to obtain both high accuracy and long

distance measurement.

When more than two measurement rulers are used for distance measurement, assuming that the

ruler lengths are 𝐿𝐿𝐿𝐿1, 𝐿𝐿𝐿𝐿2, 𝐿𝐿𝐿𝐿3 … respectively with a relationship of 𝐿𝐿𝐿𝐿1 > 𝐿𝐿𝐿𝐿2 > 𝐿𝐿𝐿𝐿3 > ⋯, in

which Ls1 is greater than the distance to be measured, therefore N1 = 0:

D1 = 𝐿𝐿𝐿𝐿1 × (𝑁𝑁1 + ∆𝑁𝑁1) = 𝐿𝐿𝐿𝐿1 × ∆𝑁𝑁1

Substitute the first ruler calculated distance in the secondary ruler equation and the result is:

D2 = 𝐿𝐿𝐿𝐿2 × �int �𝐿𝐿𝐿𝐿1 × ∆𝑁𝑁1

𝐿𝐿𝐿𝐿2� + 𝛥𝛥𝑁𝑁2�


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As a result, the ith calculation result will be:

D𝑖𝑖 = 𝐿𝐿𝐿𝐿𝑖𝑖 × �int �D𝑖𝑖−1

𝐿𝐿𝐿𝐿𝑖𝑖� + 𝛥𝛥𝑁𝑁𝑖𝑖�

In this way, the final synthesized distance can be obtained.

Operating Principles

A laser rangefinder is composed of a master control board, laser receiver board and keypad. The master

control board includes an integrated HT32F52352 MCU, which can control the laser transmission and

reception, and calculate the distance between rangefinder and object. The laser receiver board is

mainly used for laser echo signal reception and amplification, the amplified signals will be sent back

to the master MCU. Refer to the following chapters for a more detailed introduction.

Hardware Description

Figure 6

According to their functions, the laser rangefinder master board can be subdivided into several parts.

These include the Master MCU circuit, power control circuit, accelerometer circuit, APD high voltage

bias circuit, oscillator signal coupling circuit and laser modulation transmitting circuit. The master

MCU circuit contains the HT32F52352 MCU mentioned above and the peripheral basic circuits. The

power control circuit provides power-related control such as the USB power supply, battery charging

management and device power regulation, etc. The product current state can be obtained via the

accelerometer circuit, in which the angle can be calculated for calculations in other modes including


WAS-18A1EN V1.00 7 / 14 July 1, 2020

Pythagoras algorithm (2-point), Pythagoras algorithm (3-point), height measurement, level

measurement, etc. The APD high voltage bias circuit is used to provide a laser receiving required

voltage. The laser echo receiving sensitivity can be adjusted by controlling this voltage. The oscillator

signal coupling circuit will overlay a high frequency signal during laser transmitting and use a

frequency to couple with the echo signal during laser receiving. Finally, the echo signal phase can be

obtained via differential frequency phase measurement. The laser modulation circuit is used for laser

transmitting adjustment power control.

The laser rangefinder in this solution adopts a phase measurement method. Two laser emitters are

used for internal and external optical paths and the laser echo signal is received by the same laser

receiver. The distance between the rangefinder and an object can be calculated by the phase

difference of the laser signals on the internal and external optical paths. If the distance is short, the

phase difference of the internal and external optical paths is less, otherwise the phase difference

will increase if the distance is far. The phase calculation uses a Fast Fourier Transform (FFT)

algorithm. Several different frequencies are applied to measure the object with the same distance

to obtain an accurate distance value.

Layout and Hardware Considerations

Figure 7 and Figure 8 show the front view and back view of the master board PCB layout.

Figure 7. PCB Layout Front View

Figure 8. PCB Layout Back View


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PCB BOM Table


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Software Description

Start

初始化

Key scan

Measurement key is pressed?

Mode key is pressed?

Reference/Unit key is pressed?

History key is pressed?

Read history

Refresh TFT-LCD

Switch reference/unit mode

Switch measurement mode

Distance measurement

Mute key is pressed?

Disable buzzer

Clear key is pressed?

Clear data

Y

Wake up?

Key scan

N

N

Power key is pressed?

Wait for external interrupt wakeup

N N N

N N N

YY

YYYY

Y

System sleep

Initialisation

初始化 Library function

Other mode calculation

When the system is started the peripheral functions are initialised first, including ADC, PDMA, PWM

and other functions required for measurement. The measurement-related initialisation can be

implemented by calling the library function named Laser_SysInit.

The main function of laser rangefinder is distance measurement which can be implemented by directly

calling the library function named Laser_measure, where the returned value is the distance. If an error

has occurred during the distance measurement, the returned distance value will be “0”, and the

corresponding error code can be read.


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Other functions of the laser rangefinder such as area, volume, Pythagoras algorithm (2-point),

Pythagoras algorithm (3-point), height measurement, level measurement, etc., are implemented by

using the library functions to obtain the distance value first and then calculating it together with the

angle calculated by the accelerometer to get the final value.

Library Function Description

Start

GPIO Initialisation

Communication Interface Initialisation

Laser_SysInit

APC PWM Initialisation

APD PWM Initialisation

ADC Initialisation

PLL IC Initialisation

End

Laser_measure

Start

Calculate and return the distance value

APC calibration is successful?

APD calibration is successful?

First measurement is successful?

Second measurement is successful?

Third measurement is successful?

Fourth measurement is successful?

End

Return error message

N

Y

N

N

N

N

Y

Y

Y

Y

N

Y

The library function can implement APC and APD power auto adjustment and execute an ADC on

the captured echo signal. After this, an FFT algorithm is applied to obtained the echo signal phase, the

result of which will be used to calculate the actual distance. The final value is then returned to the

function. If an error has occurred during the calculation process, the returned distance value will be

zero, in such cases, the error message can be obtained through the error code, Err_Code. The distance

data can also be used together with an accelerometer to extend different measurement modes. The

accelerometer can obtain the tilt angle which is used in many other calculation modes, such as

Pythagoras algorithm (2-point), Pythagoras algorithm (3-point), height measurement, level

measurement and so on.


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Code Cause of Error Err 261 Exceed measuring range Err 501, 502 APC, APD initialisation failure Err 510 Si5351A communication failure Err 520, 521 ADC sampling error

Table 4

void Laser_SysInit(void)

Function: Initialise the measurement required peripheral functions, including GPIO, PWM, ADC,

PDMA, etc.

Input: None.

Output: None.

float Laser_measure(void)

Function: Distance measurement.

Input: None.

Output: Return the distance value (float). If the returned value is “0”, it indicates the measurement has

failed, the error code can be read from the “Err_code”.

void Laser_SysOff(void)

Function: Disable the MCU peripheral functions, including timers, ADC, PWM, etc.

Input: None.

Output: None.

Laser_On

Function: Turn on the laser.

Input: None.

Output: None.

Laser_Off

Function: Turn off the laser.

Input: None.

Output: None.


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Operation Description

Figure 9. Prototype Front View

Figure 10. Prototype Back View

1. Measurement key

After the measurement mode is selected, a short press on the measurement key will start the

measurement. If the key is held down for a longer period, the device will enter the continuous

measurement mode.

2. Function key

Pressing this key can switch to different measurement modes, including area, volume, Pythagoras

(2-point), Pythagoras (3-point) ①, Pythagoras (3-point) ②, auto level and auto height.

3. Reference/Unit key

The default rangefinder reference is rear reference, a short press on this key will switch the

reference mode, including front reference, tripod screw hole reference and rear reference. A

long press will switch the measuring unit to be meters (m), feet (ft) or inches (in).

Note: After the machine is restarted, the measuring reference will automatically switch to the

default setting, namely the rear reference.

4. Laser icon

5. 2.0 inch TFT-LCD display

Display the measurement mode and data calculation results.

6. History key

Press this key to directly check the historical measurement data. The last 20 measurements or

calculated data will be displayed in a reverse order.

7. Add/Subtract (+/-) key

In single distance measurement, area and volume measurement mode, a short press implements

the “add” function while a long press implements the “subtract” function.

8. Mute key

Press this key to turn off or turn on the buzzer.

9. Startup & shutdown/clear/return key

A long press on this key will start up or shut down the machine while a short press will clear

the previous operation or return.

10. Battery compartment

11. Tripod screw hole


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Solution Comparison

Holtek Solution Industry Solution

Function Distance measurement, angle calculation (Can be extended to other functions)

Distance measurement, angle calculation (Can be extended to other functions)

Performance Time-taken: <400ms for each time Flash: 26.5KB SRAM: 4.3KB

N/A

Cost Standard MCU + laser drive circuit Standard MCU + laser drive circuit Others Provide algorithm technology support N/A

Conclusion

This document has introduced a laser rangefinder solution implemented using the HT32F52352 master

MCU together with Holtek self-developed algorithms. The HT32F52352 provides a high speed ADC

sampling function which can implement laser signal capture and effectively reduce the error generated

in phase calculations. The MCU provides a core speed of 48MHz, allowing the laser rangefinder to

quickly calculate the object distance in 500ms.

Reference Material

Reference file: HT32F52352 Datasheet.

For more information, consult the Holtek website www.holtek.com .

Version and Revision

Date Author Issue 2019.11.29 洪瑋廷 Initial version


WAS-18A1EN V1.00 14 / 14 July 1, 2020

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