Automatic Altitude Control of Quadrotor

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Abstract

In this project, the default pressure sensor on the Hummingbird made by AscTec will be tested through several measurements. Since its performance is determined as insufficient to perform automatic altitude control of the quadrotor, other choices for distance measuring sensors will be discussed and tested, such as ultrasonic and infrared sensors. All the experimental data and conclusions drawn from the experiments will be provided for each sensor.

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Contents

Introduction....................................................................................................................................4

System Description........................................................................................................................4Original System..........................................................................................................................4Selection Criteria for Additional Distance Sensors................................................................5

Sensor Testing................................................................................................................................6Pressure Sensor Testing............................................................................................................6Ultrasonic Sensor Testing.......................................................................................................11Infrared (IR) Sensor Testing..................................................................................................14

Summary......................................................................................................................................17

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Introduction In this project, a commercial quadrotor is transformed from manual control to automatic control with a special focus on altitude control. The commercial quadrotor will need to keep the same altitude with 2 cm from minimum to maximum. The commercial quadrotor has some built-in sensors that can measure altitude, however, they do not fit the desired requirements, e.g. with respect to stability and accuracy. The task of this project is to select extra distance measuring sensors to integrate with the commercial quadrotor and to test their performance with respect to accuracy of measurement, detection range, and influence of object shape/ground level‘s surface.

System Description In this project, a quadrotor from Ascending Technologies, called AscTec Hummingbird [1], is used. Hummingbird has a few essential characteristics, e.g. the low level processor (LLP) ensures a highly stable flight behavior and the high level processor (HLP) controls the Hummingbird according to the user’s programming. On the hardware side, the Hummingbird has four 8 in long propellers, can output a maximum thrust of 20 N, weighs 0.5 kg, and has maximum takeoff weight of 0.71 kg. Moreover, the Hummingbird is equipped with the following sensors: a pressure sensor, an acceleration sensor, a yaw gyro sensor, a pitch gyro sensor, a roll gyro sensor, an AscTec 3D-MAG (compass sensor), and a GPS; however, those sensors may not be active or work in some environments or control modes, for example, the GPS does not work indoors.

The Hummingbird can be programmed using two different programming environments: the AscTec SDK to program in C language or AscTec Simulink Toolkit to work with MATLAB Simulink. In this project, the AscTec SDK is used. Furthermore, AscTec provides two communication tools: the AscTec Communication Interface and the AscTec Autopilot. Those two communication tools can display the values for different sensor readings and record them, but they still have some lacking functionality. For instance, AscTec Communication Interface only works on Linux systems and needs an additional programming to record the data. This takes additional time for a user who is not familiar with Linux. Also, Asctec Autopilot can only record certain data such as speed, battery life. Additionally, those two communication tools are working in the two different level processors: in the LLP (low level processor), the user only can use AscTec Autopilot to collect data; it does not work with AscTec Communication Interface. On the other hand, the HLP (high level processor) can only collect data using AscTec Communication Interface, not AscTec Autopilot. Nonetheless, the Hummingbird is still easy to program and monitor.

Original System

The non-functional GPS sensor in an indoor environment is the major issue in this project since the project is all about altitude control. In this case, the user can only rely on the pressure sensor

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on the Hummingbird, the MPXA6115A, made by Freescale [2]. According to the datasheet, this pressure sensor has some desirable features, such as being resistant to high humidity. It has a 1.5% maximum error over a temperature range from 0º to 85º C. It measures in a range from 15 kPa to 115 kPa and has 45 V/kPa sensitivity. It takes a 1 ms response time and 20 ms warm-up time. All measurements are taken when VS = 5.0 Vdc and Ta = 25º C. Its transfer function is given by Vout = Vs* (0.009 * P - 0.095) ± (pressure error * temp. factor*0.009*Vs ), where Vs= 5.0 ± 0.25 Vdc, see Figure 1. The pressure error band in the transfer function is ±1.5 kPa from 15 kPa to 115kPa, and the temperature factor is 1 when temperature is between 0º and 85º C. In to Figure 1, it shows the linear regression relationship between pressure and voltage with minimum, maximum and typical measuring range; however, the stability and accuracy does not show in Figure 1. The user can obtain the stability and accuracy by experiment.

Figure 1. Transfer function of pressure sensor.

Selection Criteria for Additional Distance Sensors

To fix this lacking range, it was decided to add distance-measuring sensors to the Hummingbird. Several distance measuring sensors can be found in the market, but only few sensors can be used in this project, e.g. because of take-off weight constraints.

Many sensors have the same functionality but have different ways to measure distance and have different weights. For distance measurements, the team is limited to three types of sensors: infrared sensor, ultrasonic sensor and laser sensor. The infrared sensor uses light to measure distance, and it is lightweight. Since it measures distance by light, it responds fast compared to an ultrasonic sensor that uses ultrasonic waves to measure distance. However, the infrared sensor can only work in a certain range, e.g. the Sharp infrared sensor (GP2Y0A41SK0F) can only measure from 4 to 30 cm [3]. This means that several infrared sensors would have to be added to the system to cover all the desired range of 4 to 150 cm.

The ultrasonic sensor is lightweight and can work on low voltage. Most ultrasonic sensors can have large distance measuring range, e.g. from 2 cm up to 3 m for the ultrasonic sensor

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Ping))) [4], which is more accurate than pressure sensor on the Hummingbird. Yet, it is slow to respond and it will influence the performance of Hummingbird.

Laser sensors have the most accurate measurement in the three sensors. They do not have short distance measurement limit, like ultrasonic sensor, and are as precise as infrared sensors. However, Laser sensors are heavier and also use more power, thus decreasing the operating time of the Hummingbird significantly even the Hummingbird can carry it. Given these considerations, it was decided to integrate the ultrasonic sensor and two different-range infrared sensors to reach the project requirement.

Pros ConsUltrasonic sensor - Lightweight.

- Wide detection range.- Middle distance measuring range.

- Measurement is less precise.- Slow feedback.

Infrared sensor - Lightweight.- Fast feedback.- Measurement is precise.

- Narrow detected range.- Limited distance measuring

range.

Laser sensors - Fast feedback.- Measurement is precise.- Long distance measuring range.

- Narrow detection range- Heavyweight- Use more power.

Table 1: Pros and cons of the different sensors.

Sensor Testing In this section, three different sensors will be tested. The on-board pressure sensor will be tested for 3 different experiments:

Measure height from same place/height for a certain time. Measure various heights on a small scale. Measure different height on a large scale.

The ultrasonic sensor will tested for 3 similar experiments as the pressure sensor, e.g. take different measurement for different distances, but also consider the influence of the angle and surface of an object. The infrared sensor will be tested for distance measurement, angle measurement and color detection. Based on those testing result, recommendations on additional sensors will be provided.

Pressure Sensor Testing

Since the pressure sensor is mounted on the Hummingbird, the user needs to be careful to not let the sensor get influenced by other outside variables, such as electrical flux. In this task, all of the experiments were recorded in an indoor environment with minimum environmental impact, such as low AC wind draft and nearly constant temperatures. During the testing, all of the motors and

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GPS on the Hummingbird were shut off/inactive. In Figure 2, the Hummingbird stayed in the same place and measurements were taken for five minutes using the pressure sensor. The figure depicts that the difference between the minimum and maximum measured height was close to one meter. It also shows that the sensor remains stable for the first two minutes. The measured height decreases between 2 and 3 minutes, but increases from 3 to 5 minutes. Unfortunately, there is no data to show why this happens. The mean for this experiment is 0.15 m. Based off this experiment, the team understands that the sensor used alone will not produce the desired result.

-100 100 300 500 700 900 1100 1300 15000

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measurement (1 per 200 ms)

heig

ht( m

)

Figure 2: Maximum/Minimum Distance Recorded by Sensor

The second experiment measured at different heights in a small scale. This experiment contains six different minor experiments (three minor experiments with two different directions: going up and going down). The distances of going up are from 0 m to 0.78 m, 0 m to 1.8 m and 0 m to 0.32 m to 0.73 m to 1.21 m to 1.8 m, continuously. The distances going down are the respective ones. For the first minor experiment from 0 m to 0.78 m and 0 m to -0.78 m, the measured height has a huge variation between each direction. In Figure 3 the red line shows the received measurement from 0 m to 0.78 m; however the means are 0.41 m and 1.27 m as compared to 0 m and 0.78 m. The difference between two means is 0.86 m. It is in a 10 % error. The blue line in Figure 3 shows the received data from 0 m to -0.78 m. In this data set, it shows that the means are 0.23 m at 0 m, and -0.33 m at -0.78 m. The difference between two means is only 0.56 m. It is not an accurate testing experience as the results yield an error of almost 30 %.

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0 50 100 150 200 250 300 350

-1

-0.5

0

0.5

1

1.5

2

Height_down Height_up

measurement (1 per 200 ms )

heig

ht(m

) mean: 0.41m

mean: 0.23 m mean: -0.33 m

mean: 1.27 m

Figure 3: The height measured from 0 to 0.78 m (red) and the height measured from 0 to -0.78 m (blue)

The second minor experiment is from 0 m to 1.8 m and 0 m to -1.8 m. Figure 4 shows Hummingbird going up (blue line) and down (red line). The means of 0 m and 1.8 m are 0.38 m and 2.19 m, respectively. The difference between the two means is 1.81 m, implying a good measurement. The means of 0 m and -1.8 m are -0.094 m and -1.64 m. The difference between the two means is 1.55 m, yielding a 14 % error.

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Height_up Height_down

measurement (1 per 200 ms)

heig

ht(m

)

mean: 0.38 m

mean:-0.094 m

mean:2.19 m

mean: -1.64 m

Figure 4: The height measured from 0 to 1.8 m (blue) and the height measured from 0 to -1.8 m (red)

These two minor experiments, which are difference in height between 0.78 m and 1.8 m, it demonstrated a pattern during these two experiments: measurements are more accurate when the Hummingbird is going up and less accurate when it is going down. A reason for this might lie in the sensor design: a fluorosilicate gel isolates the die surface and wire bonds from the environment, while allowing the pressure signal to be transmitted to the silicon diaphragm [2]. To

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test this hypothesis, measurements were taken with the sensor in upside down position. In this test, the result should be more accurate when the Hummingbird going down and less accurate when it goes up. However, the pressure senor did not work like the hypothesis. As a result, the direction of sensor’s position won’t affect the sensitivity when it goes up or down. It means the reason why the Hummingbird is more accurate when it goes up and down was not found.

0 100 200 300 400 500 600 700

-1.3

-0.8

-0.3

0.2

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1.2

1.7

Height_up Height_down

measurement (1 per 200 ms)

heig

ht(m

) µ:-0.038 mµ: 0.207 m

µ: 0.65 m

µ: 1.15 m

µ: 1.60 m

µ:0.23 m µ: -0.22 m µ: -0.43 mµ: -0.72 m

µ:-0.86 m

Figure 5: Mean of each different height when the quad going up (blue)\down (red)

The third minor experiment is moving from 0 m to 0.32 m to 0.73 m to 1.2 m to 1.8 m and staying at each level for 30 seconds and then repeating the same experiment in the opposite direction, i.e., from 0 m to -0.32 m to -0.73 m to -1.2 m to -1.8 m, see Figure 5. The motivation for this minor experiment was to find out if the error builds up for each increment/decrement in height. The measured averages of each height are shown in Figure 5. The difference between mean values for 0 m and 1.8 m is 1.64 m, yielding an 8 % error. It is an acceptable error range, but it is slightly higher than the measured error in the second minor experiment. The mean values from high to low (red curve) are 0.23 m, -0.22 m, -0.43 m, -0.72 m, -0.86 m. The difference between the average values of 0 m and -1.8 m is -1.09 m, which is a 44 % error. This minor experiment shows the pressure sensor has less accuracy when it goes down even when the Hummingbird goes down slowly. Figure 5 (red) also shows that this experiment has a higher error percentage than the experiment depicted in Figure 4 (red). There may be two reasons for this: firstly, the longer time may affect the measurement according to the first experiment. Secondly, the small scale measurement has less accuracy according to the second minor

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experiment. Based on those three minor experiments, the team learned that the pressure sensor does not provide a resolution that fits with desired one for small scales (from 0 meters to 2 meters).

Another set of experiments was conducted to learn how the pressure sensor operates at higher altitudes, i.e., in a large scale. The large scale heights are from 0 m to 8.1 m, 0 m to -8.1 m and from 0 m to 3.6 m to 8.1 m and back to 3.6 m and 0 m continuously. In these experiments, the team expects better accuracy compare with small scale. Figure 6 shows the experiments from 0 m to 8.1 m. The mean value from 0 m and 8.1 m are 0. 44 m and 8.23 m, respectively. The difference between this two means is 7.79 m, yielding a 3.87 % error.

0 50 100 150 200 250 30002468

1012

measurement (one per 200 ms)

heig

ht (

m)

mean : 8.23 m

mean: 0.44 m

Figure 6: The height measured from 0 to 8.1 m

Figure 7 shows the experiment from height 0 m to -8.1 m. The average values 0 m and -8.1 m are 0.31 m and -7.58 m, respectively. The difference between this two means is -7.89 m, resulting in an error of 2.59 %, which is much better than in the second minor experiment (which was from 0 m to -1.8m and had 14 % error). From these two results, the team learned that the pressure sensors perform better when measuring higher ranges.

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measurement (one per 200 ms)

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mean: 0.31 m

mean : -7.58 m

Figure 7: The height measured from 0 to -8.1 m

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The team also did an experiment regarding continuous movement at higher ranges, which is shown in Figure 8. It turns out that the sensor is quite stable compared to the results in Figure 5. The experiment was designed to go up and down twice. In this case, the team has three means at 0 m, four means at 3.6 m and two means at 8.1 m, which are shown in Figure 8. Every measurement above 0 m has an error of less than 10 %, and it is better than Figure 5.

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measurment(one per 200 ms)

heig

ht (m

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µ: 0.18 m

µ: 3.31 m

µ: 7.90 m

µ: 3.45 m

µ: 0.23m

µ: 3.50 m

µ: 7.99 m

µ: 3.52 m

µ: 0.17 m

Figure 8: The height measured from 0 m to 3.6 m to 8.1 m up and down

This pressure sensor also did not have stable and repeatability property; see Appendix 1, which shows 10 measurements at the same height/place and time duration. All measurements are in 3 sigma rule, which means that there were no outliers; however, the desired functionality is 1 cm error for same place/height. In conclusion, the pressure sensor can accurately measure distance at higher ranges. However, the experimental results imply that the pressure sensor will not be a good way to measure lower heights and that additional sensors are required for this purpose.

Ultrasonic Sensor Testing

The testing consists of two different minor tests/experiments, which evaluate the influence of an object’s surface and angular position. Each experiment collects data from 10 cm to 160 cm distance in 10 cm increments and with 50 measurements at each distance. The first test compares flat and not flat surfaces and determines whether or not the surface will influence the measurement. The second test compares different angles such that the measurement range can be determined. Before starting the first experiment, the ultrasonic sensor was tested with an object with flat surface, see Figure 9. It has R2 = 1 meaning that the linear regression equation fits perfectly with data.

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0 20 40 60 80100

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020406080

100120140160180200

f(x) = 1.19628689036499 x + 1.2335388838207R² = 0.999956100222388

measurement (cm)Linear (measurement (cm))

real distance(cm)

mea

sure

men

t (cm

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Figure 9: Measurement of a flat object

The first test is done using an object with a non-flat surface. According to Figure 10, the non-flat surface casts more outliers, resulting in R2=0.7165. However, when not accounting for the outliers, the linear regression equation is y = 1.0968x + 1.4087 and R2= 0.9997. Outliers were defined in two different ways: the physical limitation of a maximum distance of 300 m, i.e., all measurements above 350 cm are considered as outliers, and the ±3 σ rule which means that measurements in the range from µ-3σ to µ+3σ are not considered as outliers. For the distance measurement of 140 cm in Figure 10 this means that the 6 data points around 400 cm are outliers and so are the 2 data points at 205 cm and the 2 data points at 207 cm since µ±3σ ranges from 116.5 cm to 203.7 cm. Based on this experiment, the team learned that the surface will influence the sensor’s detection performance. The reason may be that the reflection of ultrasound wave did not return to the receiver on the sensor since object surface is not flat.

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f(x) = 1.09677993906 x + 1.40872430303979R² = 0.999718132847714f(x) = 1.16175882352941 x + 0.397999999999897R² = 0.716465790359825

measurement w/ outliersLinear (measurement w/ outliers)measurement w/o out-liersLinear (measurement w/o outliers)

real distance(cm)

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mt (

cm)

Figure 10: Measurement of a non-flat object

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The second test is done using the same flat object that was used in the pre-experiment (Figure 9) to determine how different degree angles, affect the measurements. These two particular angles, which are -15º and 15º, is the maximum angular range the sensor can detect according to the datasheet [4]. Figure 11 shows the result for -30 º with R2 = 0.7165 when using all data points. µ±3σ, for a distance of 100 cm is from 12.1 cm to 308.5 cm and for a distance of 110 cm is from 30.9 cm to 249.9 cm. Since the µ±3σ ranges are large at 100 cm and 110 cm the measurements around 200 cm at 100 cm/110 cm were not defined as outlier. The R2 without outliers is R2 = 0.8675. The result of 15º is better than for -30 º, see Figure 12. It has no outliers and R² =0.9993. Appendix 3 shows the measurements at 30º with R2≈ 0. Based on those three experiments, the maximum angular range should be around ±15º.

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f(x) = 1.23140294117647 x − 3.188R² = 0.71908347655404

f(x) = NaN x + NaNR² = 0

measurement w/o outlinersLinear (measurement w/o out-liners)measurement w/ outliersLinear (measurement w/ outliers)

real distance(cm)

mea

sure

men

t (cm

)

Figure 11: Measurement of object at -30º

0 20 40 60 80 100 120 140 160 1800

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f(x) = 1.0590794117647 x + 2.72450000000028R² = 0.999316679989414

measurement Linear (measurement )Linear (measurement )

real distance( cm)

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t (cm

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Figure 12: Measurement of object at 15º

The ultrasonic sensor has an angular detection range of around ±15º and it is more accurate at negative angles because of the sensor physical design. To fix the increase the range, another ultrasonic sensor may be needed. The other problem one needs to be aware of is the non-flat

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object detection. The detection will be less precise when tested in an outdoor environment since the environment contains more disorganized and non-flat surfaces. Lastly, all the tests were done by using the company’s default program which consistently resulted in measured values that were about 15% 20% too large (see slope of y in Figures 9-12)., e.g. the measurement is 60 cm when real distance is 50 cm, see Figure 9.

Infrared (IR) Sensor Testing

In this section, the infrared sensor, GP2Y0A41SK0F, with a detection range from 4 cm to 30 cm is tested [3]. The test variables include distance, color, and angle. The first experiment is distance measurement from 4 cm to 30 cm. This sensor does not come with a default program, so the voltage to distance transfer equation (v(d)) is needed. The relation between voltage and distance is provided in the datasheet, see Figure 13. The solid line is for a white object and the dashed line for a gray object with 18% reflection rate.

Figure 13 Relationship between voltage and distance for IR sensor

To get the v(d) from datasheet, the data was imported to Microsoft Excel, and curve fitting was applied, see Figure 14, resulting in

v(d)=10.853*(d)-0.943 with R2 = 0.997.

This v(d) equation does not include the part from 0 cm to 3 cm in Figure 13, which will be implemented at a later point. All experiments are taken in 4 cm increments with 50 measurements at each distance, see red line in Figure 15. The experiment revealed that the measured distance was systematically too small and the v(d) relationship was corrected to

v(d)=8.30*(distance)-0.943 - 0.7938

which was used in all following experiments, see blue line in Figure 15.

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0 5 10 15 20 25 30 350

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3f(x) = 10.8528415871314 x -̂0.943201561703772R² = 0.997027537488449

Series2Power (Series2)

distance / cm

volta

ge/

v

Figure 14: Relationship between voltage and distance for IR sensor

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f(x) = 0.764981318681321 x − 0.793839560439579R² = 0.995015773023623

f(x) = 1.04703846153846 x − 1.50403956043957R² = 0.99499950773079

after Linear (after )beforeLinear (before)

distance/ cm

mea

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t /cm

Figure 15: the liner regression before and after

The second experiment is tested on color detection with tan color which was chosen because most of the indoor environment was either white or tan. The result is precise, see Figure16.

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f(x) = 1.09264417582418 x − 1.91290813186812R² = 0.990264583241596

measurmentLinear (measurment)

distance/ cm

mea

sure

men

t / cm

Figure 16: The measurement of color

The third experiment, see Figure17, is testing the sensor performance depending on the angle of the object for angles of 8º and -8º, which is the maximum angular range from the datasheet. The measurements have some offset for the different angles because of the physical sensor design. The transmitter and receiver has 1.5 cm different and it may be the reason to cause the offset. Overall, the IR sensor is precise and stable, but the dictation range is short when compared with the ultrasonic sensor.

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f(x) = 0.815880357142857 x + 1.09465714285714R² = 0.998348489127464

f(x) = 1.13765 x − 2.3092857142857R² = 0.993144653687692

-8Linear (-8)8Linear (8)

distance / cm

mea

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ts /

cm

Figure 17: the measurement of -8º and 8º

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Summary From the sensor testing experiments in this project, it was determined that the ultrasonic sensor has wider range to detect object, but it is less precise than IR sensor. Also it needs some extra programming to correctly convert the voltage to the measured distance. The IR sensor has more precise measurements, but a shorter range of detestation than the other sensors. The pressure sensor can works on large scale distance measurements, but is less precise than the other sensors. Integrating all three sensors allows maximum measurement performance which is required for automatic height control of the Hummingbird Quadrotor..

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Appendix

Height (m)

Ave

rage

rece

ive

heig

ht( m

)

0 0.26 0.72 1.19 1.80.25 0.64 1.04 1.48 2.120.18 0.49 0.98 1.51 2.170.33 0.65 1.05 1.32 1.87

0.093 0.44 0.9 1.19 1.650.26 0.57 0.94 1.31 1.840.03 0.45 0.94 1.36 2.010.32 0.91 1.3 1.66 2.140.11 0.43 0.81 1.22 1.740.17 0.43 1.04 1.44 1.970.37 0.68 1.2 1.47 1.97

Appendix 1: stabilization testing (10 times)

-10 -8 -6 -4 -2 0 2 4 6 8 10

-10

-8

-6

-4

-2

0

2

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8

10

f(x) = 0.953487578323851 x + 0.209776358139872R² = 0.995195211907693f(x) = 1.81830188816506 ln(x) + 1.82495250949382

R² = 0.741487081839429

average received height (m)Linear (average received height (m))Logarithmic (average received height (m))

average received height (m)

heig

ht(m

)

Appendix 2: total measurement for pressure sensor (10 times)

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0 20 40 60 80 100 120 140 160 1800

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450

f(x) = 0.0276470588235302 x + 187.5825R² = 9.24429747100053E-05

measurement Linear (measurement )

real height( cm)

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sure

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t ( cm

)

Appendix 3: Measurement of object at 30º

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Reference

[1]AscTec Hummingbird, http://www.asctec.de/en/uav-uas-drone-products/asctec-hummingbird/, Ascending Technologies, 2014

[2] High Temperature Accuracy Integrated Silicon Pressure Sensor for Measuring Absolute Pressure, On-Chip Signal Conditioned, Temperature Compensated and Calibrated, http://cache.freescale.com/files/sensors/doc/data_sheet/MPXA6115A.pdf, Freescale Semiconductor,10/2012

[3]Distance Measuring Sensor Unit Measuring distance: 4 to 30 cm Analog output type, http://www.robot-electronics.co.uk/datasheets/gp2y0a41sk_e.pdf , Sharp

[4]PING))) Ultrasonic Distance Sensor, http://www.parallax.com/sites/default/files/downloads/28015-PING-Sensor-Product-Guide-v2.0.pdf, Parallax, 2/4/2013

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