Locking differential need detection system implemented in

International Journal of Emerging Trends & Technology in Computer Science (IJETTCS) Web Site: www.ijettcs.org Email: [email protected], [email protected]

Volume 7, Issue 3, May - June 2018 ISSN 2278-6856

Volume 7, Issue 3, May – June 2018 Page 43

Abstract:A digital system is implemented in a FPGA (Field Programmable Gate Array) for detecting the locking differential need detection for front or rear differentialsfor a 1:10 scale RC vehicle. The detector isbased on wheel speed difference and the drive shaft speed. First it was implemented a Data Acquisition System (DAQ) in the FPGA for data analysis for different driving situations and then it was developed the detection system. The detector could differentiate between normal driving condition (vehicle turning) and driving conditions when a locking differential is needed. The detectorwas implemented easily using a true table, and it gives the driver information of which wheel loose traction and if it is necessary to lock the differential. Keywords:locking differential, detection system, FPGA, wheel speed difference.

1. INTRODUCTION A locking differential or locker is a variation on the

standard automotivedifferential. A locking differential provides increased traction compared to a standard, or "open" differential by disallowing wheel speed differentiation between two wheels on the same axle under certain conditions [1].

A locking differential is designed to overcome the chief limitation of a standard open differential by essentially "locking" both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction (or lack thereof) available to either wheel individually.When the differential is unlocked (open differential), it allows each wheel to rotate at different speeds (such as when negotiating a turn), thus avoiding tire scuffing. An open (or unlocked) differential always provides the same torque (rotational force) to each of the two wheels, on that axle. So, although the wheels can rotate at different speeds, they apply the same rotational force, even if one is entirely stationary, and the other spinning. (Equal torque, unequal rotational speed).By contrast, a locked differential forces both left and right wheels on the same axle to rotate at the same speed under nearly all circumstances, without regard to tractional differences seen at either wheel. Therefore, each wheel can

apply as much rotational force as the traction under it will allow, and the torques on each side-shaft will be unequal. (Unequal torque, equal rotational speeds). Exceptions apply to automatic lockers, discussed below.A locked differential can provide a significant traction advantage over an open differential, but only when the traction under each wheel differs significantly [1].

a. Types of Locking Differentials

There are two main types of lockers: automatic and selectable. a) Automatic Lockers

Automatic lockers lock and unlock automatically with no direct input from the driver. Some automatic locking differential designs,regardless of traction conditions,ensure that engine power is always transmitted to both wheels, and will "unlock" only when one wheel is required to spin faster than the other during cornering. They will never allow any wheel spin slower than the differential carrier or axle as a whole. The most common example of this type would be the famous "Detroit Locker," also known as the "Detroit No-Spin". Other automatic lockers operate as an "open," or unlocked differential until wheelspin is encountered and then they lockup. This style generally uses some type of internal governor to sense a difference in wheel speeds, orthey react to torque input from the driveshaft. An example of this would be GM's "Gov-Lok." This is the type most often found on factory equipped vehicles with a locking differential [1].

b) Selectable Lockers

A "selectable" locker allows the driver to lock and unlock at will the differential from the driver's seat. This can be accomplished via compressed air (pneumatics) like ARB's "Air Locker," electronic solenoids (electromagnetics) like Eaton's "ELocker," or some type of cable operated mechanism as is employed on the "Ox Locker." This allows the differential to perform as an

Locking differential need detection system implemented in a FPGA for a 1:10 scale RC

off-road vehicle

Miguel Angel Rodríguez Fuentes1, Alejandro Escamilla Navarro2

1Unidad Profesional Interdisciplinaria en Ingeniería y Tecnologías Avanzadas, UPIITA-IPN, Instituto Politécnico Nacional (IPN)

Av. Instituto Politécnico Nacional 2580, Col. La Laguna Ticomán, CDMX 07340, México

2Unidad Profesional Interdisciplinaria en Ingeniería Campus Hidalgo, UPIIH-IPN, Instituto Politécnico Nacional (IPN)

Ciudad del Conocimiento y la Cultura, Carretera Pachuca-Actopan 1 + 500, 42162, San Agustín Tlaxiaca, Hidalgo

http://www.ijettcs.org

mailto:[email protected]




"open" differential for improved drivability, maneuverability, and reduced tire wear, while also having full locking capability for ultimate traction when it is desirable or needed. This is really the best of both worlds, but selectable lockers are more complex and therefore more expensive than their automatic counterparts [1].

Each type of locking differential has advantages and disadvantages. The purpose of this project was focused in the following factors:

a) The importance for a driver with a selectable locker to have visual simple aids to know when it is necessary to lock a specific axle and which wheel(s)is(are) loosing traction. This visual information is also important in the case of an automatic locker.

b) The importance of an easily implemented locking differential need detection system in a digital device,using only wheeland driveshaft speed information, considering thatalmost all modern vehicleshave sensor in their wheels (used for ABS, control traction).

c) The importance that the locking differential need detection system could be able to differentiate between a normal driving situation (when no lock is needed) and a difficult driving situation (when lock is needed).

d) Having a locking differential need detection system could be use in implementing an automatic locker, in the near future.

2. Methodology 2.1Test vehicle and sensors

A Traxxas TRX-4 Defender 1:10 scale RCwas used. It has off-road capability with T_LockTM manual remote Locking Differentials and High/Low Transmission[2]. For measuring wheeland driveshaft speed, it was used Hall-Effect sensors from Traxxas [3] and ¼” diameter Neodymium magnets (Figure 1). Each wheel has 6 magnets and the driveshaft has 2 magnets.

Figure 1 Wheel with magnets and left front wheel sensor mount

2.2Acquisition System For measuring speeds and analyzing data, a data

acquisition system (DAQ) was developed using NEXYS 2 development board from Digilent Inc. [4] with a Spartan 3 FPGA (Field Programmable Gate Array) and based on [5]. The Figure 2 shows the block diagram of de DAQ system and was developed using RTL (Register Transfer Level) Methodology [6].

Figure 2 Block diagram of DAQ system

The DAQ system sense independently the wheel speed

from the front wheels (LF–Left Front and RF–Right Front) and the rear wheels (LR-Left Rear and RR-Right Rear) and the Driveshaft (DS). These speeds are temporary stored in a register at the same time, then they are multiplexed (MUX) and store temporarily in a RAM memory of the development board. The task of the register, MUX and write RAM blocks are governed by aFinite State Machine (FSM) implemented as a controller. All blocks were implemented using VHDL description language. Different conditions were measure measuring data for 10s and with an acquisition time of 0.5s.

The need of a locking differential is based on the difference of wheels speed in front or rear axle in a difficult terrain driving conditions, but it is critical to remember that in normal conditions (when it is not necessary to lock the differential) not all the wheels have also the same speed, so it is very important to find the difference betweenthese two conditions, and determinate the need or not of locking the differential. This is why, different tests were made for analyzing data. These tests are summarized in Table 1.

Table 1: Tests conducted for data analysis

Only four tests are presented as reference, and are the

following.

Need of locking differential(difficult

conditions)

No need of locking differential

(normal conditions) One wheel of vehicle stop turning suddenly

(LF,RF,LR,RR)

Vehicle in straight line with constant speed

Both left or right wheels stop turning suddenly

Vehicle in straight line accelerating

LF and RR wheels stop turning suddenly

Vehicle turning left/right

RF and LR wheels stop turning suddenly

Vehicle in a slalom test






2.3 Tests for data acquisition and analysis 2.3.1 Test 1: Vehicle start in a stop condition and then accelerate gradually in a straight line

In Figure 3, the vehicle is not moving (Data 1 to 4) and then it begins to accelerate driving in a straight line. The driveshaft (DS) has a higher speed compared with the speed of wheels because the gear reduction of the differential. In straight line, the speed of the four wheels is almost the same and is related of course with the DS speed. In this case, no locking differential is needed.

Figure 3 Speeds of a vehicle accelerating in straight line

2.3.2 Test 2: All wheels are turning but suddenly LR stop turning

Figure 4 LR wheel suddenly stop

In Figure 4, all wheels are turning (Data 1 to 9) but

suddenly LR stop turning. In this test, LR wheel has all the traction from the tire to the ground (wheel speed cero) and the open differential transfers all power to the RR wheel which has no traction to the ground (increase wheel speed). In data 10 to 21, the LR wheel speed is practically cero and the RR wheel increase his speed. Locking differential in rear axle is needed.

2.3.3 Test 3: All wheels are turning but suddenly both wheels on left side stop turning

Figure 5 Left wheels stop turning

In Figure 5, all wheels are turning (Data 1 to 6) but

suddenly both left wheels (LF and LR) stop turning. In this test the left wheels have traction to the ground and the right wheels have no traction. Locking differential is needed in both axles. 2.3.4 Test 4: Vehicle turn left (normal condition)

Figure 6 Vehicle turn left

In Figure 6, the vehicle turn left in a constant speed.

Because of the different turning ratios for each wheel, the wheel speed is different for all wheels, this is a normal condition despite different speeds between left and right wheels. In this condition, locking differential is no needed. 2.4 Locking differential block detection

All test data were analyzed in order to develop the locking differential need detector that would detect when it is necessary to lock or not the differential. For analysis, here are presented only two tests to explain the detectorrequirements. Figure 10 shows the DS speed, and the difference in speed for the front and rear axle for the test when the LR wheel stop turning (data 10), here the difference between LR and RR increase, and in the front axle the differences are almost the same.






Figure 10 Speed in DS and wheel speed difference in test

when LR wheel stop turning

It was founded that for the same test the percentage of difference for each pair of wheels in the same axle is calculated using (1) or (2).

%퐹 = 푀푎푥 _ −푀푖푛 _

푀푎푥 _(1)

%푅 = 푀푎푥 _ −푀푖푛 _

푀푎푥 _(2)

and it could determinate when it is necessary to lock the differential. For the front axle the difference percentage (%F) are below 0.4 and in the rear axle the difference percentage (%R) are above 0.7, Figure 11. The DS speed is also important, because it is related with the wheel speeds.

Figure 11 Difference percentage of front and rear axle in

test when LR wheel stop turning

Figure12 shows the speed difference in a normal situation and when the vehicle turns left and the driveshaft DS increase to a constant speed. Considering the difference percentage in the previous test, when the driveshaft reaches a constant speed, the difference percentage drops below 0.4 (Figure 13). In this case it is not necessary to lock the differential because it is a normal driving condition.

Figure 12Speed in DS and wheel speed difference in test

vehicle turn left

Figure 13Difference percentage of front and rear axle in

test vehicle turn left

Thus, the locking differential need detector (F=1 indicate it is neededed) works as follow: Let A and B, be the wheels speeds for a specific axle then: 1)Determinate the maximum value (MAX) between A and B

푀퐴푋 = 푀퐴푋(퐴,퐵) (3)

2)Determinate the minimum value (MIN) between A and B

푀퐼푁 = 푀퐼푁(퐴,퐵) (4) 3)퐼푓푀퐴푋 = (0푡표3)푎푛푑푀퐼푁 = 0푡ℎ푒푛퐹 = 0(푛표푙표푐푘푖푛푔) 푒푙푠푒푖푓푀퐴푋 ≥ 4푎푛푑푀퐼푁 = 0푡ℎ푒푛퐹 = 1(푙표푐푘푖푛푔)

푒푙푠푒푖푓(푀퐴푋 −푀퐼푁)

푀퐴푋 < 0.6푡ℎ푒푛퐹 = 0(푛표푙표푐푘푖푛푔)

푒푙푠푒푖푓(푀퐴푋−푀퐼푁)

푀퐴푋 ≥ 0.6푡ℎ푒푛퐹 = 1(푙표푐푘푖푛푔)(5)






2.5Digital Implementation of the detection system

The block diagram of the detection system is shown in Figure 14. All blocks were implemented using VHDL description language.

Figure 14 General block diagram of the detection system

In 0.9229s, eachspeed block can measure the speed of

any wheel and the driveshaft, then the front wheels speedand the rear wheels speed go to the detector block. Each speed block has a binary output (an integer number) that can represent in a simple way the speed of a given wheel or the driveshaft, and these numbers are the input for the detector block. If it were necessary to calculate the rpm of each wheel or driveshaft, we can use (6) or (7).

푅푃푀 = 푆푝푒푒푑 ∗ 10.8354 (6)

푅푃푀 = 푆푝푒푒푑 ∗ 32.5662(7)

An advantage of using a programmable logic device like a FPGA is that each speed block could calculate in parallel the speed, and using description languages is possible to add blocks easily.

The detector block wasimplemented as a true table. Each detector, as it was explained previously (5), determinate if F=0 (locking differential is not needed) or if F=1 (locking differential is needed), and also it indicates which wheel turn freely indicating the loose of traction between the tire and the ground. The detection system used two detector blocks, one for each axle. The enable block controls the visual indicators LEDs (Light-Emitting Diode) and they are enable only when the DS speedis above 6 (195.3972 rpm) and below 40 (1302 rpm)Remember that, the DS has higher speeds than the wheels. The visual information LEDs function are summarized in the Table 2.

Table 2:Logic conditions of the visual indicators LOCKING DIFF.

IS NEEDED (LDN=1)

LEFT WHEEL (LF or LR)

RIGHT WHEEL (RF or RR)

FBN=1 when Front locking differential is needed

LF=1 when LF wheel loose traction

RF=1 when RF wheel loose traction

RBN=1 when rear locking differential is needed

LR=1 when LR wheel loose traction

RR=1 when RR wheel loose traction

After the design was synthetized and implemented, the

system uses 208 Flip-Flops, 253 4-inputs LUTs (Look up tables), and 149 slices of the Spartan 3E FPGA (Figure 15). The maximum propagation time is 3.985ns.

Figure 15 FPGA Technology Schematic of the detection

system

Figure 16 and Figure 17 show the test vehicle with the FPGA board with the detection system.

Figure 16Traxxas TRX-4 and FPGA board






Figure 17Traxxas TRX-4 with detection system

Results, conclusions and future work a) The system developed could detect the need of locking

differential using visual aids (LEDs) for the driver, just using speed sensor of the wheels and the driveshaft.It is important for any driver with an off-road capabilities vehicle to know which wheel loose traction and if it is necessary to active the locker.

b) Most modern vehicles have already sensors for sensing wheel speed and some driveshaft or engine speed, so all of them can be used making the according modifications, forthe locking need detection system if the vehicle has a manual locking system.

c) The system developed, based on test conditions, is able to differentiate between normal driving conditions (for example the vehicle turning and when there are speed differences between wheels) and locking differential need conditions. It works on low speeds, reducing tire wear, and it is disable when the vehicle travel at high speeds for safety reason.

d) The system must be tested in conditions not considered in the experiment and should be done the corresponding adjustmentsfor the detection system, if it is necessary.

e) The use of programmable logic devices like the FPGA could acquire speeds data in a parallel process, instead of using a dedicated processor for each wheel and a centralized control for detection algorithm. The use of a truth table is simple solution and reduce processing hardware.

f) In the future work, the detection system will be used in developing an automatic front and rear locking for the test vehicle. The test vehicle has mechanical front and rear differentials controlled by a RC servo manually.

g) The system could be used in modified vehicles with installed manual lockers to help drivers the requirements needed in a given driving situation.

h) Most mechanical auto lock differentials work based on difference on speed wheels and using centrifugal brakes, clutch lockers, the system used an electronic detection system manner.

References [1] Locking Differential. [Online] Available:

http://www.offroaders.com/tech/locking-differential.htm. [Accessed: Apr, 15, 2018].

[2] Traxxas Inc. TRX-4 Defender. [Online] Available: https://traxxas.com/products/landing/trx-4/ [Accessed: Jan, 8, 2018].

[3] Model# 6520, Sensor, RPM (long)/ 3x4mm BCS. [Online] Available: https://traxxas.com/products/parts/telemetry/6520 [Accessed: Feb, 5, 2018].

[4] Nexys 2 Board. Digilent Inc. [Online] Available: https://reference.digilentinc.com/reference/programmable-logic/nexys-2/start [Accessed Nov, 14, 2017].

[5] M. Rodríguez, A. Escamilla. A Campos. Data Acquisition System Implemented in a FPGA for analyzing a 1:10 scale car turning direction. XXI Congreso Internacional de Ingeniería Eléctrica, Electrónica, Computación y Afines. INTERCON 2015. Universidad Continental, Huancayo, Perú, Ago, 2015.

[6] F. Vahid. Digital Design with RTL Design, VHDL, and Verilog. Wiley, 2nd Ed, 2010.

AUTHORS

Miguel AngelRodríguezFuentesreceived the B.S. degreein Industrial Engineering (1998) and M.S. degree in Computer Engineering (2000) from Instituto Politécnico Nacional (IPN) in Mexico

City.He is former professor in Engineering Department (Electronics). He was faculty advisor for SAE (Society of Automotive Engineering) student chapter at IPN-UPIITA from 2002 to 2016. He has teached curses related to Programmable Logic Devices since 2000 and use FPGAs in projects involving scale vehicles.

Alejandro EscamillaNavarro received the B.S. degree in Mechanical Engineering (1998) and M.S. degree in Mechanical Design (2005), from Instituto Politécnico Nacional (IPN) in Mexico City. He has been former

Engineering Professor since 1998, and researcher specially in anisotropic behavior materials under load conditions. He is Subdirector of Students Services and Social Integration at IPN-UPIIH.



http://www.offroaders.com/tech/locking-

https://traxxas.com/products/landing/trx-4/

https://traxxas.com/products/parts/telemetry/6520

https://reference.digilentinc.com/reference/program

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Locking differential need detection system implemented in