Locking Differential Need Detection System Implemented in a FPGA for a 1:10 Scale RC Off-Road Vehicle

Home , Locking differential

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

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

Abstract:A digital system is implemented in a FPGA (Field apply as much rotational force as the traction under it will Programmable Gate Array) for detecting the locking differential allow, and the torques on each side-shaft will be unequal. need detection for front or rear differentialsfor a 1:10 scale RC (Unequal torque, equal rotational speeds). Exceptions apply vehicle. The detector isbased on wheel speed difference and the to automatic lockers, discussed below.A locked differential drive shaft speed. First it was implemented a Data Acquisition can provide a significant traction advantage over an open System (DAQ) in the FPGA for data analysis for different differential, but only when the traction under each wheel driving situations and then it was developed the detection system. The detector could differentiate between normal driving differs significantly [1]. condition (vehicle turning) and driving conditions when a a. Types of Locking Differentials 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 There are two main types of lockers: automatic and differential. selectable. a) Automatic Lockers Keywords:locking differential, detection system, FPGA, wheel speed difference. Automatic lockers lock and unlock automatically with no direct input from the driver. Some automatic locking 1. INTRODUCTION differential designs,regardless of traction conditions,ensure that engine power is always transmitted to both wheels, and A locking differential or locker is a variation on the standard automotivedifferential. A locking differential will "unlock" only when one wheel is required to spin faster provides increased traction compared to a standard, or than the other during cornering. They will never allow any wheel spin slower than the differential carrier or axle as a "open" differential by disallowing wheel speed whole. The most common example of this type would be differentiation between two wheels on the same axle under the famous "Detroit Locker," also known as the "Detroit certain conditions [1]. No-Spin". Other automatic lockers operate as an "open," or A locking differential is designed to overcome the chief limitation of a standard open differential by essentially unlocked differential until wheelspin is encountered and then they lockup. This style generally uses some type of "locking" both wheels on an axle together as if on a internal governor to sense a difference in wheel speeds, common shaft. This forces both wheels to turn in unison, orthey react to torque input from the driveshaft. An regardless of the traction (or lack thereof) available to example of this would be GM's "Gov-Lok." This is the type either wheel individually.When the differential is unlocked most often found on factory equipped vehicles with a (open differential), it allows each wheel to rotate at different speeds (such as when negotiating a turn), thus locking differential [1]. avoiding tire scuffing. An open (or unlocked) differential always provides the same torque (rotational force) to each b) Selectable Lockers of the two wheels, on that axle. So, although the wheels can rotate at different speeds, they apply the same rotational A "selectable" locker allows the driver to lock and force, even if one is entirely stationary, and the other unlock at will the differential from the driver's seat. This spinning. (Equal torque, unequal rotational speed).By can be accomplished via compressed air (pneumatics) like contrast, a locked differential forces both left and right ARB's "Air Locker," electronic solenoids wheels on the same axle to rotate at the same speed under (electromagnetics) like Eaton's "ELocker," or some type of nearly all circumstances, without regard to tractional cable operated mechanism as is employed on the "Ox differences seen at either wheel. Therefore, each wheel can Locker." This allows the differential to perform as an

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"open" differential for improved drivability, Figure 1 Wheel with magnets and left front wheel sensor maneuverability, and reduced tire wear, while also having mount full locking capability for ultimate traction when it is 2.2Acquisition System desirable or needed. This is really the best of both worlds, For measuring speeds and analyzing data, a data but selectable lockers are more complex and therefore more acquisition system (DAQ) was developed using NEXYS 2 expensive than their automatic counterparts [1]. development board from Digilent Inc. [4] with a Spartan 3 Each type of locking differential has advantages and FPGA (Field Programmable Gate Array) and based on [5]. disadvantages. The purpose of this project was focused in The Figure 2 shows the block diagram of de DAQ system the following factors: and was developed using RTL (Register Transfer Level) Methodology [6]. 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 Figure 2 Block diagram of DAQ system vehicleshave sensor in their wheels (used for ABS, control traction). The DAQ system sense independently the wheel speed from the front wheels (LF–Left Front and RF–Right Front) c) The importance that the locking differential need and the rear wheels (LR-Left Rear and RR-Right Rear) and detection system could be able to differentiate the Driveshaft (DS). These speeds are temporary stored in between a normal driving situation (when no lock a register at the same time, then they are multiplexed is needed) and a difficult driving situation (when (MUX) and store temporarily in a RAM memory of the lock is needed). development board. The task of the register, MUX and d) Having a locking differential need detection write RAM blocks are governed by aFinite State Machine system could be use in implementing an automatic (FSM) implemented as a controller. All blocks were locker, in the near future. implemented using VHDL description language. Different conditions were measure measuring data for 10s and with 2. Methodology an acquisition time of 0.5s.

2.1Test vehicle and sensors The need of a locking differential is based on the A Traxxas TRX-4 Defender 1:10 scale RCwas used. It difference of wheels speed in front or rear axle in a difficult has off-road capability with T_LockTM manual remote terrain driving conditions, but it is critical to remember that Locking Differentials and High/Low Transmission[2]. For in normal conditions (when it is not necessary to lock the measuring wheeland driveshaft speed, it was used Hall- differential) not all the wheels have also the same speed, so Effect sensors from Traxxas [3] and ¼” diameter it is very important to find the difference betweenthese two Neodymium magnets (Figure 1). Each wheel has 6 magnets conditions, and determinate the need or not of locking the and the driveshaft has 2 magnets. differential. This is why, different tests were made for analyzing data. These tests are summarized in Table 1. Need of locking No need of locking differential(difficult differential conditions) (normal conditions) One wheel of vehicle Vehicle in straight line with stop turning suddenly constant speed (LF,RF,LR,RR) Both left or right wheels Vehicle in straight line stop turning suddenly accelerating LF and RR wheels stop Vehicle turning left/right turning suddenly RF and LR wheels stop Vehicle in a slalom test turning suddenly

Table 1: Tests conducted for data analysis

Only four tests are presented as reference, and are the following.

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2.3 Tests for data acquisition and analysis 2.3.3 Test 3: All wheels are turning but suddenly both 2.3.1 Test 1: Vehicle start in a stop condition and then wheels on left side stop turning 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 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 3 Speeds of a vehicle accelerating in straight line

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

Figure 6 Vehicle turn left

In Figure 6, the vehicle turn left in a constant speed. Figure 4 LR wheel suddenly stop Because of the different turning ratios for each wheel, the

wheel speed is different for all wheels, this is a normal In Figure 4, all wheels are turning (Data 1 to 9) but condition despite different speeds between left and right suddenly LR stop turning. In this test, LR wheel has all the wheels. In this condition, locking differential is no needed. traction from the tire to the ground (wheel speed cero) and the open differential transfers all power to the RR wheel 2.4 Locking differential block detection which has no traction to the ground (increase wheel speed). All test data were analyzed in order to develop the In data 10 to 21, the LR wheel speed is practically cero and locking differential need detector that would detect when it the RR wheel increase his speed. Locking differential in is necessary to lock or not the differential. For analysis, rear axle is needed. 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.

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Figure 10 Speed in DS and wheel speed difference in test when LR wheel stop turning Figure 12Speed in DS and wheel speed difference in test vehicle turn left 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).

− % = _ _ (1) _

− % = _ _ (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 Figure 13Difference percentage of front and rear axle in also important, because it is related with the wheel speeds. 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

= (, ) (3)

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

= (, ) (4) Figure 11 Difference percentage of front and rear axle in test when LR wheel stop turning 3) = (0 3) = 0 ℎ = 0 ( )

Figure12 shows the speed difference in a normal ≥ 4 = 0 ℎ = 1 () situation and when the vehicle turns left and the driveshaft DS increase to a constant speed. Considering the difference ( − ) < 0.6 ℎ = 0 ( ) 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 ( − ) ≥ 0.6 ℎ = 1 () (5) differential because it is a normal driving condition.

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2.5Digital Implementation of the detection system After the design was synthetized and implemented, the The block diagram of the detection system is shown in system uses 208 Flip-Flops, 253 4-inputs LUTs (Look up Figure 14. All blocks were implemented using VHDL tables), and 149 slices of the Spartan 3E FPGA (Figure 15). description language. The maximum propagation time is 3.985ns.

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) Figure 15 FPGA Technology Schematic of the detection that can represent in a simple way the speed of a given system wheel or the driveshaft, and these numbers are the input for the detector block. If it were necessary to calculate the rpm Figure 16 and Figure 17 show the test vehicle with the of each wheel or driveshaft, we can use (6) or (7). FPGA board with the detection system.

= ∗ 10.8354 (6)

= ∗ 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 Figure 16Traxxas TRX-4 and FPGA board 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. LEFT WHEEL RIGHT WHEEL IS NEEDED (LF or LR) (RF or RR) (LDN=1) FBN=1 when Front LF=1 when LF RF=1 when RF locking differential wheel loose wheel loose is needed traction traction RBN=1 when rear LR=1 when LR RR=1 when RR locking differential wheel loose wheel loose is needed traction traction

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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/program mable-logic/nexys-2/start [Accessed Nov, 14, 2017]. [5] M. Rodríguez, A. Escamilla. A Campos. Data Figure 17Traxxas TRX-4 with detection system Acquisition System Implemented in a FPGA for Results, conclusions and future work analyzing a 1:10 scale car turning direction. XXI Congreso Internacional de Ingeniería Eléctrica, a) The system developed could detect the need of locking Electrónica, Computación y Afines. INTERCON differential using visual aids (LEDs) for the driver, just 2015. Universidad Continental, Huancayo, Perú, using speed sensor of the wheels and the driveshaft.It is Ago, 2015. [6] F. Vahid. Digital Design with RTL Design, VHDL, important for any driver with an off-road capabilities nd vehicle to know which wheel loose traction and if it is and Verilog. Wiley, 2 Ed, 2010. necessary to active the locker. AUTHORS b) Most modern vehicles have already sensors for sensing

wheel speed and some driveshaft or engine speed, so all Miguel AngelRodríguezFuentesreceived of them can be used making the according the B.S. degreein Industrial Engineering modifications, forthe locking need detection system if (1998) and M.S. degree in Computer the vehicle has a manual locking system. Engineering (2000) from Instituto c) The system developed, based on test conditions, is able Politécnico Nacional (IPN) in Mexico to differentiate between normal driving conditions (for City.He is former professor in Engineering Department example the vehicle turning and when there are speed (Electronics). He was faculty advisor for SAE (Society of differences between wheels) and locking differential Automotive Engineering) student chapter at IPN-UPIITA need conditions. It works on low speeds, reducing tire from 2002 to 2016. He has teached curses related to wear, and it is disable when the vehicle travel at high Programmable Logic Devices since 2000 and use FPGAs in speeds for safety reason. projects involving scale vehicles. d) The system must be tested in conditions not considered in the experiment and should be done the corresponding Alejandro EscamillaNavarro received the adjustmentsfor the detection system, if it is necessary. B.S. degree in Mechanical Engineering e) The use of programmable logic devices like the FPGA (1998) and M.S. degree in Mechanical Design could acquire speeds data in a parallel process, instead (2005), from Instituto Politécnico Nacional of using a dedicated processor for each wheel and a (IPN) in Mexico City. He has been former centralized control for detection algorithm. The use of a Engineering Professor since 1998, and researcher specially truth table is simple solution and reduce processing in anisotropic behavior materials under load conditions. He is Subdirector of Students Services and Social Integration hardware. at IPN-UPIIH. 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.

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