Locking Differential Need Detection System Implemented in a FPGA for a 1:10 Scale RC Off-Road Vehicle
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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 Volume 7, Issue 3, May – June 2018 Page 43 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 "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. Volume 7, Issue 3, May – June 2018 Page 44 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 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.