SATISFACCIÓN DEL CLIENTE

In the late 1940, Magneto-Rheological (MR) fluids were discovered as materials with abil- ity to change their shear strength within few milliseconds in response to external magnetic fields [13]. MR fluids are typically made from water or oil carrying suspended micron-sized particles. The viscosity of the fluid can be controlled with an external magnetic field through the formation of particles into columns aligned in the direction of the field. The use of MR flu- ids has been extended into compliant actuation mechanisms in that a clutch-like device filled with MR fluids, hence the name MR clutch, is used as the complaint element. MR clutches have the capability of transmitting input mechanical power to their output in proportion to the intensity of an applied magnetic field. In a Hybrid MR clutch (HMR clutch), a combination of a permanent magnet and an electromagnetic coil is used to develop the required magnetic field [14]. Fig. 4.1 depicts a cross-section of a HMR clutch along with the magnetic field distribution inside the clutch. The HMR clutch consists of an interchanging stack of input and output disks arranged around the rotational axis of the clutch with the space between them filled with MR fluid. As shown in Fig. 4.1 A, the magnetic fields generated by the permanent magnet crosses the MR fluid and sets the MR fluids resulting in partial torque generation in the HMR clutch. An electromagnetic coil is used to adjust the output torque by tuning the magnetic field strength. Fig. 4.1 B and C depict two boundary cases where the applied current to the electromagnetic coil results in the maximum and minimum, i.e., absolute zero, output torque in the HMR clutch, respectively. One can note that the electromagnetic coil is arranged such that its magnetic flux either adds or completely negates the flux of the permanent magnet and its hysteresis remanence (depending on the polarity of the current) within the MR fluids.

4.2.1

Prototype Hybrid MR clutch

To investigate the characteristics of the proposed HMR clutch, a prototype as shown in Fig. 4.2 was designed, developed, and fabricated. This design is composed of input/output magnetic disks that are held apart with non-magnetic spacers. The inner disks are mechanically coupled to the shaft, while the outer disks are fixed to the clutch casing. A permanent magnet ring is integrated inside the HMR clutch to produce a bias electromagnetic flux. N45 Neodymium magnet was chosen during optimization process to allow achieving absolute zero and maximum torques using minimal magnetic flux supplements from the electromagnetic coil. When the

4.2. HybridMR Actuator 69

(a)

(b)

(c)

Input/ Output Disk

Shaft

Housing

Permanent Magnet

Coil

Magnetic Flux

Figure 4.1: Magnetic field distribution inside a hybrid MR clutch: (a) magnetic flux density (B field) distribution inside the MR clutch due to the permanent magnet (off-state), (b) the field enhancement in the MR clutch pack with an applied magnetic field, and (c) the field cancellation in the MR clutch pack due to an applied magnetic field in reverse direction.

70 Chapter4. HysteresisModeling of aHybridMagneto-RheologicalActuator

(a) (b)

Figure 4.2: Prototype of the hybrid MR clutch. (a) Components of the prototype. (b) Assembly of the prototype.

electromagnetic coil is energized (depending on the polarity of the applied current), the shear stress of the MR fluid is altered (increased or decreased) within the magnetic path, allowing the HMR clutch to fully transmit or disengage the input mechanical power. Using this combination much higher torque-to-mass ratios can be obtained in comparison to coil-based MR clutch. The specifications of the prototype HMR clutch are presented in Table 4.1.

4.2.2

Integration of the Magnetic Sensor

The location of the magnetic sensors should be carefully chosen to accurate measurement of the magnetic flux inside the clutch. The flat surface of the magnetic sensor (Hall sensor) should be perpendicular to the path of the magnetic field to measure both the direction and magnitude of the flux. Given their comparable permeability to air, magnetic sensors must be placed in a location within the magnetic path that forces the magnetic flux to pass through the sensors. Fig. 4.3 shows the locations of the magnetic (Hall) sensors inside the prototype HMR clutch. The Hall sensors are sandwiched in a pack of two steel disks and an aluminum disk. The aluminum disk is chosen for its similar reluctance to the Hall sensors, allowing the magnetic flux density to be distributed equally through the sandwich pack. The clutch is designed to integrate two analogue and two digital Hall sensor assemblies. Two unipolar ratio-metric analog Hall sensors (Infinion TLE4990) faced in opposite directions were used to measure ±400 mT. Also, two bipolar digital Hall sensors (Infinion TLE4998S) were integrated inside the clutch to measure ±200 mT. The digital sensors provide a Single Edge Nibble Transmission (SENT) signal based

4.3. Modeling andControl ofHybridMR Actuator 71

Table 4.1: Specifications of the Hybrid MR Clutch

Diameter [mm] 155

Width [mm] 59

MR fluid gap thickness [mm] 0.425

No. of input disks 8

Off-state torque [Nm] 4.5

Nominal torque(at 2 A) [Nm] 45

Maximum torque [Nm] 65

Total mass [kg] 3.1

Torque density [Nm/kg] 21

on the SAE J2716 standard [15], which consists of a sequence of pulses. Each transmission has a constant number of nibbles containing the flux value, the temperature, and status information of the sensor. These digital sensors are used to monitor the magnetic field and temperature of the MR fluid simultaneously.