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 Archiwum Process Control Club 2002, poz.20

 
 

Construction and Operation of
Magnetorheological Rotary Brake

 

Sławomir Bydoń MSc.
Department of Process Control
University of Mining and Metallurgy 30-059 Kraków, al. Mickiewicza 30, Poland
 

Abstract - Value of braking torque of magnetorheological (MR) rotary brake can be controlled by setting of the electric current in its coil. Advantages of MR brake are: smooth-acting, simple construction and ease of control. The paper presents construction, operation and basic technical data of the rotary brake filled with magnetorheological fluid. Exemplary application, in which motion of the rotary pneumatic actuator is controlled with help of MR brake is also described.


TABLE OF CONTENTS


1. Introduction
2. Construction of MR rotary brake
3. Operation of MR rotary brake
4. Technical data of MR brake
5. Velocity control system with pneumatic actuator and MR brake
6. Results of simulation tests
7. Conclusions
References


1. Introduction

Devices using MR fluid are getting more popular, because they have a lot of advantages. Most important of them are: ease of control, not sophisticated construction, durability, low power consumption, smooth-acting, stepless torque control, fast reaction, ease of installation (control signal is led by two wire cable) and almost linear characteristics [1]. Because of advantages, wide range of usage possibilities and despite of big cost of MR devices they will probably be very often used in different kind of control systems. Magnetorheological devices are used in a lot of applications like: vehicle suspensions, pneumatic actuators, vibration damping systems, prosthesis, exercise machines and many others. In this paper construction and operation of MR brake produced by Lord Corporation is described.


2. Construction of MR rotary brake

Construction of MR rotary brake was shown in fig. 1. It consists of rotor (3) fixed to the shaft (6), which is placed in bearings (5) and can rotate in relation to housing (4). Wires (1) allow supplying electric current to the coil (2). Between rotor and housing there is a gap (7) filled with MR fluid (8). Construction of the brake and location of the coil makes the gap to be in the magnetic field generated by the coil.

Fig. 1
Fig. 1. Sheer draft of magnetorheological rotary brake. 1 - wires, 2 - coil, 3 - rotor, 
4 - housing, 5 - bearing, 6 - shaft, 7 - gap, 8 - MR fluid


3. Operation of MR rotary brake

MR fluid being in the brake is suspension of magnetic particles in the liquid (water, oil or some other kind of liquid). Magnetic particles are dissipated in the liquid when field strength is equal to zero (H = 0) but when the field strength is grater then zero (H0) particles are magnetised, they gather in chains and make liquid flow through them to be difficult (fig. 2.).

Fig. 2
Fig. 2. Behaviour of MR fluid without (H = 0) and with (H ą 0) magnetic field.

Operation of MR brake is based on the effect (called MR effect) described above [2, 3 ]. MR fluid changes viscosity when it is in magnetic field. Fig. 3. shows MR fluid in the gap during working in clutch (direct-sheer) mode, when there is magnetic field applied. This kind of working takes place in the brake. Fluid is cut by two surfaces moving in the different directions. In the brake these are surfaces of housing and rotor.

Fig. 3
Fig. 3. Direct-sheer mode of MR fluid.

Current in the coil creates magnetic field in the gap. Coil is supplied with 12 VDC. Value of the current can be set from 0 to 1 A. Magnetic field strength and following it fluid viscosity depend on the current in the coil. Viscosity of the fluid influence torque that brakes the rotor. When the current in the coil is equal to zero there is no magnetic field and brake has torque equal to minimum Mmin. Mmin is equal to the torque caused by bearings, seal and viscosity of carrier liquid. When current I = 1 A then H 0 and brake has highest possible value of the torque Mmax, that is limited only by maximum current in the coil Imax and construction of the brake. 
To control the brake voltage-to-current transducer can be used. With its help it is possible to change standard voltage signal (05) V to current signal (01) A, that is needed to supply coil. Torque can be set stepless, because the brake is controlled by an analog signal. In practice number of steps depend on used digital-to-analog converter.


4. Technical data of MR brake

Fig. 4. shows the static characteristic (dependence between torque M and coil current I) of MR brake. Torque can change from Mmin = 0.34 Nm to Mmax = 5.65 Nm. Time-constant is (1030) ms. Power consumption at maximum torque is 12 W. Rotation speed is limited to 1000 RPM. Working temperature is form -30 to 70 °C. Coil resistance is 8 . Important feature is smooth working (seal and bearings cause very small static friction).

Fig. 4
Fig. 4. Dependence of current I in the coil on torque M.

 


5. Velocity control system with pneumatic actuator and MR brake

Magnetorheological rotary brake was used in velocity control system with pneumatic rotary actuator. Schematic of experimental setup was shown in fig. 5. and its view is shown in fig. 6. Pneumatic actuator (4) is set in motion by pressurised air flowing through two pneumatic valves (6). Shaft of actuator is connected with shaft of brake (2) with the help of rigid coupling (3). Additional inertia load (1) is connected to the shaft of the brake. Angular position measuring system is based on resistance transducer (5). System is controlled by PC with I/O board (fig. 5.). By changing torque of the brake it is possible to control angular velocity of the shaft.

Fig. 5
Fig. 5. Schematic of control system with MR brake and pneumatic actuator.

 

Fig. 6
Fig. 6. Experimental setup.


6. Results of simulation tests

Angular velocity and angular position of the shaft being a response to the step of velocity command
(z = 3.5 rad/s) is shown in fig. 7.

Fig. 7
Fig. 7. Angular velocity and angular position of the shaft, 
the response to the step of velocity command (z = 3.5 rad/s)


7. Conclusions

Exemplary application shows, that MR brake can be used in control system. Accuracy of velocity control is about 15% and setting time is about 0.1 s. Improvement of control algorithm and determining of accurate static and dynamic characteristics of brake is being planed. 


References

[1]    Carlson J.D., Spronston J.L., 19-21 June 2000, Controllable Fluids in 2000 Status of ER and MR Fluid Technology, "Actuator 2000"- 7-th International Conference on New Actuators.
[2]    Kordonski W., January 1993, Elements and Devices Based on Magnetorheological Effect, Journal of Intelligent Systems and Structures, 4, 65 - 69.
[3] Sapiński B., 2001, Wpływ fluktuacji pola magnetycznego na charakterystyki mechaniczne liniowego tłumika MR, Zeszyty Naukowe Politechniki Krakowskiej - Mechanika, 83, 243 - 252.
[4]  Szenajch W., 1992, Napędy i Sterowanie Pneumatyczne, WNT, Warsaw.