Other actuators:

A few less common magnetic actuators exist which do not fall into any of the categories previously discussed. One such device is an impact printer head, used on high-speed printers for computers. A small, powerful magnet attached to a spring- loaded arm hammer is held against a magnetically permeable platen by its own field. When the platen is magnetized by current in a surrounding coil, the head is driven forward by repulsion with great speed and force, printing a character on the paper. It then bounces back, to be recaptured again by the now de-energized platen.

Figure 11.24 Servo valve control

In one type of servo valve, pilot chambers push a spool which controls the main flow (Figure 11.24). Two holes, close

together, lead to the pilot chambers. A jet of fluid is aimed at the two holes by a magnetic tilt mechanism, so that a small rotation of the jet assembly directs the jet to one or other of the holes, or somewhere in between. The jet is rotated by a permeable bar between two sets of biasing magnets. With no current, the fields are just balanced at the center rest position but current through the coil causes the stable

position to shift in one direction or the other, rotating the nozzle. Deflection versus current is linear over a small

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Symbol Definition Units Units

SI CGS

a no. of winding parallel paths -- --

A area

B flux-density Tesla Gauss

C capacitance farad farad

E emf, voltage Volt Volt

d distance m in

D diameter m in

f frequency Hz Hz

F force Newton lb

h distance m in

H magnetic coercivity At/m Oersted

i current Ampere A

I current Ampere A

J moment of inertia

J current density

kl leakage constant -- --

eddy current constant -- --

emf constant V/rad/s V/krpm

hysteris constant -- --

reluctance coefficient -- --

torque constant Nm/A oz in

l length m in

L inductance Henry Henry

m number of coils/phase -- --

m number of phases -- --

mmf magneto motive force At Gilbert

p number of poles -- --

P permeance Wb/At M/B

R _{reluctance At/Wb B/M}

R resistance Ohm Ohm

s speed rpm rpm

S steps/rev -- --

t time s s

T torque Nm lb ft

v velocity m/s in/s

V supply voltage Volt Volt

W energy, work Joule Joule

z no. winding conductors -- --

thermal resistance 1/°C 1/°C

skin depth m in

wave length m in

flux-linkage Weber Maxwell

permeability Henry/m Henry/in

damping factor -- --

mechanical time constant s s

resistivity Ohm-m Ohm-in

flux Weber Maxwell