Angular velocity units, such as rad/s
, deg/s
,
and rpm
, can also be used to measure frequency
for cyclical processes. This is consistent with frequency defined
as revolutions per second in a mechanical context, or cycles per second
in an electrical context, and lets you write frequency-dependent equations
without requiring the 2*pi
conversion factor. In
the SI unit system, however, the unit of frequency is hertz (Hz),
defined as 1/s
.
Simscape™ software defines the unit hertz (Hz
)
as 1/s
, in compliance with the SI unit system.
This definition works well when frequency refers to a nonrotational
periodic signal such as the frequency of a PWM source. For cyclical
processes, however, the block equations have to contain the 2*pi
conversion
factor, to convert the numerical value specified in Hz
,
or s
^{-1}, to angular frequency.
As a result, frequency units (based on Hz
)
and angular velocity units (based on rpm
) are not
directly convertible, and using one instead of the other may result
in unexpected conversion factors applied to the numerical values by
the block equations. For example, the AC
Voltage Source block explicitly multiplies the value you specify
for its Frequency parameter by 2*pi
,
to convert it to angular frequency before calculating the sine function.
Drop-down lists of suggested units in block dialogs reflect
this distinction. For example, if a block has a Frequency parameter
with the default unit of Hz
, the drop-down list
for this parameter contains only units directly convertible to Hz
(such as kHz
, MHz
, and GHz
)
and does not contain the angular velocity units. Conversely, if you
define a custom block where the Frequency parameter
has the default unit of rpm
, its drop-down list
of suggested units will include deg/s
and rad/s
,
but will not contain Hz, kHz
, MHz
,
or GHz
.
When you type a unit expression in the parameter units combo
box (instead of selecting a value from the drop-down list), the Simscape unit
manager considers the units of frequency and angular velocity to be
commensurate. For example, when the default parameter unit is Hz
,
you are able to type not only 1/s
, but also expressions
such as deg/s
and rad/s
. This
behavior is consistent with the Simscape implementation of angular
units (see Angular Units). It is
your responsibility to verify that the unit expression you typed works
correctly with the block equations and reflects your design intent.
Note:
Prior to Release R2013a, the unit definition for Hz was |