TMC457 DATASHEET (V. 1.16 / 2009-Nov-25)
8
5 Functional Blocks and Registers
5.1
Ramp Generator
The ramp generator is the heart of the motion controller. It runs either ramp with linear velocity profile
or ramp with s-shape velocity profile. The selection is done by the bow parameter. Setting bow to 0
selects linear velocity profile. Linear ramps perform the quickest motion, by using the maximum
available acceleration at all times. But, since the acceleration becomes switched on and off abruptly,
system resonances can occur. They appear like an additional load on the motor, thus reducing the
available useful portion of motor torque. Further, system resonances need some time to fade away,
and this can costs valuable system time, if a complete stand still is required, before other actions can
start. With the S-shaped ramp, resonances can be reduced. However, it is advised to choose the bow
parameter as high as possible, in order to optimize positioning time.
The ramp generator provides four modes of operation:
[…]
It should be noted, that the choice of the microstep resolution directly influences the complete ramp
generator parameter settings, because a higher microstep resolution means a higher end velocity
setting, and thus a higher acceleration and a higher bow parameter to yield the same results. This way,
the settings are scaled in a huge range, e.g. when changing between fullstep and highest resolution
microstep.
Attention:
At all times, all parameters may be changed, but it should be noted, that unexpected results may
occur, when changing the bow parameter to a lower value during an acceleration phase, or when
changing the acceleration or deceleration parameter to a lower value. In these cases, the maximum
positioning velocity, respectively the target position could be exceeded, in case the new values do not
allow decelerating quickly enough. Even an overrun of the register value could occur and lead to
unexpected results. Under normal circumstances, the bow parameter will be fixed in an application.
position
A
B
N
-4 -3 -2 -1
0
1
2
3
4
5
6
7
t
Figure 5 : Outline of ABN Signals of an Incremental Encoder
5.2
ABN Incremental Encoder Interface
The TMC457 is equipped with an incremental encoder interface for ABN encoders that gives positions
via digital incremental quadrature signals (usually named A and B) and a clear signal (usually named N
for null of Z for zero). The N signal can be used to clear a position. It might be necessary to disable the
clearing of the encoder position after the first N signal event because the encoder gives this signal
once for each revolution and for most applications a motor turns more than one revolution.
The encoder constant named
enc_const
is added or subtracted on each position change of the
quadrature signals AB of the incremental encoder. The encoder constant
enc_const
represents an
unsigned fixed point number (16.16) to facilitate the generic adaption between motors and encoders. In
Copyright © 2009 TRINAMIC Motion Control GmbH & Co. KG
TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
