Page 530 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c07
516 MECHATRONICS Printer: Yet to Come October 9, 2014 8:41 254mm×178mm
P. S.
Controller
ENC ENC
P1 ADC 1
P2 ADC 2 DSP DAC/ Amp
PRX 1 DI 1 PWM
PRX 2 DI 2

P1
Cylinder

Electric motor or Pump Valve PRX 2
engine
E/M P

Tank PRX 1
ENC
P2

FIGURE 7.87: Control system components and circuit diagram for a single axis EH control
system which operates in two modes using closed loop control: 1. speed mode, 2. force mode.

The position range of the actuator around which that is expected is known. When a certain
pressure differential is sensed, the control system is supposed to automatically switch to
force (pressure) regulation mode, maintain the desired pressure until a certain amount of
time is passed or another discrete ON/OFF state sensor input is high. Let us assume that
we will use the time period option to decide how long to apply the pressure. Then reverse
the motion, and return to the original position under a programmed speed profile. Such EH
motion control systems are used in mechanical testing of products, injection molding, nip
roll positioning, and press applications.
(i) Modify the design of the EH system used in the previous example.
(ii) Draw a block diagram of the control logic and closed loop servo algorithms for both
position and force servo modes. Suggest ways to make a “bumpless transfer” between
the two modes.

There are two closed loop control modes: 1. position servo mode where the actual
position is sensed using an encoder (ENC, PRX1, PRX2) and the desired position is
programmed, 2. force servo mode where the force information is derived from the measured
differential pressure (P1 and P2). The desired force profile (constant or time varying) is pre-
programmed and its magnitude can be changed for each product. Therefore, we need two
servo control algorithms (i.e., of PID type). Each loop has its commanded signal (desired
position or desired force) and feedback signal (encoder signal or differential pressure sensor
signal). At the power-up, the cylinder makes a home search motion sequence in order to
find an absolute reference position (also called home position) so that every cycle is started
from the same position. Using an absolute encoder, the controller can read the current
position at power-up, and command the axis to move to the desired position for the home
reference (using the position servo loop). If an incremental encoder is used, then an external