FIGURE 2. This is the mechatronic version of a DPDT switch.
We’ll flip this switch with electrons.
actuator has a 30 mm stroke with a gear reduction ratio of 50. The actuator’s working voltage is 12
volts and the controller is a two-wire, open-loop interface with limit switching at the stroke
endpoints. Actually, this actuator does not house an electronic “controller” since power is applied
directly to its motor. The only control we have over this actuator is to reduce the voltage to the
motor which will result in slower actuator movement and reduced load handling capability. Motor
voltage reduction is usually only considered in battery-powered systems.
The two-wire interface is actually a power interface with the red wire representing MotorV+ and
the black wire acting as the Motor ground input. An internal limit switch will remove power to the
motor when the actuator is within 0.5 mm from either stroke extent. Once an extent is reached and
power is removed from the actuator by the limit switch, the only way to move the actuator is to
reverse the polarity of the voltage applied to the actuator power interface. The actuator will drive in
the opposite direction until it is within 0.5 mm of the extent it is approaching.
A purely mechanical way of extending and retracting the actuator is depicted in Figure 1. A
DPDT mechanical switch is all we need to reverse the polarity at the motor interface. We can write
the firmware behind the mechanics you can build to throw the switch, or we can convert mechanics
in Figure 1 to mechatronics.
The mechatronic alternative is drawn up in Figure 2. Now we have something to hang some
code around as the H-bridge device has three logic inputs. We need to rough out an H-bridge/PIC
SCHEMATIC 1. The H-bridge only requires a pair of
control lines. That leaves us plenty of expansion GPIO.
28 RB6/PGC RB7/PGD
SERVO 08.2010 69