Page 801 - Mechatronics with Experiments

LABORATORY EXPERIMENTS 787
11.9 EXPERIMENT 9: SOLENOID CONTROL USING
A TRANSISTOR AND PIC MICROCONTROLLER

Objectives
To control a DC solenoid using a power transistor and PIC 18F452 (or PIC 18F4331)
microcontroller.

Components

Item Quantity Part No. Supplier
On/Off (pull) type DC solenoid 1 142463 www.jameco.com
On/Off (push-pull) type DC solenoid 1 145314 www.jameco.com
Transistor: IRF510 (MOSFET) 1 06F8238 www.newark.com
Diode 1 76970 www.jameco.com
PIC Demo Board/Connectors 1 set DM163022 or www.microchipdirect.com
DV164006

Theory
A solenoid is a linear displacement actuator. It has a coil, a plunger, and a core to guide
the electromagnetic field between the coil (stator) and the plunger (rotor). When current is
applied to the coil, force is generated in the direction to minimize the magnetic reluctance.
The direction of the current does not affect the direction of the force. The force generated
is proportional to the square of the current and inversely proportional to the square of the
air gap between the plunger and the stopper. By design, some solenoids are designed to
be operated in ON/OFF mode and some are designed to be operated in proportional mode.
In the ON/OFF mode of operation, the plunger is intended to take one of two positions
(fully OPEN or fully CLOSED) based on the current in its coil. In the proportional mode of
operation, the solenoid can take intermediate positions as a function of the coil current. In
this experiment we use a solenoid designed to be operated in ON/OFF mode. With careful
real-time control software, it can still be operated in the proportional mode. To control the
solenoid in ON/OFF mode, the output signal from the microcontroller to the transistor,
which switches the load current to the solenoid, can be a digital output signal. If we wanted
to control it in proportional mode, then a PWM output pin of the PIC should be used to drive
the transistor. We can also use a digital output channel and drive it with a PWM type signal
under software control, since the PWM frequency to drive the solenoid does not have to be
very high and such low frequency PWM signals can be generated under software control
at a standard digital output pin. For instance, if we wanted to generate a PWM signal at
50 Hz with 5% duty cycle control resolution, it requires us to update the digital output pin
at the following rate: 50 ⋅ 100 = 1000 Hz.
5

Hardware
The circuit is designed to switch the voltage across the solenoid during the ON/OFF periods
of the cycle. In every cycle, when the voltage is high (12 V) the coil is energized by the
current flowing through it. A magnetic field, and the resulting actuation force, is produced
due to the tendency of the ferromagnetic plunger and coil generated magnetic flux to seek
the minimum reluctance point. This magnetic field pulls the plunger in towards the stopper.
When the voltage is low (0 V) for the remainder of the cycle (OFF period) the base current of