5.6 Igniters and Pyrotechnic Actuators
Igniters activate pyrotechnic actuators by inputting electrical power and outputting heat. Pyrotechnic devices operate by burning an explosive material that generates heat, causing pressure. Combustion pressure causes motion of mechanical pistons or other such devices, for staging separation, deployment of parachutes or other one-time events. Igniters are also called squibs or initiators.
Pyrotechnic actuation is commonly used for chute deployment and stage separation. Because of the lower atmospheric pressure at deployment altitude, the amount of pyrotechnic charge used in flight must be less than that used in ground tests, adjusted to compensate for the ambient pressure difference. At higher altitudes, not as much pressure (pyro charge) is required to eject the chutes, and over-pressuring them can cause deployment failure.
Pyrotechnic charge is usually ignited by the same kind of igniters used to light solid-propellant engines. One of the potential problems of igniters is their unpredictable electrical characteristics during and after ignition. While igniters are usually electrically open after igniting, some igniters end in a shorted state instead. If a simple relay-actuated circuit is used in series with a battery, the battery can remain shorted through the igniter, thereby discharging the battery.
Igniters are designed to ensure that the pyrotechnic charge ignites if the igniter is excited correctly electrically. The result of ignition can either be deflagration (burning) or detonation (explosion). The difference is that detonation is a burn rapid enough to cause a shock wave. Shock waves form when the burn-front moves supersonically.
Igniters have an electrical resistance of typically about 1 W. Current through the resistance causes ohmic loss of power, and it heats. When the temperature reaches the combustion temperature of the pyrotechnic material surrounding it, the material burns, further raising the temperature, until the main charge (if any) is set off.
Igniter heat output can be calculated from Watt’s Law for electrical power:
P = v× i
where v is voltage across the igniter and i is current through it. Power can be calculated from either voltage or current and igniter resistance by substituting into Watt’s Law from Ohm’s Law:
v = i× R
where R is resistance. Then substituting,
For example, a 1 W igniter conducting 2 A will dissipate 4 W (watts) of power. The resulting temperature also depends on thermal resistance, Rq , of the igniter and surrounding material. The "Ohm’s Law" of thermal "circuits" is:
T = Rq × P + TA
where T is temperature and TA is the ambient (surrounding) temperature. Then, the igniter temperature is equal to
T = Rq × i2× R + TA
For a thermal resistance of 50 °C/W and a power dissipation of 4 W, the temperature rise above ambient will be 200 °C.
Igniters can be driven by simple electrical circuits, such as the one shown below.
The battery is the electrical power source, controlled by a switch. The igniter will change resistance as it heats, and when burning occurs, will either open (infinite resistance) or short (zero resistance). If it shorts, and the switch remains closed, the battery will be shorted.
To remedy a possible battery short, a resistance, Rs, can be placed in series in the circuit so that the current is limited to the battery voltage, Vs, over Rs, by Ohm’s Law. Even then, excessive discharge of the battery can occur.
The better approach is to use pyro pulse generators to drive igniters. A pulse generator consists of a pulse source and a pulse amplifier. The source generates a pulse of a specified amplitude and duration (or pulse width) and the amplifier provides a controlled current or voltage to the load (igniter) proportional to the input pulse amplitude.
A better-designed pyro-actuation circuit is a constant-current source that is pulsed on for a programmed duration. A predictable amount of battery charge is then used, equal to
Q = i× D t
where Q is the charge (in coulombs º ampere-seconds), i is the igniter current, in amperes, and Dt is the pulse duration, in seconds.
A current-source amplifier provides a constant current for the pulse duration. This requires a controlled amount of battery charge, which is
Dq = i× D t
where i is the current-pulse amplitude and Dt is the pulse duration. Knowing the amount of charge required for pyro actuation makes it possible to accurately budget the total charge required from the battery for the mission. The unit of charge is the coulomb (C º A×s). Batteries are rated in charge, but usually in units of A×h (amp-hours) instead, where 1 A×h = 3600 C.
An example similar to a commercial pulse amplifier, the Innovatia PG2, is shown in the schematic diagram below. It is a dual current-source feedback amplifier. Only one of the two similar amplifiers is described here.
The on-board voltage regulator (U2) supplies U1 with a constant (ripple-free) and accurate 12 V. The supply input to the PG2 is an unregulated voltage above 12 V (+12V(U)).
A pulse is generated by computer and input at connector J1, PG IN. D1 provides input protection against negative voltage inputs. U1:A is an op-amp amplifier stage which drives the power output transistor, Q1. Output current (to the igniter), iO, flows through sense resistor R7, of 0.2 W value. By Ohm’s Law, the voltage appearing across it is iO×(0.2 W). This voltage is amplified by differential amplifier U1:B, which senses across R7 and amplifies the difference in voltage across it. This stage is also the H block of the feedback topology.
Output current develops a voltage across sense resistor R7 that is differentially input to the op-amp U1:B. R8-R11 also divide the output voltage down to within the input range of the op-amp. This one-op-amp diff-amp has a gain of 8.2. The voltage drop across R7 is 0.5 V for 2.5 A through it. Multiplied by 8.2, the output of U1:B is 4.1 V, the full-scale PG IN voltage.
In general, the transconductance (current out/voltage in) of the pulse amplifier is:
For R7 = 0.2 W, the input voltage times 0.61 A/V gives the output current. Consequently, the output of H (U1:B, pin 6) is
R7×AvH ×iO= (0.2 W)× (8.2)×iO @ (1.64 W)×i = (1.64 V/A)×iO
The output of U1:B is scaled to 1.64 V/A. And that is the scale-factor at the input. The difference between input and feedback voltages is amplified by U1:A as the error voltage.
This pulse amplifier outputs 2.5 A for an input voltage (the high level of the input pulse) of 4.1 V. R7 can be changed in value to achieve other scale factors (transconductances). For 0.4 W, a 4.1 V full-scale input will output 1.25 A instead. For accuracy, R8 - R12 should be ± 1 % tolerance resistors. A 5 V CMOS digital output bit line has close to a 5 V high level. By placing a 22 kW resistor in series with the digital output at the PG IN input, a voltage divider is formed with R1 that attenuates by ´ 0.82, resulting in a 4.1 V input.
C2 dynamically compensates the amplifier so that its output pulse is free of overshoot, ringing and slow transition times – in other words, it optimizes the response so that the pulse is as "square" in shape as possible. Diode D2 at the output protects the amplifier from accidental application of the supply voltage to it and also prevents small currents from flowing into the load when the pulse is off. It ensures that the output current is zero for an input pulse low level near zero volts.
All the parts of this amplifier are commodity components, widely available from many electronics component distributors.