This issue's articles: Boeing patents force field • Segway PTs • Digital interface selection • PWM explained
• Fire fighting water cannons • Feature video: Linear Position Sensors • To view newsletter as webpage click here.

Boeing Patents Force Field

From science fiction to reality, Boeing’s shockwave attenuator patent claims to lessen or protect “assets” from damage due to shockwaves. The patent defines shockwaves as “traveling discontinuities in pressure, temperature, density, and other physical qualities through a medium, such as the ambient atmosphere.” It further states protection is achieved by detecting shockwaves and attenuating them using an electromagnetic arc.

It lists several methods this arc could be generated by resulting in, “relatively hot and ionized air to form the second, transient medium ... such that the shockwave contacts the second, transient medium and is attenuated in energy density by ... (various techniques to deflect or absorb energy)”. To view the patent click link and enter the patent number: 8,981,261

Digital Interface Selection and Application
Editor's note: this section continues a multi-part series covering output interface selection and applications of the most frequently used interfaces. This issue focuses on reasons various digital options are selected and describes PWM.

There are five digital interface types typically used for position sensor outputs. These are PWM (pulse width modulation), SPI (Serial Peripheral Interface), SSI (Synchronous Serial Interface), CANopen (Controller Area Network bus), and Incremental output types.

PWM is chosen for an interface in applications that include brake pad feedback and forklift steering. It can add a layer of data integrity to the measurement system because the signal will be processed as full - on or off based on output level. Duty cycle is the only determinate of percentage of full scale output.

This contrasts with voltage output where a bad contact can cause the appearance of a fluctuating reading while the input remains fixed.

PWM is just what its name implies. It is a square wave but instead of a fixed 50% Duty Cycle of off and on time, the amount of time during one cycle that the signal is high (on) of low (off) is variable. While the on and off times can be varied, the frequency is fixed.

By varying tON and/or tOFF to change the Duty Cycle, the average voltage over time can be changed. These times are controlled by a moving rod (linear position sensor) or turning shaft (rotary position sensor) on an application to apply various position inputs to the position sensor.

In one example, a position sensor with PWM output can be applied to a motor controller to control amount and direction of motor movement. The motor controller translates duty cycle to an average voltage.

This results in, VAVG = VIN • Duty Cycle

With a motor controller output of 0 V when a PWM output at 50% Duty Cycle, it would leave the motor motionless with no rotation in either direction. Changing from above to below that duty cycle would mean the controller ouputs a voltage that ranges from positive to negative voltage applied to the motor. Increasing Duty Cycle to >50% would turn the motor in one direction, say clockwise, and <50% Duty Cycle would turn it in the other direction. Both 0% and 100% Duty Cycles would drive the motor at maximum speed in clockwise or counter-clockwise directions respectively.

Positioning Water Cannons

Best known for fire fighting equipment, Elkhart Brass manufactures firefighting and fire protection equipment for fire departments, building systems, offshore drilling sites and other applications. One of their product lines are Sidewinder EXM electronically remote controlled water canons. Key components in the control system are Vert-X 1300 absolute angle sensors measuring the horizontal and vertical positions of "monitors".

Fire departments and industry use these in manual and automated modes to spray water at a precise flow rate over a 0 to 350° travel range. Water pressure in these systems can go from zero to 1,500 psi and flow rate from 15 to 3,000 GPM. Since the angle sensors are absolute sensors, they retain position information throughout any power loss so the water cannon can stay on target even after a loss of power.

The water cannons are controlled via a joystick and display system connected to a central programmable controller using CANBus protocol. This system also controls the fire engine’s engine, pump, valves, lights as well as camera and aerial ladder.