Motion Control Applications | CNC Control

Photodiodes: A smart motion control solution

Koren, Brock

The automotive industry is increasingly using photodiodes to provide greater levels of automation and product sophistication.

Mechanical and piezoelectric motion control has already reached nanometer-level precision and repeatability. While some further improvement in these areas can still be expected, a more commercially significant trend is towards "smart" systems, where feedback signals control motion using microprocessors and DSP-based controllers. The feedback signal is often derived from some type of optical sensor, frequently utilizing a photodiode.

This article briefly reviews the technology and advantages of photodiodes, and then examines some emerging applications in the automotive industry, which is one of the fastest-growing markets for smart motion control systems.

A photodiode is a special type of semiconductor diode that converts incident light into a useful electronic signal. The majority of photodiodes are silicon-based and respond to light in the 350- to 1000-nm range, i.e., the near-UV through near-IR. Like other solid-state components, photodiodes are small, simple, rugged, and reliable. Moreover, advances in wafer fabrication technology means they can be manufactured in high volume for relatively low unit costs. Obviously, all these advantages are direct benefits in a motion control system.

Just as important for OEMs, photodiode functionality and performance are highly customizable. For example, by fabricating small, low-capacitance devices, the response can be as short as 300 ps. On the other hand, other devices can offer over 10 decades of linear response. Also, photodiodes can be fabricated as single elements or multielement arrays, as well as position-sensing detectors, where a duplex electrical signal indicates the x-y centroid position of an incoming light beam.

Photodiode manufacturers have also supported the development of sophisticated "smart" applications by offering custom assemblies that incorporate optics and/or amplifier and signal-processing electronics. Indeed, many OEM customers have found it more cost-effective to incorporate a photodetector subassembly, in which the individual components are all preassembled, aligned, and fully tested. Modern packaging techniques, such as plastic encapsulation and surface-mount interconnects, enable this integration to be provided in an economical manner.

Two recent advances, the large-area avalanche photodiode (LAAPD) and the Filtrode(R), both patented by Advanced Photonix, offer specialized performance benefits for motion control applications. The LAAPD uses internal amplification to generate multiple charge carriers from a single incoming photon, yielding high sensitivity and low-light performance characteristics formerly only found in bulky, fragile vacuum-tube-based detectors. The Filtrode(R) is a photodiode in which a thin-film optical coating is deposited directly onto the detector's surface, enabling it to reject or accept specific wavelengths (Figure 1).

Automotive Applications

Automotive applications place unique demands on photonic devices. Simple economics dictates that these components must provide high reliability, meaning years of service, at a low cost. But at the same time they must withstand constant vibration over a range of frequencies. Also, devices outside the passenger compartment must endure extreme changes in temperature and humidity, as well as exposure to grit, oil, and dirt. Obviously this necessitates excellent bonding and packaging techniques. In fact, to supply devices to the automotive market, component manufacturers must meet a very strict set of qualifications referred to as QS 9000. These are similar in philosophy to the widely used ISO 9000, but even more stringent.

An interesting smart motion control problem in the automotive industry involves power windows. At present, power windows offer automatic travel only in the opening direction: this feature allows a window to be fully opened without the switch having to be continuously held down. This "hands-free" operation is obviously an advantage at toll booths, drive-through services, etc. Clearly, it would be just as advantageous to have hands-free closing as well. At present, this feature is not offered because of the danger of causing trauma to an object-e.g., a child's finger-accidentally placed in the window's opening; the switch must therefore be actively held down throughout the entire range of travel.

Unfortunately, a simple mechanical interrupt based on resistance pressure is not a viable solution to this problem. In order to force a good wind-rain seal, the closing window is typically accelerated over the last inch or two of travel. Consequently, the reaction time of a resistive-force-based loop would not be fast enough to completely eliminate the potential damage to a small finger. Engineers are therefore developing potential optical solutions.

One approach that has been investigated uses a near-infrared light-emitting diode (LED) located in the corner of the window. A low-cost cylinder lens is used to fan the LED's output to produce a flat sheet of light that is parallel to the window and close to its inner surface (see Figure 2). A photodiode-based sensor is located in the same module, with wide-field optics to gather light from the entire window area. The system is set up so that any sudden change-increase or decrease -in the photodiode signal will momentarily disable the automatic window-closing system.

Eliminating False Positives

A significant limitation of this approach is the number of false positives: the system can be fooled by sudden changes in the ambient light level, such as would occur when passing through the shadow of a building or a large tree. The goal of system designers is to reduce background effects while still delivering one hundred percent response to the industry-standard test: a 4-mm-diameter opaque tube placed in the furthest corner of the window.

A possible solution that is currently being investigated relies on wavelength discrimination. Advanced Photonix's Filtrode(R) technology offers an economical route to this goal. The light source is an LED that emits over a narrow wavelength band, and the Filtrode(R) coating is designed to transmit only this same wavelength band. This approach not only improves discrimination by orders of magnitude, but preserves the key goals of simplicity and low cost.

Fully automated window operation will also allow for smarter climate control. For example, the windows could be programmed to close automatically in response to rain falling on the car. Also, when the temperature of a parked car reaches a certain level, the windows would open and close to regulate the interior temperature. Photodiodes would also be used to determine the temperature by measuring the ambient near-infrared intensity at two different wavelengths.

Photodiodes and Cruise Control

As cars become smarter, photodiode-- based systems may also play an increasing role in controlling the motion of the car itself. An early example of this is the smart cruise control, already available on some German luxury models. Here a photodiode is mounted on the front of the car, together with an LED sending unit. The photodiode detects light that is reflected from objects in front of the car, allowing continuous ranging by measuring time or phase delay.

In practice, the cruise control is set at the desired speed. When an object enters the lane in front of the car, the cruise speed is lowered as necessary to maintain a minimum distance. When the vehicle in front accelerates or exits the lane, the original cruise speed is restored. In contrast to the automated window problem, this collision avoidance application utilizes basic highspeed photodiodes, where rugged packaging and low cost are the most important design parameters.

Finally, looking even further into the future, automotive engineers are exploring the concept of "drive by wire" and "drive by fiber." Analogous to the systems used in some airplanes, the controls would no longer be mechanically linked to the wheels and transmission, etc. Photodiodes are expected to play a key role, not only in transmitting information, but also as optical sensors to monitor the steering and transmission functions.

For more information, please contact the author of this article, Brock Koren, president of Advanced Photonix, 1240 Avenida Acaso, Camarillo, CA 93012; (805) 987-0146; fax: (805) 484-9935; e-mail: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it ; www.advancedphotonix.com.

Copyright Associated Business Publications Oct 2000
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