Exploring optical and magnetic sensors - Embedded.com

Exploring optical and magnetic sensors

Sensors help microprocessor-based systems find their bearings. Where would embedded systems be without them?

Sensors provide the windows through which microprocessor-based systems can observe their environment. Optical and magnetic sensors are used to detect the presence, absence, or motion of an object. Typical applications include sensing when a door is open or closed, or measuring the speed of a rotating shaft. This article looks at these two broad classes of sensors, their applications, and how they interface to processors.


Figure 1: Optical switches

Optical sensors
Figure 1a shows a slotted optical switch. An LED is mounted in a plastic housing, facing a phototransistor, but separated by a gap. If something moves into the gap, it blocks the light path between the LED and the phototransistor.

Slotted switches are often used to detect motor speed by placing a slotted wheel on the motor shaft; as the shaft rotates, it alternately blocks and unblocks the light path. Slotted switches are also used as indicators when a door or hood is open or closed. A flag on the door drops into the slot and blocks the light when the door is closed.

Figure 1b shows a reflective sensor, which works similarly. The phototransistor in a reflective sensor picks up reflected light from whatever is in front of the switch. Most reflective sensors have a focal length, which is the optimum distance for the object to be detected to be placed, usually between 0.1 and 0.5 inches. A reflective sensor is typically used to detect motor motion by painting or anodizing the motor shaft black, then placing a strip of reflective material on the shaft. As the shaft rotates, the sensor sees no reflection from the part of the shaft that is black, then high reflection from the reflective strip.

Slotted and reflective optical sensors share a schematic symbol, which is shown in Figure 1c. Both types also have some common characteristics that must be taken into account when designing them into a system.

Current transfer ratio
The LED and phototransistor pair have limited gain, usually less than one. The amount of current generated in the phototransistor collector for a given current through the LED is called the current transfer ratio (CTR). A typical CTR for a slotted switch is 0.1. This means that 10mA of current in the LED will result in 1mA of current in the collector. The CTR is sometimes specified as a ratio and sometimes in a table that shows the collector current for various values of LED current. The CTR is dependent on the LED and phototransistor characteristics and can vary widely from one optical sensor to the next.


Figure 2: Circuit for connecting an optical sensor to a digital I/O pin

The current transfer ratio has several implications when you want to interface an optical sensor to a processor. First, if you want to connect the switch directly to a digital input as shown in Figure 2, the transistor output has to swing between valid logic levels. To ensure that the phototransistor saturates, the value of the pullup resistor is limited. For example, if you're driving the LED with 10mA and the CTR has a minimum value of 0.1, then the pullup resistor will need to be around 5kΩ.

A smaller resistor would provide better noise immunity (lower impedance) and possibly faster speed, but wouldn't work with all optical sensors. The transistor must be able to sink enough current to ensure a valid logic low level. If you want to use a smaller pullup, you could either use an optical switch with a higher CTR or drive the LED with more current.

Optical switches are available with darlington transistor outputs, which often have a CTR higher than 1. However, they are typically only 20% as fast as a single transistor output and have a higher saturation voltage.

Reflective sensors also have a CTR. Since the sensor depends on reflected light, the CTR is dependent on the type of surface and the distance of that surface from the sensor. The CTR of a reflective sensor is normally established with a standard reflective surface, placed at the specified focal length from the sensor.

The CTR of a reflective sensor varies from device to device and with the application. If your sensor is aimed at a surface that switches between gray and black, you will not get the same CTR you get with the white reference used by the manufacturer. Your design has to accommodate the actual CTR resulting from your application of the sensor. One way to determine the range of CTR is to measure the CTR in your application, then compare that to the CTR of the same sensor using the same white reference used by the sensor manufacturer. This will give you an idea of the CTR range you can expect to see.

Since the CTR of an optical sensor has a wide range, you may want to connect the output of the sensor to an analog-to-digital converter (ADC). This allows the software to look for changes in the output level, rather than depending on the ability of the part to generate digital logic levels. The price for this capability, of course, is the cost of adding an ADC and the need for more time for ADC sampling.

Speed
The phototransistor in any optical sensor is fairly slow. This limits the maximum speed that can be detected. Typical numbers are 8ms turnon time and 50ms turnoff time.

If the drive LED in a sensor is software-controlled, the software must take the turnon and turnoff delay into account when reading the output of the sensor.

Mechanical instability
Mechanical jitter can cause strange results with reflective sensors. I saw a system once that used a reflective sensor to count rotations by looking at a shiny strip on a flat black motor shaft. The output of this sensor circuit generated an interrupt. Occasionally, the motor would stop with the shiny strip right at the edge of the detection area for the sensor. Machine vibration would then generate enormous numbers of interrupts to the processor, effectively shutting it down.

You could envision a similar situation with a slotted sensor, if the flag that interrupts the light path only partially obscures the phototransistor. This could leave the phototransistor halfway on, causing an ambiguous output. Good hardware design, such as the use of hysteresis in comparator circuits, will minimize these problems.

Reflective sensors require some additional considerations. Reflective sensors are frequently used to sense objects of differing types. A good example would be paper in a high-speed sorting mechanism. The paper has varying quality, color, and reflective properties. The sensor system must be designed to handle all the types of material used. In the motor speed application, what happens if a film of oil coats the flat black portion of the shaft? What does that do to the sensor's ability to detect rotation?

In some cases, you may have to add hardware or software (or both) to detect unusual conditions. In the example, where a reflective sensor generated excessive interrupts, the software might have a timer that keeps track of the time between interrupts. If the sensor interrupt service routine is exited and immediately reentered, the interrupt service routine could disable the interrupt and set a flag to tell the rest of the system that something is wrong.

Failed LEDs
In systems where safety is an issue, be sure that a failed sensor doesn't cause the system to operate in an unsafe manner. A typical example would be a safety hood that must be closed before the machine can start. The idea is that all the dangerous moving parts are under the hood, so if the hood is closed you know the operator's hands are out of the way.

You could solve this problem with a slotted optical switch and a flag that blocks the light path when the hood is closed. You then connect the phototransistor emitter to ground and pull the collector up with a resistor. When the flag is blocking the sensor, the transistor is off and the output is high.

The problem with this approach is that an open or disconnected LED would appear the same to the system as a closed hood. The system might then try to start with the hood open. In a case like this, use a flag that instead clears the path when the hood is closed. A failed LED then looks like an open hood and everything is safe.

An even safer method would use two sensors, one that's blocked when the hood is open and one that's blocked when the hood is closed. For complete operator safety, the machine isn't allowed to start unless both sensors are in the correct (hood closed) state.

You occasionally need to know if the LED in an optical sensor has failed. Say you use a slotted switch to determine if a motor is turning. If the motor appears to stop, you might need to know if the motor is jammed or if the sensor LED has failed (or been disconnected) so you can display the correct diagnostic message.


Figure 3: Detecting a failed LED

Figure 3 illustrates a simple way to detect a failed LED. A comparator senses the voltage at the LED anode. When the LED is on, it will have a voltage drop around 1.2V (typical), so the comparator output will be high. If the LED opens, the voltage at the anode will rise to Vcc. (For this to work, Vcc must be greater than 3V.)

The circuit as shown is for an LED that's on all the time. You can also use this method for a switched LED, but you've got to take the voltage drop across the switching transistor into account when selecting the reference voltage. The software must ignore the comparator output when the LED is turned off.

Although a disconnected LED is much more likely than a shorted LED, you could also add a second comparator to detect the shorted condition. The reference voltage would be around 0.6V, and the software would declare an error if the voltage were to drop below the reference.

Optical options
In addition to slotted switches and reflective sensors, optical sensors are also used in optical isolators, and discrete optical emitters/receivers are also available.

Optical isolators
An optical isolator (also called an optoisolator or optocoupler ) houses an LED and a phototransistor in a package like an IC. An optical isolator isn't used to detect mechanical motion. Instead it provides electrical isolation between two electrical circuits. The optical isolator is sealed, so there's no way to break the light path.

A common use for optoisolators is to isolate a high-voltage circuit from the microprocessor that controls it. Musical instrument digital interface (MIDI) technology uses optical isolation to interconnect electronic musical instruments, preventing problems caused by differing ground potentials.


Figure 4: An optical isolator

Figure 4 shows how an optoisolator can be used to pass signals from one circuit to another. The ground and power connections for the system may be completely separate. Even in a single system where the grounds are nominally the same, an optoisolator may be used to prevent ground loops, or to keep noise in one ground (such as pulse-width modulation motor noise) out of the logic/analog ground.

Some optoisolators are available with logic outputs instead of phototransistor outputs. These devices have additional circuitry inside the IC to convert the analog output into a digital level.

Optoisolators have the same gain and speed issues that optical sensors do. However, the CTR of an optoisolator can be higher, typically in the 0.2 to 1 range, because the LED is closer to the phototransistor base.

The speed of an optoisolator is usually better than for an optical switch. The common 4N35 optoisolator has turnon and turnoff times of 10ms each, so it can pass signals over 10kHz. For high-speed isolation, a fast optoisolator is normally used. The 6N136 is capable of speeds up to about 1MHz. This part uses a photodiode coupled to a transistor to achieve high speed.

Discrete optical sensors
A design occasionally calls for the use of discrete optical parts—an LED and a phototransistor. These are usually infrared parts, like those in packaged optical switches. They are normally used to detect when an object is blocking the light between the LED and phototransistor, but in places where physical characteristics preclude using a slotted switch.

Discrete parts are connected and used in the same way as an optical switch or optoisolator, but with a few additional considerations. Since the distance between sensor and phototransistor is usually larger, the CTR is lower. The circuit often needs a software adjustment for LED current or sensing threshold for reliable and repeatable operation. In some cases, a lens may be required on one of the parts to focus the light. A software adjustment/gain scheme can also compensate for cases where the LED and phototransistor are on separate mechanical assemblies and the tolerance stackup causes misalignment.

In a packaged optical switch, the LED and phototransistor are matched to the same infrared (IR) wavelength. Although most IR phototransistors and LEDs will work together, these parts do operate at different peak wavelengths in the IR range. When using discrete parts, it's best to select an LED and phototransistor that are designed for the same IR range. If the parts have different ranges, then an LED at one end of its range and a phototransistor at the other end of its range may result in a system with a lowered CTR.

Other optical sensors
Other optical sensors include photodiodes and charged coupled device arrays. We unfortunately don't have room here to discuss those devices, but you can obtain more information about them in my book, Analog Interfacing to Embedded Microprocessors, (Woburn, MA: Newnes, 2001).

Magnetic sensors
Probably the simplest magnetic sensor to use in an embedded application is a Hall effect sensor. The Hall effect, discovered by Dr. Edwin Hall in 1879, is the production of a voltage in a current-carrying semiconductor in the presence of a magnetic field. This voltage is proportional to the current flowing in the sheet and the magnetic flux density.

A Hall effect sensor is made from silicon, and the voltage produced is only a few microvolts per gauss. Consequently, a high gain amplifier is required to bring the signal from the Hall element to a useable range. Hall effect sensors integrate the amplifier into the same package as the sensor element.

Hall effect sensors are available as sensors that produce an output proportional to the magnetic field, or as switches that change state when the magnetic field exceeds a certain level. Analog Hall effect sensors are suited to applications where you need to know how close a magnet is to the sensor—such as sensing whether an oscillating arm is really moving. Hall effect switches are best for applications where you just need to know if a magnet is near the sensor, such as sensing whether a safety hood is closed or open.

The output of an analog Hall effect sensor can be connected to a comparator or ADC like any other voltage-output sensor. One caution—some analog output sensors provide an output that's proportional to the supply voltage. For an accurate noise-free output, you must power the sensor from a noise-free, well-regulated supply. A typical analog Hall effect sensor will produce an output that is halfway between the supply voltage and ground when no magnetic field is present. When a north pole is near the sensor, the voltage moves toward ground, and when a south pole is near the sensor the voltage moves toward the positive supply.

Hall effect switches produce a digital output to indicate the presence of a magnetic field. They drive the output active when a certain magnetic strength (the operate point ) is sensed, then drive the output inactive when the magnetic field drops below a certain level (the release point ). There's some hysteresis in the range, where the release point is less than the operate value.

Hall effect switches come in two varieties—unipolar and bipolar, sometimes called nonlatching and latching. Bipolar switches have a positive (south pole) operate point and a negative (north pole) release point. Unipolar switches have a positive (south pole) operate point and a less-positive release point. In both cases, the actual operate and release points vary with temperature. Both bipolar and unipolar switches typically have an open-collector output that has to be pulled up with an external resistor.

Hall effect sensors are commonly available in 3-lead packages similar to the TO-92 transistor package. The three leads are power, ground, and output. Typical supply voltages are 5 to 10V, although some sensors operate up to 30V or more. When using a Hall effect sensor, remember to account for stray magnetic fields. If using a magnet on, say, a rotating shaft, be sure that the magnet doesn't excessively magnetize the shaft itself, or this may affect the output of the sensor.

Remember that the magnetic field falls off with the approximate square of the distance. The output of an analog Hall effect sensor may be linear with respect to the strength of the magnetic field, but it will not be linear with respect to distance.


Figure 5: A geartooth Hall effect sensor

Geartooth Hall effect sensors include a magnet and Hall effect sensor in one package. They're designed to measure the rotation of a geared device by placing the sensor near the gearteeth as shown in Figure 5. As each geartooth moves past the sensor, it affects the magnetic field between the magnet and the Hall effect sensor, causing an output pulse to be generated.

Other magnetic sensors include the variable reluctance sensor (VRS), linear variable differential transformer (LVDT), and devices such as a Hall effect potentiometer made by Clarostat.

Super sensors
Optoisolators and magnetic sensors provide a versatile solution to numerous position and motion sensing needs, and there's probably one that's well-suited to your application.

Stuart Ball is an electrical engineer with over 20 years of experience designing embedded systems. He is the author of three books, Embedded Microprocessor Systems: Real World Design, Debugging Embedded Microprocessor Systems, and Analog Interfacing to Embedded Systems, all published by Butterworth-Heinemann. His e-mail address is .

Further reading
Analog sensors:
Ball, Stuart R. Analog Interfacing to Embedded Microprocessors, Woburn, MA: Newnes, 2001.
Kodak CCD Primer
www.kodak.com/US/en/digital/pdf/ccdPrimerPart2.pdf
Photodiode information, from CentroVision Inc.
www.centrovision.com/tech2.htm

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