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Linearize optical distance sensors with a voltage-to-frequency converter

Jordan Dimitrov, Toronto, ON, Canada -April 19, 2012

A popular series of inexpensive distance sensors integrates an infrared emitting diode, a linear charge-coupled-device array, and a signal-processing circuit in one unit. The output is a dc voltage, VS, that depends on the distance, D, in a nonlinear manner (Figure 1).

Linearize optical distance sensors with a voltage-to-frequency converter figure 1To improve linearity, the manufacturer suggests using the relationship between the output voltage and the inverse value of the distance (Figure 2). You can use the curve-fitting utility of Excel software to calculate two or three coefficients of this alternative relationship, and a microcontroller can then use the coefficients to calculate distance from VS. The calculation requires floating-point arithmetic, which results in a large amount of machine-language code, a difficulty for many microcontrollers due to their limited memory size.

This Design Idea describes a way to present the sensor response with better linearity and a circuit that eliminates the need for complex calculations to find the distance. The built-and-tested unit uses the Sharp GP2D120 sensor (Reference 1), which measures distances of 4 to 30 cm (40 to 300 mm). This sensor is currently out of production but may be available through some sources. If not, a similar but untested replacement is the Sharp GP2Y0A21YK0F (Reference 2), which measures distances of 10 to 80 cm (100 to 800 mm).

Linearize optical distance sensors with a voltage-to-frequency converter figure 2Linearize optical distance sensors with a voltage-to-frequency converter figure 3

Figure 3
shows the linearity improvement you gain by using the inverse value of the voltage, VS, versus distance. Figure 4 shows the circuit that provides a linear relationship between distance and another variable. The key component is a voltage-to-frequency converter, such as the AD654, between the sensor and the microcontroller (references 3 and 4). The sensor’s response is 1/VS=aD+b, where a and b are coefficients. The VFC has a linear response, f=SFVS, where SF is a coefficient. The pulse period is T=1/f. The microcontroller defines the period as a number of internal clock pulses, N= T/TCLK. The period of clock pulses is 0.5 μsec, and it defines the values of the frequency-determining components of the VFC. From these equations, you can build a relationship between N and D: N=(aD+b)/(SF×TCLK), which is a straight line. The hardware circuit’s design performs the calculations; they do not take place when the microcontroller calculates distance.

Linearize optical distance sensors with a voltage-to-frequency converter figure 4

The RC network at the sensor output matches the sensor-voltage swing to the VFC’s input range and attenuates the 1-kHz noise riding on the sensor signal. The resistor divider modifies the system response to the form N=(aD+b)/ (kD×SF×TCLK)=α×D+β, where kD is the transfer ratio of the divider, α is the slope, and β is the offset.

Read more design ideasListing 1 shows the subroutine code for measuring and calculating the distance. Calibration is somewhat tedious because the sensor cannot measure zero distance. You adjust the slope of the last equation by using two reference distances and tweaking the 500Ω trimming potentiometer at the VFC. If the reference distances are 80 and 220 mm, you must adjust for a difference of 140 between the corresponding numbers on the display. When you finish that task, use any of the reference numbers to calculate the offset. In the code, subtract the offset from the measured value of N. A test of the assembled circuit covers the whole measurement range in steps of 20 mm. The nonlinearity error is ±3 mm, 2.7 times smaller than the error of the VS-versus-1/D response.

Linearize optical distance sensors with a voltage-to-frequency converter listing 1

Editor’s note: The author teaches a course on microcontrollers at a large community college in Toronto, ON, Canada. The course inspired this Design Idea. The Sharp distance sensor is an opportunity to show students that they can perform linearization using software or hardware, and they can compare the two approaches.

References
  1. GP2D120 Optoelectronic Device,” Sharp Microelectronics, 2006.
  2. GP2Y0A21YK0F Distance Measuring Sensor Unit,” Sharp Microelectronics, Dec 1, 2006.
  3. HC11 MC68HC11F1 Technical Data,” Freescale Semiconductor Inc.
  4. AD654 Low Cost Monolithic Voltage-to-Frequency Converter,” Analog Devices.

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