AN 1000 – Electrolytic Tilt Sensor Excitation

Electrolytic Tilt Sensor Excitation

Description

This document explains the minimum hardware and software requirements for obtaining an angular position measurement from a Fredericks electrolytic tilt sensor.

Basic Design and Sensor Excitation

Figure 1 shows the major components required to create a functioning tilt measurement unit using an electrolytic tilt sensor.

Figure 1 High level view of tilt measurement unit

 

As you will see later on, it is important to avoid direct current to the sensor. We therefore suggest using an operational amplifier with high input impedance such as the LMC6482 from Texas Instruments. This will limit current leakage to ground and in turn, direct current to the sensor. Figure 2 shows the circuit equivalent of a single axis electrolytic tilt sensor.

 

Figure 2 Circuit representation of a single axis electrolytic sensor

 

Let’s continue with the excitation signal for the sensor. The excitation signal is provided by the microprocessor with the simplest case being a single axis sensor. A single axis sensor requires two output ports of the microprocessor which are connected to the outer pins of the sensor. The ports are toggled with a 50% duty cycle and a period of between 200 and 1000 hertz. A schematic of what this circuit may look like is shown in figure 3.

Precise timing in software is required to maintain a 50% duty cycle to prevent asymmetry in the sensor excitation signal. See Figure 4. Asymmetry is defined as direct current to the sensor which causes drift and ultimately permanent and irreversible damage to the sensor. This can be explained chemically; direct current causes electrolysis to take place, eventually rendering the sensor inert.

Let’s examine the process of creating an excitation signal in software for a PIC18F microprocessor using the CCS PCW C Compiler. These instructions will differ significantly depending on your specific microprocessor and compiler. We’ll start by defining which pins will be used for output. We will use PORTB for output so we will need to define it as such. This is accomplished by setting the 8 bit data direction register, called TRISB. Bits in TRISB set to 0 will define the corresponding PORTB pin as an output, so we will use the following line of code to use all PORTB pins for output:

// use all PORTB pins for output
set_tris_b(0b00000000);

In order to quickly change the PORTB pin values, we will need to access it in memory. On this particular microprocessor, PORTB is a special function register (SFR) which is located at memory address 0xf81. We will therefore store the location of PORTB in a variable to allow easy access. This is done with the following line of code:

// store local portion of PORTB in a variable
#byte port_b = 0xf81;

Each bit in port_b now corresponds to a pin on the chip. The most significant bit is pin B8 and the least significant bit is pin B0. Let’s assume a single axis sensor is connected to pins B0 and B1. See Figure 3. If you are using a dual axis sensor, let’s assume it is connected to pins B0, B1, B2, and B3.

Figure 3 Circuit connecting electrolytic sensor to PIC18F

 

For a single axis sensor, there are two states for the excitation signal. See Figure 4. To mimic this timing diagram, the two states can be stored in an array of 8 bit numbers. This is done with the following line of code:

int8 output_arr[2] = { 0b00000010, 0b00000001 };

 

Figure 4 Excitation states of a single axis sensor

 

For a dual axis sensor, there are four states for the excitation signal (shown in Figure 5). To mimic this timing diagram, the four states can be stored in an array of 8 bit numbers. This is done with the following line of code:

int8 output_arr[4] = { 0b00000011, 0b00001100, 0b00001010, 0b00000101 };

 

Figure 5 Excitation states of a dual axis sensor

Timers are a commonly used tool to generate interrupts at precise intervals in software and they can be used to easily generate an excitation signal. It is important that we use a high priority interrupt so that the excitation signal maintains a 50% duty cycle. The following code creates a high priority interrupt which toggles the ports to generate an excitation signal for a dual axis sensor:

// interrupt to generate excitation signal 
int isr_counter = 0;

#int_timer1 HIGH
void timer1_isr(void) {
     set_timer1(64535);
     port_b =  output_arr[ (isr_counter++) % 4];    // this is for a dual axis sensor
}

In this example, timer1 is a 16-bit counter (0 to 65535). This means that it will count up from the value set by set_timer() and when it overflows (reaches 65535), an interrupt will be generated and the code within the timer1_isr() function will be executed.

The amount of time that it takes to overflow is defined by a variety of factors including the microprocessor clock speed and the timer1 setup parameters, among others. It is important to maintain an interrupt time of between 1 and 5 milliseconds to ensure proper excitation of the sensor. Refer to the manual of your individual microprocessor for this information.

The port toggling works by using the modulus function. Each time an interrupt is generated, isr_counter is incremented and modulus 4 is performed on it. This essentially makes a counter that counts from 0 to 3, incrementing with each interrupt. This accesses each of the four states stored in output_arr that comprise the excitation signal.

Sensor Output

Now that we’ve created an excitation signal, let’s examine the sensor’s output. The output is an analog voltage referenced to the excitation voltage. The output must be read in phase with the excitation signal. It is suggested to take multiple samples during each of the two/four states (two states for each axis).

 

Figure 6 Waveform output from dual axis sensor with positive X & Y tilt

 

The waveform in Figure 6 shows an example output from the analog to digital converter connected to the output of a dual axis sensor. This particular output indicates a positive tilt angle for both the x and y axis. See Figure 7 for an output indicating a negative tilt angle.

 

Figure 7 Waveform output from dual axis sensor with negative X & Y tilt

 

In order to read the output we will need to use the microprocessor’s analog to digital converter (ADC). Looking at Figure 3 we will need to set up pin A0 as an ADC input and then set A0 as the current ADC port. We can then execute the following code to read the output from a dual axis sensor in phase with the excitation signal:

 

int samples = 0;
int index = 0;
int xpos[128], xneg[128], ypos[128], yneg[128] = {0,0,0, … ,0,0,0};
int xpCount, xnCount, ypCount, ynCount = 0;

while(true) {
     while( (isr_counter % 8) != 0 );
     delay_us(150);
     while( (isr_counter % 8) == 0 ) { // loop to read x positive state
          if(index < 128) {
               xpCount++;
               xpos[index] = Read_ADC();
          }
          index++;
}
index = 0;

while( (isr_counter % 8) != 1 );
delay_us(150);
while( (isr_counter % 8) == 1 ) { // loop to read x negative state
     if(index < 128) {
          xnCount++;
          xneg[index] = Read_ADC();
     }
     index++;
}
index = 0;

while( (isr_counter % 8) != 2 );
delay_us(150);
while( (isr_counter % 8) == 2 ) { // loop to read y positive state
     if(index < 128) {
          ypCount++;
          ypos[index] = Read_ADC();
      }
      index++;
}
index = 0;

while( (isr_counter % 8) != 3 );
delay_us(150);
while( (isr_counter % 8) == 3 ) { // loop to read y negative state
     if(index < 128) {
          ynCount++;
          yneg[index] = Read_ADC();
     }
     index++;
}
index = 0;
// data processing and analysis would then be done here
}

This code will take up to 128 samples in each state of the excitation signal. The 150 μs delay allows the output to settle after the excitation signal changes. The samples can be averaged in the data analysis code to provide a more stable result. This will also introduce oversampling, which will improve precision.

Note that it is important to ensure the data analysis code finishes running before the excitation frequency reutrns to the first cycle. In the code above, the processing code allows one full cycle of excitation frequencies to calculate; this could be increased by changing the modulus in the while statements to a different multiple of 4. For example, a modulus of 12 would leave 2 cycles. Inversely, a modulus of 4 would mean there would be no time for data processing in the code.

Generating the Output

The tilt angle of the sensor can be derived by comparing the difference between the voltages for each excitation state:

X axis tilt = (X+) – (X-)
Y axis tilt = (Y+) – (Y-)

Let’s say we have a 16-bit (0 to 65535 counts) analog to digital converter reading the output from a ±20° dual axis sensor. This means that our range of output is ±65535 counts using the definitions of X and Y axis tilt above. Let’s examine the following output for one cycle:

X+ = 45535
X- = 20000
Y+ = 30000
Y- = 35535

We can then conclude the following:

X Axis Tilt = 45535 – 20000 = 25535
Y Axis Tilt = 30000 – 35535 = -5535

This value is directly related to the angle of tilt, increasing as the tilt becomes more positive and decreasing as the tilt becomes more negative. This can be used directly, or it can be converted into degrees. See Application Note 1005 for information on the process of converting these raw measurements to degrees

Software Considerations

As with all software, there are many different ways to accomplish the same goal. This document simply presents one example of how electrolytic tilt sensor signal conditioning can be achieved through the use of a specific microprocessor and compiler.

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The Fredericks Company
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Huntingdon Valley, PA 19006
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