Assuming that the interval between two heartbeats is t seconds, the instantaneous heart rate can be expressed as:
IHR=60 times/minute
That is to say, the time between two adjacent R waves is measured for t seconds (that is, the instantaneous heart rate period), and then this heart rate period is converted into the number of heartbeats per minute.
If the clock pulse with frequency f is used as the reference signal for measuring time, and the clock pulse is counted within t seconds, and the design value is n, then T=N/f, then the calculation formula of instantaneous heart rate can be expressed as:
IHR=60f/N (times/minute)
As long as the pulse number is calculated, the instantaneous heart rate [1] can be calculated according to the above formula.
2 circuit composition and working principle
2. 1 Circuit composition block diagram
The weak human ECG signal (about 0.5mV~3 mV) is obtained by sampling circuit, and the ECG signal is amplified to about 2V by an amplifier with low-pass filtering characteristics to filter out the audio interference contained in the ECG signal. The shaping circuit first converts the pulse wave into a square wave through a comparator, and then shapes the irregular square wave into a "clean" rectangular pulse with the same width and less than one clock period through a monostable trigger. A synchronous crystal multivibrator generates a clock pulse with stable frequency as a time base signal. The counter counts the number of clock pulses input in the heart rate period of t seconds, decodes the storage circuit to get the BCD code of instantaneous heart rate value, and then sends it to the display circuit for display by the latch driving circuit. The rectangular narrow pulse obtained by amplifying and shaping the ECG signal is used as the latch enable signal of the latch, and after being delayed, it is used as the clear signal of the counter to ensure that the counter counts in the instantaneous heart rate period, and the latch displays the rate of the last heartbeat.
2.2 The working principle of the circuit and each module
2.2. 1 sampling sensor
A sensor is a physical device or biological organ, which can detect and feel external signals, physical conditions (such as light, heat and humidity) or chemical components (such as smoke), and transmit the detected information to other devices or organs.
Classification of sensor working principle Physical sensors apply physical effects, such as piezoelectric effect, magnetostrictive phenomenon, ionization, polarization, thermoelectric, photoelectric, magnetoelectric and other effects. Small changes in the measured signal amount will be converted into electrical signals. Chemical sensors include sensors with chemical adsorption, electrochemical reaction and other phenomena as causal relations, and small changes in the measured signal amount will also be converted into electrical signals. The sensor is provided with 15V power supply, and the crystal oscillator in the excitation circuit generates a square wave of 400Hz, which is generated by the TDA2030 power amplifier, and is transmitted from the static primary coil to the rotating secondary coil through the energy loop transformer T 1, and the obtained AC power supply is obtained by the on-axis rectification filter circuit as the working power supply of the operational amplifier AD822; A high-precision regulated power supply consisting of a reference power supply AD589 and a dual-channel operational amplifier AD822 generates a precision DC power supply of 4.5V, which is used as both a bridge power supply and a working power supply for amplifiers and V/F converters.
The sampling circuit adopts corrosion-resistant ceramic pressure sensor CPS 182. The corrosion-resistant ceramic pressure sensor has no liquid transmission, and the pressure directly acts on the front of the ceramic diaphragm, which makes the diaphragm slightly deformed. Thick film resistor is printed on the back of ceramic diaphragm and connected into Wheatstone bridge. Through laser calibration, the sensor has high temperature stability and time stability.
2.2.2 amplification shaping circuit
The amplifier consists of a first-stage amplifier A 1, a second-stage amplifier A2 and an RC low-pass filter, as shown in Figure 2. The design requirements of this part of the circuit are: (1) high input impedance and low output impedance; (2) the magnification is large enough; (3) Good low frequency response; (4) the temperature drift is small; (5) Strong anti-interference ability [2].
The resistor R 1 and the capacitor C 1, the resistor R3 and the capacitor C2 form a low-pass filter to filter out the audio noise in the pulse wave. The resistance values of R 1 and R3 are 2kω and c 1 respectively, and the capacity of C2 is subject to the measured value. The first-stage operational amplifier A 1 adopts differential proportional amplification circuit (because the bridge circuit is in front), and the second-stage operational amplifier A2 adopts voltage series negative feedback, which improves the stability of amplification, and the input resistance of the operational amplifier stage is larger and the output resistance is smaller. The first stage voltage magnification is
Obviously, the above formula ignores the influence of capacitance C 1. In fact, the total voltage is amplified by a
The ECG signal output by the amplifier can not be directly used for heart rate measurement, but must be converted into pulse signal by shaping circuit. The shaping circuit is shown in Figure 3. The shaping circuit in this design first converts the pulse wave into a square wave through a comparator, and then converts the square wave into a narrow pulse with the same width through a monostable composed of R 10 and C6, which is used as a latch enable signal and a counter delay clearing signal.
2.2.3 Counting decoding display circuit
The counting decoding display circuit of this design is shown in Figure 4. 16 bit binary counter is composed of 12 bit binary counter 74HC4040 and 4 bit binary counter 74LS 163. Its counting value is between 0 and 65535, and the counted binary number is used as the address code of EPROM uPD24C 1024D, and the data stored in this address unit is BCD code corresponding to the instantaneous heart rate value. The data output by EPROM is sent to three chips CC45 1 1 with latch driving function, which drives the LED display to display unit, tens and hundreds [3].
Assuming that the heart rate is 150 beats/min, the counter can count the number of pulses in a period t, n = 60×1000/150 = 400 (pieces), which is converted into hexadecimal number 190, that is,/kloc-in EPROM. D7, D6, D5 and D4 are 0, 1, 0,1respectively; D3, D2, D 1 and D0 are all 0. If the heart rate is 80 beats/min, the number of pulses counted by the counter in a cycle is N=60× 1 000/80=750 (pieces), which is expressed in hexadecimal as 2EE, that is, the 80-bit heart rate value is filled in the address cell of 2EE of EPROM. D7, D6, D5 and D4 are 1, 0, 0, 0 respectively; D3, D2, D 1 and D0 are all 0. If the heart rate is 10 beats/min, the number of pulses counted by the counter in a period is n = 60×1000/10 = 6 000 (pieces), which is expressed in hexadecimal as 65438+. D7, D6, D5 and D4 are 0, 0, 0,1respectively; D3, D2, D 1 and D0 are all 0. Code accordingly. Because of the difference between the actual count value and the theoretical value, n values in a certain range can represent the same heart rate value, so multiple address units of EPROM may store the same heart rate value, and the lower the heart rate, the more corresponding addresses.
power circuit
The power supply circuit is designed by traditional methods [3]. After rectification, filtering and amplitude stabilization, the 220 V alternating current outputs a stable DC voltage to supply power to all circuits. Correctly set the turns ratio of the primary and secondary windings of the transformer, so that the power supply circuit can provide DC voltages of+15V and +5V. The circuit diagram is shown in Figure 5. Power supply circuit refers to the circuit design, circuit form and characteristics of the power supply part provided for power supply of electrical equipment. There are AC power and DC power. DC power supply circuit is divided into switching power supply and non-switching power supply, and the circuits are also very different. Switching power supply generally does not need a transformer; Switching-free power supply is a traditional design method, and transformers are often used in power supply circuits to transform and then rectify and filter. Several elements are interconnected by wires to form a "circuit", which can also be called a "network". More specifically, the circuit is a network that can form a closed loop. "Branch" is a part of the circuit, and each component has its own branch. The intersection of any two or more branches is called a node. Several elements are interconnected by wires to form a "circuit", which can also be called a "network". More specifically, the circuit is a network that can form a closed loop.
The data output by EPBOM reflects the instantaneous heart rate value and is displayed after being latched. The latch enable signal is taken from the narrow pulse of the ECG signal output at 555. When the rising edge of the pulse comes, the latch latches the current value. The delay circuit ensures that the latches CC45 1 1 have enough latch time before the counter is cleared. This ensures that the counter counts in the instantaneous heart rate cycle, and the latch shows the rate of the previous heartbeat.
According to the instantaneous heart rate calculation formula: IHR=60f /N (times/minute), when the cycle count value n is small, the instantaneous heart rate value corresponding to the change of n by one unit (increase or decrease 1) changes greatly. Therefore, considering the resolution requirement and the address range of EPROM (0~65 535), this design adopts a high stable clock pulse with frequency f= 1000 Hz as the time base signal. The stability of clock pulse frequency directly determines the timing accuracy. The clock circuit in this design adopts a synchronous crystal multivibrator with high oscillation frequency stability. Adjust the capacitance of capacitor C4 to keep the clock frequency of the crystal oscillator at f= 1000 Hz.
The instantaneous heart rate is calculated by the formula IHR=60f/N (times/minute).
N = 6 0 f /IHR=60× 1000/IHR
Therefore, the relationship between the instantaneous heart rate value IHR and the count value n is shown in table 1 [4].