Application of current sensors in control and protection of door entry systems

Door entry systems must provide safety and protection for the users. Imagine a garage door that automatically locks a person or car inside. Current measurement can provide this protection function.

Electronically controlled garages, gates, shutters and awnings are becoming part of everyday life, and protection is an important feature that must be considered when designing these systems. Current measurement can provide this protection.

The DC brush motors used in garage doors, electronic gates, shutters and awnings produce their own repeatable current profile each time the door is opened and closed normally. This current waveform can be stored in the microcontroller. The current sensor can provide feedback on the current when the DC brush motor is operating, which is compared with the current waveform stored in the microcontroller within the tolerance range defined by the door entry system manufacturer. If the values ​​do not match, there is a safety issue. The microcontroller will then sound an alarm and shut down the system or reverse the motor to close the door again. This provides protection against obstacles, people or other objects, while protecting the motor from damage caused by excessive current.

The protection function required is to detect if a person or object is blocking the correct opening or closing of a door, gate, shutter or awning. This can be achieved by monitoring the current consumed by the motor that powers the door.

The space available for electronics on these devices is very limited, so it is essential to use compact current sensors, such as the HTS10 sensor from LEM, which is only 17x19mm. In such applications, accuracy is not the most important parameter, because the purpose is not to control and regulate the motor, but to ensure its protection through detection. The current sensor is used to detect whether the motor is delivering too high a current at a specific moment, such as when an electronic door encounters an obstacle during the opening and closing process. In this application, the HTS sensor acts as a mechanical limit switch.

If a pump is clogged, the motor connected to the pump will draw more current and the HTS sensor can be used to detect the current overcurrent and then shut down the unit and/or start a backup motor or pump. HTS provides similar functionality for grinders and crushers.

When the motor rotor is blocked, the current sensor can provide early detection warning, thus providing safety protection. This can also prevent the motor from being damaged.

In lighting applications, this sensor integration can detect current overloads to protect electronic ballasts. The output of the current sensor is coupled to a circuit that manages alarm levels or triggers a circuit breaker.

Other applications for HTS include power seats and overload protection for low-end motors such as those in household appliances, including washing machines.

The HTS 10 sensor can measure bidirectional currents when powered by a single-ended +5V. Its sensitivity and offset vary in proportion to the power supply. This ratiometric output and single-ended 5V supply make it ideal for use with ratiometric microcontrollers. Microcontrollers are often used in control and protection circuits, and the ratiometric output of the sensor allows the use of low-end current sensors while avoiding the effects of power supply variations on the output accuracy. Billy's microcontroller uses the same power supply as the current sensor and performs an 8-bit analog-to-digital conversion on the sensor's output signal, so there can be a ±10% tolerance for different devices and the use of a +5V power supply for the ratiometric current sensor.

This tolerance, together with the tolerances of sensitivity and initial offset, has an influence on the measuring range. Example:

When the +5V supply has a tolerance of ±0% (in this case it is 0%), a gain tolerance of ±30%, and an offset tolerance of ±12%, the maximum initial bias of the HTS 10-P sensor can reach 2.8V and a maximum sensitivity of 130mV/A (+25°C). In this case, the sensor can provide a measurement range from +13A to -17A. The full-scale output is limited to Vdd-0.5V (= 4.5V) for the positive pole and Vss+0.5V (= +0.5V) for the negative pole.

Positive measurement range: (+4.5V-initial bias)/sensitivity = (4.5V-2.8V)/0.13 = +13Apk.

Negative measurement range: (+0.5V-initial bias)/sensitivity = (+0.5V-2.8V)/0.13 =-17.7Apk.

The HTS 10-P/SP1 sensor has a higher sensitivity and offset tolerance (±20% and ±3%, respectively), which results in less fluctuation in the measurement results (thermal drift is also improved). When the sensor is used as an accessory to a microcontroller, the device is calibrated so that tolerances can be easily eliminated or adjusted without affecting the calculation accuracy, ensuring repeatable results. Only the linearity error needs to be considered.

The HTS 10-P and 10-P/SP1 can measure 10ARMS rated current (15Apk) with a maximum linearity accuracy of 1% (25°C), which is suitable for the detection requirements of this type of equipment. The maximum deviation of the voltage including the hysteresis offset when drifting to 1 x Ip is the maximum linearity error and is a repeatable value.

Linear curve

This product is designed for PCB applications and has primary and secondary connector pins. However, there are two ways to connect the main conductors:

1. When the PCB trace represents the main conductor to be measured, the two sensor pins of the HTS can be fixed to the trace. These two pins are connected to a three-turn coil built near the magnetic circuit. In this application, the rated primary current of the permanent magnet of 10 Arms (30 ampere-turns) can be measured with a peak of 15A (45 ampere-turns). This configuration requires cutting the main conductor.

2. If non-contact measurement is required without cutting the main conductor, the sensor aperture can be used. When the conductor passes through the aperture, a rated current measurement range of 30Arms and 45Apk can be provided. In this application, the current measurement range of the sensor can be changed by changing the number of times the main conductor passes through the aperture. For example, 3 turns around the aperture can measure 10Arms, and 5 turns around can measure 6Arms. In order to achieve the same output accuracy as the internal coil, the cumulative effective current of the conductor must be 30Arms regardless of how many turns the conductor passes through the sensor aperture. Due to the power supply and voltage output, the sensor still needs to be soldered on the PCB board.

Based on the use of open-loop Hall effect and ASIC (Application Specific Integrated Circuit), HTS sensors measure current frequencies from DC to 16kHz (signal attenuation -3dB) and to 10kHz (-1dB).

Bandwidth Curve

HTS has another advantage over current transformers in that, in addition to measuring AC signals, it can also measure current signals containing DC components.