Motor-driven automation in smart home systems: how to achieve efficiency and cost savings

My last technical article, “Automating smart home systems with motor drivers,” explored how motorized applications like video doorbells and smart locks provide a sense of security and convenience. In this article, I’ll cover how motorized systems in window blinds and smart thermostats can help make heating, ventilation and air-conditioning (HVAC) systems more efficient and reduce overall energy costs.

Smart thermostats

Smart thermostat manufacturers claim upwards of 25% energy and cost savings on HVAC system expenses compared to traditional thermostats. These savings are possible because smart thermostats adapt to usage patterns, which optimize when the HVAC is on or off. Remote control via wireless connectivity is another factor behind the growth of smart thermostats. For example, consumers can turn on their AC when commuting home from work and arrive to a cool house.

Traditional thermostats use electromechanical relays to switch power on and off to HVAC compressors, fans, evaporators, the furnace and other parts of the system. The loud clicking noise when the AC turns on is from the traditional relay actuating. There is virtually no audible noise coming from a metal-oxide semiconductor field-effect transistor (MOSFET) H-bridge integrated circuit (IC). Because electromechanical relays have large contacts, they are also typically larger compared to ICs, taking up some serious board space.

Another major flaw with traditional electromagnetic relays is their limited number of actuations – and thus, their life span. A typical relay has between 50,000 to 100,000 cycles, so in buildings designed to last a lifetime or more, relays may not be the best solution. In comparison, actuation with an H-bridge is in the order of multiple millions or more cycles, as long as there is protection from overheating and overcurrent events. Devices like TI’s DRV8837 and DRV8837C <12-V H-bridge driver and DRV8876 <40-V H-bridge driver include the necessary protections.

Another not-so-obvious way to use an H-bridge motor driver like the DRV8837C is to drive a piezo speaker or buzzer in a thermostat. Piezo buzzers are commonly used as an audible feedback mechanism when programming the thermostat or changing the temperature. In its basic architecture, a piezo buzzer is a disk of piezoelectric material with electrodes connected to both faces of the disk. Applying a voltage deforms the disk, creating sound in the buzzer. Depending on the configuration of the OUT1 and OUT2 pins of the DRV8837C, applying a positive or negative voltage to the electrodes gives full range of the piezo speaker, as shown in Figure 1.

Figure 1: A piezo buzzer driven by DRV8837C outputs

Some thermostat models have a rotary dial controlled by a small motor, while the temperature is set remotely from a smart home app. A brushed-DC motor with positional feedback can be a good fit, with the DRV8837C driving the motor to a specified dial position. A stepper motor driver like the DRV8847 can drive this motor in full step mode with only two general-purpose inputs/outputs (GPIOs), whereas most drivers require four GPIOs.

Motorized window blinds, shades, shutters and curtains

According to the U.S. Department of Energy, in the warmer seasons, “76% of sunlight that falls on double-pane windows becomes heat.” One way to prevent heat entering the home is to add motorized awnings or shutters external to the home that prevent sunlight from reaching windows at all. Another, perhaps more common method, is to use interior motorized blinds that can adapt to temperature, changing seasons, the sun’s various positions and other variables. Motorized blinds can also help reduce cost and energy usage of HVAC systems.

A couple of different motor-driver topologies are suitable for raising and lowering blinds, depending on the load’s weight, desired efficiency and overall system complexity. For small blinds or individual installations, a motor driver with integrated MOSFETs will help keep overall system size and bill of materials (BOM) small. For heavier loads or to better distribute heat dissipation, a gate driver with external MOSFETs enables higher performance and greater flexibility by choosing the on-resistance of the MOSFET’s RDS(on) according to how much current and torque the blinds, shutters, shades or curtains require.

One other important aspect of the motor driver in these systems is current sensing and regulation. In brushed-DC-driven systems, when the motor starts up there is a spike of current, sometimes called inrush current. A large inrush current can draw too much power away from the battery, limiting supply to other critical parts of the system. The motor’s torque is also dependent on the amount of current delivered to the motor. The ability to sense and regulate current with a motor driver can enable torque control.

Another benefit of current sensing is the detection of stall conditions in the motor. When the blinds or shades reach an end-of-travel point or mechanical stop, the current in the motor will rise to a level typically much higher than the continuous current required during steady motion. By measuring the current and sending a scaled-down analog signal back to the microcontroller’s analog-to-digital converter (ADC), the microcontroller can detect this drastic change in current, assume a stalled position, and stop sending a drive signal to the motor driver, as shown in Figures 2 and 3.

Figure 2: Flow chart of a microcontroller and motor-drive operation during a stall event

Figure 3: A typical motor current profile during startup (inrush) current, continuous current and stall event current

The DRV8876 (3.5-A peak) and DRV8874 (6-A peak) are mid-voltage-range, integrated MOSFET motor drivers with integrated current sensing and feedback (through the IPROPI pin). The DRV8873 integrated MOSFET motor driver has the same integrated current sensing and feedback (IPROPI) features, but 150-mΩ RDS(on) MOSFETs enable a higher peak current of 10 A, which means that the device will work well for heavier blinds and shades with more torque capability.

For even heavier types of motorized blinds, shades, shutters and curtains, a gate driver like the DRV8701 is a good fit. You can select the RDS(on) of the external MOSFETs depending on the appropriate amount of current needed to drive the load. Separating the gate driver from the MOSFETs also more evenly spreads heat dissipation. The DRV8701 has an integrated current-shunt amplifier with an output pin that can send the sensed current back to the microcontroller to detect a stall, as shown in Figure 4.

Figure 4: The DRV8876 integrated MOSFET driver and DRV8701 gate driver for brushed-DC motors, with integrated current sensing and feedback

Smart home systems often have connectivity features to control subsystems wirelessly. Brushed-DC motors can generate noise on these communication lines and cause interference in these systems. By controlling the slew rate of the MOSFETs’ switching, it’s possible to achieve a balanced trade-off between electromagnetic interference radiation and thermal dissipation. The DRV8701 has smart gate drive technology from TI that integrates this slew rate control function. Normally this is a discrete circuit between the gate driver and the MOSFETs (Figure 5). Smart gate drivers enable more robust performance without having to increase the BOM cost and board size.

Figure 5: A half-bridge discrete solution for slew rate control vs. TI’s smart gate drive

Whether a home or office building has smart locks, video doorbells, smart thermostats, motorized window closures or other motor- and H-bridge-driven systems, TI has several motor drive devices and technologies to enhance system performance and reliability while reducing cost and size.

Additional resources