Motorizing the IoT With Battery-powered Stepper Motors
This article looks at how stepper motors work well in IoT-oriented tasks such as positioning security cameras and remote sensors or actuating air vents, valves, and window covers.
Sensor-enabled smart objects are already playing vital roles as the “eyes and ears” of the IoT. But, until recently, there have been few practical solutions for giving IoT applications practical, affordable “arms and hands” that can reach back out across the internet and react to what they see or sense in a physical manner. This is changing, however, with the emergence of cost-effective IoT-capable electronic drivers that can use small battery packs to power motors, stepper motors, solenoids, and other types of actuators that translate the cyber world’s virtual intent into real-world actions.
Figure 1a. Stepper motors are finding a growing number of IoT applications, such as this remotely activated radiator controller.
Figure 1b. Radiator controller shown with a Microchip AVR IoT development board.
In this article, we will focus on stepper motors because their segmented rotor and armature structures allow them to rotate in small, precise, discrete steps and maintain their position while unpowered. This makes them work well in IoT-oriented tasks such as positioning security cameras and remote sensors or actuating air vents, valves, and window covers.
Working with a Limited Power Source
While some motorized IoT devices are line-powered, a growing number of applications are now required to operate in remote locations, often using relatively small, low-voltage energy sources, such as a single Li-ion cell or an AA or AAA battery. In the case of many IoT applications around the home and office, these applications are supposed to blend into the environment, meaning they can’t have a power cable.
In theory, battery power will work for many of these applications because they use the motor infrequently, so their impact on the battery’s limited capacity is relatively low. But the battery may not be able to provide the higher drive voltage and relatively large current pulses a stepper motor requires to energize its coils. As Table 1 shows, most commonly available batteries have a significant amount of internal resistance that reduces their output voltage as their output current increases.
Table 1. Small Battery Characteristics
Fortunately, a few simple strategies exist for overcoming these limitations including supply buffering, step-up converters, and custom-wound steppers. Let's looks at how each of these strategies work.
A simple technique known as “supply buffering” can be used to supplement a small battery’s limited output by adding a supercapacitor that can deliver a short high-current pulse.
The size of the supercapacitor can be calculated using the formula:
C = dU*I/t
dU = the battery’s maximum permissible internal voltage drop,
I = the current needed to supplement the battery’s output, and
t = desired operating time
At present, supercapacitors tolerate a maximum working voltage of only 2.7V and require a protection circuit if the supply voltage can exceed this value. Where higher voltages are required, two or more supercapacitors may be placed in series, but the circuit must include a Zener diode or some other device for balancing the voltages (Figure 2).
Figure 2. A supercapacitor balancing circuit with Zener overvoltage protection (2.5V).
Supercapacitors suitable for these types of applications are now widely available from many component manufacturers, including Maxwell, Skeleton, and Vishay.
Some ICs, including many popular motor drivers, have difficulty running on the low voltages available from small battery packs, especially when they are near the end of their service life. Step-up converters are low-cost ICs that can be used to boost a battery’s voltage by as much as three to four times and to maintain your system’s supply voltage at an even level toward the end of a battery’s life. These converters are very efficient (90%-95%) at high loads, but their efficiency drops somewhat when they are lightly loaded. They can be used as a standalone solution or in conjunction with a supercapacitor.
IC manufacturers producing step-up converters include Analog Devices, Maxim Integrated, and Texas Instruments. One of the most frequently used converters for this type of application is Maxim’s MAX8969.
Most steppers, even small ones, are designed to operate on voltages ranging from 5V to 12V, whereas most small battery stacks produce 1.5V to 5V. To operate on these lower voltages, steppers need windings with fewer turns of thicker, lower-resistance wire. Fortunately, most manufacturers are set up to accommodate custom orders for a reasonable charge, or at no charge.
To order a custom-wound motor, you will need to specify a coil current (ICOIL), defined as the RMS motor current, that gives the nominal torque at standstill. For these types of applications, it is good to use a motor that will deliver the necessary torque at 50%-70% of its maximum rated current to minimize resistive losses and to provide some headroom.
The first step in specifying the windings is to use the manufacturer’s original motor specifications to calculate the current needed to generate the torque required for your application. Use this value to calculate the voltage requirement for the motor type using the formula below.
This calculation for standstill conditions is also reasonably accurate for slow-motion operation where there is little back-EMF. At higher speeds, the motor’s specific back-EMF constant CBEMF should also be accounted for using the following:
This formula uses the quotient of holding torque and assigned coil current. Note that in either situation, reducing the stepper’s RCOIL allows for a lower UBAT.
If you find that the voltage requirement of the stepper you’ve selected exceeds the voltage available from your supply, contact the motor manufacturer about a custom-wound version that will operate on a lower voltage using a higher current.
Putting It All Together
If you are interested in learning more about the techniques introduced in this article, you can download Trinamic Application Note #57, How to Make a Thermostat with the TMC2300, where practical examples are used to explore many of these topics in more depth.
The theory behind these techniques is explained in even greater detail in the paper Low Voltage Motor Control System Design for Mobile and Wireless IoT Device, which I presented at Embedded World 2020.
- The Inventables Workshop: Stepper Motors Basics
- Choosing the Right Motor for Your Project – DC vs Stepper vs Servo Motors
- Driving a Stepper – Adafruit Industries
- TMC2300-THERMO-BOB evaluation kit (PDF)
- Datasheet: Trinamic TMC2300 low voltage stepper driver
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