How to Adapt Solenoid and Stepper Motor Drivers for Industrial Applications

Edge device applications, such as factory floor control systems, automotive, and lab equipment, increasingly utilize Internet of Things (IoT) and artificial intelligence (AI) capabilities for low-latency decision making, higher performance, lower cost, and greater safety and productivity. Drivers for solenoids and stepper motors need to evolve to incorporate more onboard sensing and intelligence to facilitate their integration into this rapidly evolving environment, and to further improve precision, reliability, closed-loop control, cost, footprint, and ease of use.

This article summarizes the basic operation of solenoids and stepper motors and outlines the benefits of driver ICs designed for the intelligent edge. It then introduces and explains how to start designing with sample drivers from Analog Devices.

Solenoids and steppers: similar yet different

Solenoids and stepper motors convert electrical current to physical motion via a wound coil acting as an electromagnet. Despite the differences in appearance and function, the coil commonality allows the same driver IC to be used for both actuators in some circumstances.

Solenoids are relatively simple components that develop linear mechanical motion with applied current. They comprise an electrical coil wound around a cylindrical tube with a ferromagnetic actuator (also called the plunger or armature) in the hollow core that is free to move within the body of the coil (Figure 1, left).

In contrast, stepper motors use multiple stator coils arranged around the circumference of the motor body (Figure 1, right). The motor also has a set of permanent magnets attached to its rotor.

Figure 1: Solenoid construction comprises a wound coil with an internal sliding plunger (left); stepper motors are more complicated, with permanent magnets on the rotor and electromagnetic coils arranged on the stator (right). (Image sources: Analog Devices, Monolithic Power Systems)

For solenoids, the motion of the plunger is a single “punch” impact that occurs when a current is applied, slamming the plunger to its extreme position. When the power is removed, most solenoids use a spring to return the plunger to its nominal rest position.

In the most basic drive scheme, the solenoid is controlled by a crisp on/off current pulse. While this is simple and direct, its drawbacks include high impact force, vibration, audible and electrical noise, electrical inefficiency, and little control over the plunger action or its return.

The rotational action is activated for the stepper motor when the stator coils are energized in sequence, and the resultant rotating magnetic field pulls on the armature magnets. By controlling the sequencing, the stepper’s rotor can be made to rotate continuously, stop, or reverse direction.

Unlike the solenoid, which has no timing considerations, the stator coils must be energized sequentially and with the correct pulse width among other attributes.

Smart drivers overcome limitations, enhance performance

By carefully controlling the current driving the coils of solenoids and stepper motors, including waveform profile shape, up and down ramp rate, and other parameters, an intelligent driver can provide many benefits, including:

• Enhanced smoothness of motion and rotation with minimal chattering
• Reduced vibration and impact, especially for solenoids
• More precise positioning for the stepper motor start/stop/reverse motion
• Consistent performance and accommodation of transient or varying load conditions
• Improved efficiency
• Less physical wear
• Generation of less audible and electrical noise
• Ease of interfacing with a supervisory processor, essential for IoT installations

The Analog Devices MAX22200, an integrated, serial-controlled solenoid and motor driver, shows what a sophisticated driver can do for solenoids (Figure 2). The eight, 1 ampere (A) half-bridge drivers in this 36 volt IC can be paralleled to double the drive current, or configured as full bridges to drive up to four latched valves (also called bi-stable valves).

Figure 2: The Analog Devices MAX22200 is an integrated, serial-controlled solenoid and motor driver with eight half-bridge drivers that can be arranged in different configurations. (Image source: Analog Devices)

This driver supports two control methods: voltage drive regulation (VDR) and current drive regulation (CDR). With VDR, the device outputs a pulse width modulated (PWM) voltage in which the duty cycle is programmed using its SPI interface. The output current is proportional to the programmed duty cycle for a given supply voltage and solenoid resistor. CDR is a form of closed-loop control where an integrated, lossless current-sensing circuit senses the output current and compares it with an internal programmable reference current.

Unlike a simplistic current-source driver, the MAX22200 offers tailoring of the current-drive profile. To optimize power management in solenoid drive applications, the excitation drive level (I_HIT), the hold drive level (I_HOLD), and the excitation drive time (t_HIT) can be individually configured for each channel. It also offers multiple protection and fault-related features, including:

• Overcurrent protection (OCP)
• Open-load (OL) detection
• Thermal shutdown (TSD)
• Undervoltage lockout (UVLO)
• Detection of plunger movement (DPM) verification

The first four features are standard and well understood. DPM requires further explanation. For instance, if the valve works correctly when the solenoid is activated in a solenoid-controlled valve, the current profile is not monotonic (Figure 3, black curve). Instead, it shows a drop due to the back electromotive force (BEMF) generated by the movement of the plunger (Figure 3, blue curve).

Figure 3: When driving a solenoid, the MAX22200 can detect a stuck solenoid or valve by looking for the expected BEMF-driven current drop versus the threshold value (IDPM_TH) as the solenoid is driven from the start current (I_START) to the final excitation drive level (I_HIT). (Image source: Analog Devices)

When set up and used for solenoids, the DPM function of the MAX22200 detects the presence of the BEMF drop during the excitation phase. If the drop is undetected, an indication is set on the FAULT pin and in the internal fault register.

Evaluation kits ease the process

To resolve issues related to the system's performance under different static and dynamic demands and load conditions, Analog Devices offers the MAX22200EVKIT# Solenoid Control Power Management Evaluation Board for the MAX22200 (Figure 4). This evaluation kit (EVK) enables serial control of the MAX22200 and fault monitoring through an onboard USB-to-SPI interface via a MAX32625 microcontroller. It includes a Windows-compatible graphical user interface (GUI) for exercising the features of the MAX22200 IC, making it a complete PC-based evaluation system.

Figure 4: The MAX22200EVKIT# Solenoid Control Power Management Evaluation Board for the MAX22200 facilitates full exercise of the IC and its load using a Windows-based GUI. (Image source: Analog Devices)

This fully assembled and tested board is configurable as a high-side/low-side solenoid, and for latched valves (often driven by solenoids) or brushed DC motors.

Stepper motors: more degrees of freedom to control

Stepper motors are more complicated than solenoids and have more control requirements. This is seen in the features of the Analog Devices TMC5240 (Figure 5), an integrated, high-performance stepper motor controller and driver IC with serial communication interfaces (SPI, UART), extensive diagnostic capabilities, and embedded algorithms.

Figure 5: The TMC5240 high-performance stepper-motor controller and driver IC embeds sophisticated algorithms to deliver optimum performance with solenoids and stepper motors. (Image source: Analog Devices)

This IC combines a flexible eight-point ramp generator for minimum jerk in automatic target positioning. Jerk is the rate of change of acceleration, and excessive jerk can cause many system problems and performance issues. This stepper-motor driver integrates 36 volt, 3 A H-bridges with 0.23 ohms (Ω) on-resistance and non-dissipative integrated current sensing (ICS). The TMC5240 is available in a small, 5 × 5 millimeter (mm) TQFN32 package and a thermally optimized 9.7 × 4.4 mm TSSOP38 package with an exposed pad.

The TMC5240 implements unique and advanced features that enable enhanced precision, high energy efficiency, high reliability, smooth motion, and cool operation. These features include:

• StealthChop2: A no-noise, high-precision chopper algorithm for inaudible motion and standstill of the motor, allowing for faster motor acceleration and deceleration than the simpler StealthChop
• SpreadCycle: High-precision, cycle-by-cycle current control for the highest dynamic movements
• StallGuard2: Provides sensorless stall detection and mechanical load measurement for SpreadCycle
• StallGuard4: Offers sensorless stall detection and mechanical load measurement for StealthChop
• CoolStep: Uses StallGuard measurement to adapt the motor current for the best efficiency and lowest heat-up of the motor and driver

These features can be pre-set and invoked during the motor’s operating cycle. In addition, torque can be controlled in conjunction with acceleration to develop the desired value while providing efficient and smooth acceleration and deceleration.

For example, a set of three acceleration and deceleration segments can be used in two ways: for adaptation to the motor torque curve by using higher acceleration values at a lower velocity, or to reduce the jerk when transitioning from one acceleration segment to the next. To address both, the TMC5240’s eight-point motion-profile generator allows the controller to maintain a constant-velocity segment while the desired target position changes in real time, resulting in bumpless mode transfers (Figure 6).

Figure 6: The TMC5240 offers an eight-point ramp supporting on-the-fly target position change, resulting in bumpless mode transfers. (Image source: Analog Devices)

Given the flexibility, versatility, and complexity of this driver IC, the TMC5240-EVAL evaluation board is a welcome adjunct (Figure 7). It uses the standard schematic diagram for the IC and offers several options in its software, allowing designers to test different modes of operation.

Figure 7: Using the TMC5240-EVAL evaluation board and associated GUI, designers can investigate and tune the performance of the TMC5240 to their specific actuator and load combination. (Image source: Analog Devices)

For designers with less complex evaluation and design requirements, Analog Devices also offers the TMC5240-BOB. This basic IC breakout board brings the physical pin connections of the TMC5240 out to user-accessible header rows.

Conclusion

Adding intelligence to solenoid and stepper motor drivers provides better control and fault detection, enables real-time decision making, and allows communication with higher-level control or AI-based productivity systems. Highly integrated drivers, such as the Analog Devices MAX22200 and TMC5240, allow users to quickly get up and running with advanced algorithms to optimize solenoid and stepper motor performance for their application.