Introduction

Acro Associates’ solenoid pinch valves employ a spring-loaded linear actuator to control the motion of the pinching surfaces. Most linear solenoid actuators are closing-the-air-gap devices, where the force characteristics vary substantially throughout the stroke. Generally, the actuators are designed so that the force to power ratio increases by a factor of 3-4 as the unit moves towards the fully energized position. Utilizing these characteristics of solenoid actuators, the “Pulse and Hold” technique can hold a pinch valve in the energized state at reduced power with respect to the applied power required for initial activation. Power consumption is optimized, heat generation minimized and the result is increased performance: higher actuation forces, longer strokes, and extended duty cycles.

Model 900R Modular Solenoid Controller

Acro’s Model 900R is a rugged, cost effective control module designed for interfacing high-performance solenoid actuators (such as Acro’s Models 951, 955 and 958 Solenoid Pinch Valves) to computer systems and digital logic. Its primary purpose is to provide “Pulse & Hold” actuation, which is required for hard to pinch tubing materials, heat sensitive and high duty cycle applications.

The Circuit Model of a Solenoid

A solenoid actuator can be modeled as fixed resistance in series with a variable inductance (an RL circuit). The resistivity of the wire used to wind the solenoid’s armature determines the DC resistance of the solenoid. The arrangement of the windings and materials used to construct the actuator, in addition to the position of the ferrite plunger; all contribute to the inductance of the solenoid. Because the solenoid is resistive, any current passing through the windings produces heat. Excessive heat may damage the actuator and consume unnecessary power. In general, power management devices, such as Acro’s Model 900R, modulate the armature current so that the desired mechanical performance is achieved, while significantly reducing energy consumption and waste-heat production.

Adaptive Pulse and Hold

“Pulse and Hold” (also known as “Spike and Hold”, “Pick and Hold”, and “Hit and Hold”) is a technique whereby a strong initial timed DC pulse is passed through the solenoid actuator to close the air gap; this is then followed by a reduced current or hold state. In the hold state, Acro’s Model 900R, controller operates the power switch in PWM mode and the reduced cycle maintains the solenoid’s energized state. When the power is removed, the power switch shut offs and the cycle ends.

As an enhancement to standard “Pulse and Hold” techniques, Acro utilizes an integrated microprocessor, which analyzes load current for additional power savings and for short-circuits protection. This exclusive adaptive technology makes our products 3 to 10 times more power efficient than other solenoid control products on the market.

Pulse Width Modulation

PWM is used to vary the total amount of power delivered to a load without resistive waste and is a method commonly used in DC motor speed control. It is also useful in varying the current through a solenoid because it keeps the power switch either in pinch-off or saturation mode most of the time, which minimizes the heat generated by the switch. Acro’s Model 900R contains a low-side Field-Effect Transistor (FET) power switch employing a PWM output.

A PWM power signal, which consists of a fixed frequency square wave with a varying duty cycle, is fed to the gate of the power switch. If the frequency of the signal is much higher than the response time of the armature (usually out of the audio range), then the effective current through the solenoid is the time average of the current provided by the power switch. The low-side MOSFET power switch employed, in Acro’s Model 900R Modular Solenoid Controller, operates at a fixed PWM frequency
of 28.5k Hz.

Conclusion

The power savings realized by making use of the “Pulse and Hold” technique is dependent upon the pinch valve’s intended application. Generally, if the actuator is energized for long periods, standard “Pulse and Hold” will reduce waste energy significantly. Acro’s “Adaptive Pulse and Hold”, as utilized in the Model 900R, provides for additional power savings, which is highly recommended for applications where the pinch valve cycles at a high rate.

“Pulse and Hold” power management increases the capacity of the user’s power system and the life of the solenoid actuators by reducing the required energy consumption and emitted waste heat. Higher actuation forces, longer strokes, and extended duty cycles are the results for your pinch valve.