For safety reasons, a motor that drives a safety-critical electromechanical assembly often includes an electromagnetic brake on its drive shaft. The brake typically comprises a solenoid coil that actuates a mechanical clutch, and, when you power it, the brake allows the drive shaft to rotate. Although simple and robust, the brake requires a lot of energy to release the clutch and then much less energy to remain actuated.
Measurements show that a brake rated for 24 V dc requires a minimum of 18 V to release and as little as 8 V holding voltage. Substituting those numbers into the equation
shows that, while actuated, the brake consumes less than a quarter of the power required for its initial release. Conversion of excess release power into heat normally doesn't pose problems.
However, a precision positioner that uses a brake mounted on a long drive screw can suffer from unacceptable errors if the heat expands the drive screw and alters the assembly's position.
One method of solving the problem involves actuating the brake by applying 24 V dc for a brief interval and then reducing the holding voltage to 12 V. Under these conditions, the brake dissipates only a quarter of the initial power and thus operates at a reasonable temperature. Figure 1 shows the influence of actuation voltage on the brake's temperature. As expected, lowering the voltage after actuation drastically lowers the brake's temperature and therefore its effects on the positioning screw.
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| Figure 1. | Under continuous operation with 24 V applied, the brake’s temperature stabilizes at 75 °C, or 53 °C above ambient temperature (Curve A). Applying a 24 V actuation pulse for a few seconds and then applying a 12 V holding voltage stabilizes the brake at 34 °C, or only 12 °C above ambient temperature (Curve B). |
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Figure 2 shows one obvious voltage-reduction approach, which uses relays and a power resistor to halve the voltage applied to the brake. Setting the current-limiting resistance, RPOWER, equal to the brake's solenoid resistance, RBRAKE, reveals a few problems. First, the power resistor must dissipate as much power as the brake solenoid's coil. Second, the relays and power resistor occupy considerable space on a pc board. Third, proportioning the values of the R1 C1 delay circuit's components to achieve a few seconds' delay can prove difficult.
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| Figure 2. | Actuating the brake release trips relay K1 and applies 24 V to the brake. An RC network delays K2’s actuation. When normally closed relay K2 opens, resistor RPOWER reduces the voltage applied to the brake to the holding level. |
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Figure 3 shows another approach, which uses the actuator coil's inductance and replaces relays with an IC. The voltage you apply to the brake need not be continuous, and applying a PWM (pulse-width-modulated) voltage works as well as applying a dc holding voltage because the coil's inductance integrates the current pulses.
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| Figure 3. | Applying the command input switches on PWM regulator IC1, and capacitor C1 holds IC1’s feedback input low, applying a maximum output voltage of 24 V to the brake until C1 fully charges. As the feedback voltage slowly rises to 1.23 V, the regulator’s output voltage decreases to approximately 12 V, the brake’s nominal holding voltage. |
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A switched-mode voltage regulator can provide an inexpensive and effective PWM-drive voltage. For example, LM2575 adjustable regulator, IC1, operates over a 7 to 40 V range and includes an on/off-control input and a high-impedance feedback input, but any other switching-regulator IC with these two characteristics would also serve. Resistors R1 and R2 determine the holding voltage (Figure 4). Capacitor C3 filters the PWM signal to a dc voltage at the feedback input and also maintains the feedback input for a few seconds during start-up at ground, forcing the regulator to deliver the full input voltage to actuate the brake. Diode D1 quickly discharges the capacitor when the regulator switches off, diode D2 clamps the switch-off transient voltage that the brake's actuating coil produces, and diode D3 protects IC1 against reverse voltage. Photo coupler IC2 isolates the brake controller from the control circuit.
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| Figure 4. | After the actuation pulse applies full voltage to the brake, the regulator’s output gradually decreases to the nominal holding voltage. |
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During start-up, the duration of the regulator's 24 V actuation-pulse output fluctuates from 1 to 4 seconds (Figure 4). Fortunately, the variation has no impact on the circuit's function but could present a problem if another application requires a precisely timed actuation pulse. After start-up, the regulator delivers a 12 V holding voltage, reducing the power demand to one-quarter of the start-up value. As a bonus, the circuit uses inexpensive components, occupies only a few square centimeters of pc-board area, and eliminates the need for two electromechanical relays. Wiring for the PWM-drive voltage can radiate electrical noise unless the circuit is adjacent to the brake. For remote installation, use a shielded twisted-pair cable to minimize noise radiation.
