Putting On The Brakes

September 26, 2005

9 Min Read
Putting On The Brakes

Dynamic spring-set brakes come in two ways. One style holds the load with power applied to the electric brake solenoid. A second type holds with the power disconnected. Power-on brakes secure the load when the specified voltage energizes the solenoid, but lose their grip during a power failure. Power-off brakes are safe by default. Any power loss engages the brake to hold the load with a mechanical spring. In addition, a power-off brake is safer and preferred for use on machines where loads move vertically.

Engaging and disengaging the brake takes a finite amount of time, so the equations that compute the time delay determine the final position at which a machine member will either arrive or depart, or the time for a shaft to start or stop moving when commanded by an electrical signal. The two choices available in the brake selection process determine whether the machine stops within a specified time or distance.

Worst-Case Selection Parameters

Consider the following parameters before determining brake inertia, brake torque, and operating time delays. Then select a family of brakes from the manufacturers catalog based on these parameters.

  • Brake mode: power on or power off

  • Temperature range

  • Voltage/current range for the brake solenoid

  • Allowable solenoid temperature rise

  • Duty cycle—continuous or intermittent

  • Mechanical brake mounting restrictions or preferences

  • Stopping operations/requirements: emergency dynamic only, static hold only, or dynamic stop every cycle (total number of dynamic stops required by the application)

  • Maximum speed and direction of rotation

  • Mounting position of brake: vertical or horizontal

  • System drag or friction torque

  • System inertia

  • Allowable deceleration time and allowable number of shaft revolutions after issuing a stop command (overshoot)

  • Cycle-to-cycle stopping tolerance and variation over life

A power-off brake will hold a load even when power fails because springs supply the actuating force.

A power-on brake needs electricity to stop or hold a load.

Power-Off Operation

Consider the power-off mode first. The brake engages and stops the load within a few milliseconds after the brake solenoid receives the command to release or de-energize. Braking component makers usually supply a graph of the solenoid current versus the time delays. The amount of time delay between de-energizing the solenoid and stopping the load depends on the total system inertia, which is composed of the load inertia and the brake inertia.

Stopping time adds de-energize time and decel time:

Ts = Te + Td

Where:

Ts = time to stop, sec.

Te = energize time, sec

Td = deceleration time, sec

The deceleration rate, dù/dt (rad/sec²), depends upon the total torque required to stop the load and the system inertia:

dù/dt = Tt/Js

Where:

Tt = total torque, lb-inch

Js = system inertia = Jl + Jb (load inertia + brake inertia), lb-inch-sec²

The brake-engage time is typically different from the release time.

The time required for a power-off brake to engage depends in part on how quickly the armature releases.


Armature release time depends on the circuit controlling it.

All the pieces of the system add up in determining just how quickly motion stops. Stop Within Specified Time First, estimate the torque required to stop the system inertia within an interval that is half the time that the servomotor can safely hold the load without assistance. At this point in the analysis only the load inertia is known, so a rule of thumb is to add about 25 percent to the load inertia to estimate the brake rotor inertia, that is, Js = 1.25Jl. This information helps determine a candidate brake. After calculating the brake inertia, the suppliers data sheet provides the response time specifications for that brake. The equation for the estimated torque Tt is: Tt = 0.1047Js(dù/dt) Where: 0.1047 is a factor that converts rad/sec to rpm Js = system inertia, 1.25Jl, lb-in.-sec² When the system has substantial drag, Td, it aids in deceleration, so it subtracts from the brake torque rating, Tb: Tb = Tt - Td On the other hand, when an overrunning torque develops during deceleration, the brake torque rating increases: Tb = Tt + Td Next, select a suitable brake from the catalog, and run through the calculations again using the actual load torque and the specified brake rotor inertia for a new system inertia value. Verify that the brake selected can handle the system torque. If it is unsatisfactory, select the next higher size. When it proves to be satisfactory, calculate the deceleration time, dt, using the new total system inertia: dt = 0.1047Js(dù/Tt). Stop Within a Specified Distance The second option is selecting a brake to stop the load within a specified distance. First, find a brake that fits all the other selection criteria given above, and then calculate the total travel allowed after the command signal is issued to stop the load. This includes the travel while the armature engages, plus the travel during load deceleration. The total travel, S, given in the manufacturers graph of Figure 3 is: S = [(t4-t3)+(t5-t4)/2]ù/60 When the travel does not meet specifications, select another brake assembly and repeat the calculations. Or, consult with the brake maker about changing or adding an arc suppression circuit to quicken the decay time. After the travel is acceptable, calculate the energy absorption. Energy Absorption The above calculations address only a single cycle. Now verify that the brake can dissipate the kinetic energy that it absorbs per cycle for as long as it takes to perform the entire operation. In addition, repeated cycling builds up heat, which the solenoid must be able to withstand continuously. First, calculate the per cycle energy absorption, Eb: Eb = 4.6(Jù²)10-4ft-lb/cycle When the friction drag is significant compared to brake torque, modify the energy calculation by the ratio of brake torque to total torque: Eb = [Tb/Tb +Td][4.6(Jù²)10-4ft-lb/cycle When braking at a relatively rapid rate, multiply Eb per cycle by the cycle rate, N: Ebmin = [Tb/Tb +Td][4.6(Jù²)10-4(N)ft-lb/min

Most brake manufacturers furnish wear life data for determining expected life based on the number of operations. Compare the calculated values of energy per cycle and energy per minute with the values given in the product data sheets. The values should be equal to or less than the catalog ratings to ensure that the brakes survive over a long lifetime. Most manufacturers provide wear life data, which help determine the expected life, considering the total number of cycles expected. This is usually estimated by the equation: Tn = Te/Tc Where: Tn = maintenance-free life, cycles Te = total allowable energy absorption, ft-lb Tc = calculated energy absorbed per cycle, ft-lb/cycle

A sweeper benefits from power-off brakes because they can't be released unless the engine is turning. Final Proofing All solenoid-operated brake systems designed for dynamic braking also apply to static-holding systems. The wear factors for friction materials are more critical for the dynamic mode, so in all cases, the brake component manufacturer should verify the final selection. Not all relevant information and data (including some engineering units of measure) that might be critical to each unique installation is available in catalog data sheets. For example, some information regarding friction materials are proprietary and the life data given in catalog sheets and graphs are computed and collected under optimum laboratory testing conditions. This is because not all real-world installation details can be anticipated and evaluated. The manufacturers calculations may show that your selection proves to have a much greater life expectancy than initially expected. Instead it may show that the next higher rating is the optimal brake assembly for your machine. In most cases the component manufacturer does not charge for this service, so in the end, it is less expensive and shortens project time by obtaining the verification.  

Various arc suppression circuits can be built to change the decay time for a solenoid's magnetic field. Decay time is one factor affecting brake engagement time. Web Resources For more info on Danaher Motion power-off brakes, go tohttp://rbi.ims.ca/4397-557.

Sign up for the Design News Daily newsletter.

You May Also Like