Power windows: now much more than just a motor and switch, Part 1

The automotive power window has evolved from a basic on/off switch and motor to a sophisticated motion-control subsystem with dedicated ICs.

Figure 1. The manual, crank-operated car window and door latch was the only approach available for many years (Image: How a Car Works).

In just a few decades, we’ve come a very long way from those bygone days of manually operated car windows, outside mirrors, and door locks. Now they are all electrically powered, with no manual backup linkage. Managing and delivering controlled power for the various motors and mechanisms used for the complicated door system and adjuncts — and it is truly a system — is a substantial design challenge.

A look at car windows shows the change and how it has evolved. About 50 years ago, all car windows were manually operated via a mechanical linkage actuated by a simple “hand-powered” rotary crank, formally called the “window winder” by designers. Each occupant of the car had to manage his or her own window using their arm as the power source (Figure 1); the car’s driver could only do the adjacent window rather than all four.

You could buy a plain replacement crank (window winder) at any auto-supply store in case yours broke, and decorative aftermarket cranks were also popular (Figure 2).

Figure 2. Plain replacement cranks as well as decorative versions were standard auto-supply store items when windows were crank-operated (Image: SIA Joom — Latvia).

Power windows were first introduced in the 1940s and initially used an electrically controlled, hydraulic-powered system since small electric-motor technology (size, power, and control) was not advanced enough to allow motors to be embedded into the door. The mechanism that moved the window up and down is called the window regulator.

In the 1960s, the Cadillac Fleetwood came standard with windows powered solely by electric motors. Within about a decade, power windows were a standard feature on most cars, which is a fairly quick rate of mass-market adoption. By the start of the 21st century, the car door was transformed into a complex electromechanical subsystem with all functions activated electronically and implemented electrically without mechanical backup systems (Figure 3) (there was an intermediate period when doors and windows had both electrical and mechanical control, to overcome customer hesitation and provide a comfort factor).

Figure 3. The high-level overview of a modern car’s driver-side door shows some of the complexity and sophistication packed in the available space (Image: CareParts.com via Facebook).

Two basic mechanisms were used for what is formally termed a window regulator: the Ford cable-type regulator (Figure 5), and the Toyota gear-drive-type window regulator (Figure 4).

Figure 4. The “Toyota” power mechanism is based on a geared scissor arrangement (Image: Samarins).

Figure 5. The “Ford” power mechanism for the window regulator uses a cable and sliding assembly (Image: Samarins).

Regardless of which mechanical design was used, both had one common electrical aspect. Unregulated 12-V DC power went directly from the car battery for the motor via the individual driver/passenger control switches of the respective window motors. As a result, incorporating this “convenience feature” into the car added to the ever-increasing burden of thicker cable harnesses and the challenges of routing them around the vehicle.

The next part looks at two application-specific ICs for automotive power windows and door functions.